Rapid Analysis of Silicate, 

Carbonate, and Phosphate RocksRevised Edition 

GEOLOGICAL SURVEY BULLETIN 1401 

Rapid Analysis of Silicate, 

Carbonate, and Phosphate RocksRevised Edition 

By LEONARD SHAPIRO 

GEOLOGICAL SURVEY BULLETIN 1401 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1975 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director 

First printing 1975 

Second printing 1978 

For sale by the Branch of Distribution, U.S. Geological Survey, 

1200 South Eads Street, Arlington, VA 22202 
CONTENTS 

Page 

Abstract -------------------------------------------------------- 1 

Introduction ---------------------------------------------------- 1 

Apparatus and instruments --------------------------------------- 4 

Teflon beakers ---------------------------------------------- 4 

Water-repellent coated pipets ---------------------------------- 5 

Stopcock-pipet device ---------------------------------------- 6 

Tilting pipet ------------------------------------------------ 6 

Squeeze pipet ------------------------------------------------ 8 

High-precision piston pipet ----------------------------------- 8 

Pipetting machine ------------------------------------------- 9 

One-litre water dispenser ------------------------------------ 10 

Fusion equipment -------------------------~------------------ 11 

Multiple magnetic stirrers ------------------------------------ 11 

Sequential heating device for FeO ---------------------------- 11 

Carbon dioxide scanning test tube ------------------------------ 12 

Middle-range carbonate tube ---------------------------------- 13 

Evolution-absorption apparatus for C02 ---------------------- 13 

Manual spectrophotometer _ __ _ _ _ _ __ _ _ __ _ _ _ _ _ _ __ __ ___ _ _ _ ___ _ _ _ _ 13 

Automated spectrophotometer -------------------------------- 16 

Automated atomic absorption equipment ------------------------ 21 

Automatic sample changer -------------------------------- 22 

Preparation of powdered samples ---------------------------------- 25 

Two-solution procedure ------------------------------------------ 25 

Preparation of solution A ------------------------------------ 25 

Preparation of solution B ------------------------------------ 27 

Determination of constituents ---------------------------------- 29 

Si02 in silicate, carbonate. and phosphate rocks______________ 29 

Al20a in silicate and carbonate rocks ----------------------- 30 

Al20a in phosphate rocks -------------------------------- 32 

Total iron in silicate, carbonate, and phosphate rocks -------- 32 

Ti02 in silicate, carbonate, and phosphate rocks -------------- 34 

MnO in silicate, carbonate, and phosphate rocks ------------- 35 

Pz05 in silicate rocks ------------------------------------- 37 

Pz05 in carbonate rocks ---------------------------------- 38 

P205 in phosphate rocks ---------------------------------- 38 

MgO, N azO, and K20 in silicate, carbonate, and phosphate 

rocks and CaO in silicate rocks -------------------------- 39 

Total CaO+MgO in carbonate and phosphate rocks ---------- 40 

Single-solution procedure _________________________________ _,__ _ __ _ __ 43 

Preparation of solution -------------------------------------- 43 

Determination of constituents ------------------------------..,.- 46 

Si02 in silicate, carbonate, and phosphate rocks -------------- 46 

III 
IV CONTENTS 

Page 

Single-solution procedure-Continued 

Determination of constituents-Continued 

Al20a in silicate, carbonate, and phosphate rocks ----------- 47 

Total iron in silicate, carbonate, and phosphate rocks -------- 47 

Ti02 in silicate, carbonate, and phosphate rocks -------------- 48 

MnO in silicate, carbonate, and phosphate rocks ------------ 49 

P20s in silicate and carbonate rocks ------------------------ 50 

P20s in phosphate rocks ---------------------------------- 51 

MgO, N~O, and K20 ·in silicate, carbonate, and phosphate 

rocks and CaO in silicate rocks -------------------------- 53 

Total CaO+MgO in carbonate and phosphate rocks ---------- 54 

Determinations on separate samples -------------------------------- 54 

H20 in silicate, carbonate, and phosphate rocks ------------------ 54 

FeO in silicate, carbonate, and phosphate rocks ------------------ 57 

C02 in silicate rocks ------------------------------------------ 58 

Rapid scanning technique for low levels of C02 -------------- 59 

Technique for C02 content above 0.1 percent ---------------- 60 

C02 in carbonate rocks by evolution ---------------------------- 62 

C02 in phosphate rocks- -----------·--------------------------- 64 

C02 in limestone and dolomite by titration ---------------------- 64 

F in silicate, carbonate, and phosphate rocks -------------------- 66 

S in silicate, carbonate, and phosphate rocks -------------------- 68 

Sulfate S in silicate, carbonate, and phosphate rocks ------------ 70 

Powder density ---------------------------------------------- 71 

Bulk density ------------------------------------------------ 73 

References cited ------------------------------------------------- 74 

ILLUSTRATIONS 

Page 

FIGURE 1. Schematic diagram of a complete rock analysis -------- 3 

2-11. Diagrams showing: 

2. Teflon beaker and cover ---------------------- 5 

3. Stopcock pipet device ------------------------ 7 

4. Tilting pipet -------------------------------- 8 

5. Squeeze pipet --------------------,----------- 9 

6. One-litre dispenser -------------------------- 10 

7. Sequential heater for FeO -------------------- 12 

8. C02 scanning test tube ----------------------- 13 

9. Middle-range carbonate tube _____ ------------- 14 

10. Evolution-absorption apparatus for C02 -------- 15 

11. Cell and cell adapter for the spectrophotometer _ 16 

12. Photograph showing automated spectrophotometer assembly ------------------------------------------- 18 

13-16. Diagrams showing: 

13. Schematic sketch of dipper arm and shifter 

mechanism of sample changer -------------- 20 

14. Schematic sketch of pin drive of Geneva 

mechanism __________________________ - _ _ _ _ 23 

15. Tube in position for water determination -------- 56 

16. C02 tube in heating block -------------------- 62 
CONTENTS v 

TABLES 

Page 

TABLE 1. Conversion of percent transmission ( T) to absorbance (A) -- 17 

2. Heavy-liquid mixtures ---------------------------------- 71 
Any use of trade names and trademarks in this publication is for 

descriptive purposes only and does not constitute endorsement 

by the U.S. Geological Survey. 
RAPID ANALYSIS OF SILICATE, 

CARBONATE, AND PHOSPHATE ROCKSREVISED EDITION 

By LEONARD SHAPIRO 

ABSTRACT 

The rapid methods previously used by the U.S. Geological Survey to determine the major constituents of rocks have been modified to introduce atomic 

absorption spectrometry (AAS) where applicable. Two procedures are available 

for determining 10 constituents: one, from a single solution prepared by a 

nitric-acid dissolution of a lithium metaborate-lithium tetraborate fusion, and 

the other, a two-solution method in which one portion of sample is dissolved in 

an HF-H2SO,-HNOa mixture and another portion is fused with NaOH. In both 

techniques, Si02, Al20a, F~Oa, Ti02, P20s, and MnO are determined spectrophotometrically, and CaO, MgO, Na20, and K20 are determined by AAS. 

Separate portions of samples are used for the following determinations: FeO 

by titration with K2Cr201 after decomposition with HF and H2SO,; total H20 

by its weight when evolved on heating a mixture of sample plus flux; H20 

by loss of weight at 110° overnight; C02 by its volume upon evolution with 

acid; fluorine by a new indirect measurement of Si02 evolved with fluorine on 

heating; and. sulfur by a new procedure based on a turbidimetric measurement 

of BaSO, after an aqua regia attack. Several mechanical aids and automated 

devices are used for the analyses. 

INTRODUCTION 

The methods described here are those currently in use by the 

U.S. Geological Survey for rapid rock ana.lysi·s. They are an outgrowth of the scheme of analysis originally presented in 1952 

(Shapiro and Brannock, 1952), revised in 1956 (Shapiro and 

Brannock, 1956) and again revi'Sed in 1962 (Shapiro and Brannock, 1962). Since 1962 approximately 2,500 samples per year 

have been analyzed. During the last few years atomic absorption 

spectrometry ( AAS) techniques have been introduced into the 

procedure to replace the disodium ethylenediamine tetra-acetate 

(EDTA) automatic titration of GaO and MgO and the flame 

photometric determination of Na20 and K20. During this time 

a single-solution procedure, based on fusion with a lithium 

metaborate-tetraborate mixture, was developed for which the 

1 
2 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

same 10 constituents are determined as in the two-solution procedure. This report provides a detailed description of both the 

single- and two-solution methods as both are actually used in the 

laboratory. Some chemists currently using the two-solution procedure may prefer to continue to do so, with the modifications 

described. Others :may prefer the single-solution procedure which 

is somewhat faster, being capable of providing a sample solution 

in two hours. However, because it is based on the use of a 200-mg 

sample, the sampling error of the single-solution method may be 

larger than that of the two-solution method, which requires 500 

mg of sample. 

In the presence of much fluorine, as is common in phosphate 

rocks, separate provision is made in the two-solution procedure 

for the removal of fluorine, which would interfere in the determination of aluminum. This is not required in the single-solution 

method. 

Methods are described for the determination of Si02, Al203, 

total iron, FeO, MgO, CaO, Ti02, MnO, P20i'i, K20, Na20, H20+, 

H20-, C02, F, and S. The complete scheme for the rapid analysis 

of rocks is shown in a flow diagram in figure 1. In the U.S. Geological Survey the methods described are generally applied to 50-

to 100-sample batches in order to benefit from the advantages of 

automation and mechanization. 

To be acceptable the methods must give results which sum to 

100±1 percent. The methods are designed so that major constituents, those 10 percent or above, a,re expected to be accurate 

to 1 percent of the amount present; these are reported to the 

nearest 0.1 percent. Constituents between 1 and 10 percent are 

also reported to the nearest 0.1 percent but are accurate to ± 0.1 

percent absolute. Below 1 percent, results are reported to 0.01 

percent and are accurate to ± 0.02 percent absolute. These accuracies have been confirmed by comparing the results of these 

methods with those obtained by careful conventional analysis 

and by the use of primary and secondary standards. The collaborative study of the accuracy and precision of conventional analysis 

(Fairbairn and others, 1951), which provided the incentive for 

the development of rapid procedures, and the followup study 

(Stevens and others, 1960) both provide guidelines for accuracy 

and precision that we are well able to meet. 

The methods for preparing solutions of samples are adequate 

for all but a few rare refractory rock types. In some cases very 

refractory silicates can be dissolved by HF decomposition in 

sealed vessels under heat and pressure (May and Rowe, 1965; 
Two solution procedure 

l

"" ~ . 1000 

ml [] 

Solution A 

Sample+ 

NaOH 

~Teflon 

I liJitl ___ Sample + 

'~~·I HF,H2S04.HN03 

E~ 

--c:~ Vycor &· '· 1 -50. 

ml 

Solution B 

INTRODUCTION 

Si02-Molybdenum Blue 

AI 20 3-Aiizarin RedS 

Ti02- Tiron 

Total Fe 

as Fe20 3-0rthophenanthroline 

MnO-Persulfate 

P20 5-Vanadomolybdate 

Muffle l 

1 hr at 

1000°C 

+HN03 

Smgle >oloUon--+ .. 

LnJ 

Magnetic stirrer 

Spectrophotometer 

Low level C0 2 

Atomic absorption spectrophotometer 

Medium level C02 

FIGURE !.-Schematic diagram of a complete rock analysis. 

3 

Langmyhr and Paus, 1968; Bernas, 1968). Other refractories 

such as chromite require procedures described earlier by Dinnin 

(1959). 
4 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

The methods for determining ferrous iron and water are of 

limited applicability in the presence of significant amounts of 

organic matter, such as may be found in phosphate and carbonate 

rocks and shales. 

The procedure for determination of sulfur is not suitable for 

glassy rocks such as obsidian. It also fails to detect sulfur in 

samples containing more than 0.5 percent Ba. In addition, a 

few rare sulfur minerals are not attacked by aqua regia. Where 

organic matter is present the sulfur often is not brought into 

solution and the sulfur determination can be expected to be in 

error by approximately 0.01 percent S per percent organic matter. 

APPARATUS AND INSTRUMENTS 

In addition to the commonplace items such as beakers, pipets, 

and so forth, the following specialized equipment is used: Teflon 

beakers, water-repellent coated pip·ets, a spectrophotometer, an 

automated spectrophotometer, a 1-1 water dispenser, atomic 

absorption spectrometric equipment, a carbon dioxide scanning 

test tube, a middle-range carbonate tube, evolution-absorption 

apparatus for C02, stopcock pipets, a pipetting machine, tilting 

pipets, squeeze pipets, high-precision piston pipets, fusion equipment, multiple m·agnetic stirrers, and a sequential heating device 

for FeO. Automatic sample changers attachable to both the atomic 

absorption apparatus and the spectrophotometer are useful but 

not essential. 

TEFLON BEAKERS 

Teflon beakers are used in the preparation of solution B. In 

recent years, Teflon beakers of various sizes and shapes have 

become readily available, and they may be adapted to the procedure described. It is better, however, to use beakers specifically 

designed for the purpose. Such beakers may be made from Teflon 

bar stock and the covers from sheet Teflon (Shapiro, 1959). 

Teflon is unaffected by acids ·at steam-bath temperatures, but, 

unlike platinum, it conducts heat poorly. To assure adequate 

heating for the decomposition of powdered samples, a beaker 

with a tightly fitting cover was designed which could be heated 

on the sides as well as the bottom by the stea.m of a steam bath 

(fig. 2). The beakers are suspended through holes 46 mm in diameter in a sheet of polypropylene, which is used to replace the 

top of a steam bath. 
APPARATUS AND INSTRUMENTS 

I 

I 

lj_ t::::;:=======~;:::=:::::::l• 3 m m 

I• 40 mm ---~~~ t 

r+- 50mm : ~1___1 

6mm 

1.5 mm 

Beaker 

I 

I 

I 

I 

I 

I 

I 

47 mm I I I__L I 

~3mm 

~------! __ -~ 

FIGURE 2.-Teflon beaker and cover. 

WATER-REPELLENT COATED PIPETS 

5 

The use of water-repellent coated pipets saves time. Rinsing 

between samples is eliminated because coated pipets retain only 

negligible quantities of liquid. With a coated pipet, aliquots from 

30 samples can be easily taken in about 7 min. The precision of 

this operation is comparable to that using conventional pipets, 

but as the volume delivered is slightly altered, it may be necessary 

to pipet both the samples and the standard with the same pipet, 

or to use pipets known to deliver practically equal volumes of 

solution. Using a 10 ml pipet, a delivery rate of about 10 sec may 

be obtained by enlarging the orifice by cutting and fire-polishing 

the tip. Such rapid flow does not affect the reproducibility of the 

volume delivered by a coated pipet. 
6 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

The pipet is coated with water repellent according to the directions of the manufacturer. Dimethyl dichlorosilane is a repellent 

that is very convenient to use, as it need be only passed through 

a clean pipet, which is then allowed to dry. 

STOPCOCK PIPET DEVICE 

The reproducibility of 8-·ml pipets is generally within 0.5 percent of the volume. If the concentration of the constituent to be 

determined is high, as for example Si02 in silicate rocks, greater 

pipetting reproducibility is· desirable. With the stopcock-pipet 

device illustrated in figure 3, sample portions can be taken with 

a precision of about 0.1 percent. 

The device consists of a two-way stopcock fused to a waterrepellent 8-ml pipet in .such a way that the volume of liquid 

delivered by the pipet, after it has been filled to the stopcock, 

is nearly 8 mi. On . one arm opposite the pipet, a 150-ml flask is 

fitted with a squeeze bulb and valve to provide for overflow and 

suction. Another squeeze bulb, with a small hole or a valve in it, 

is attached to the other arm. 

The stopcock-pipet is used in the following manner: With the 

stopcock in a position so that all openings are closed, the bulb 

attached to the overflow flask is squeezed to force air from the 

exhaust valve. The pipet is inserted into the liquid to be pipetted, 

and the stopcock is rotated so that the passage from the pipet 

to the overflow flask is open. The liquid rises to fill the pipet 

and then overflows past the stopcock. When the liquid begins to 

pass the stopcock, the stopcock is closed. The tip of the pipet is 

wiped with a tissue and placed over the receiving vessel. The 

stopcock is rotated to allow air to enter through the hole or valve 

in the bulb on the other arm of the stopcock and to allow the 

liquid to drain from the pipet. When the flow has stopped, a 

finger is placed over the hole in the bulb and the bulb is squeezed 

to force out the drop of liquid remaining in the tip. The process 

is repeated for the next solution. 

The pipet is self-rinsing in that it is coated with a water 

repellent and the overflow of the next liquid carries the slight 

residual liquid over into the overflow flask. 

TILTING PIPET 

Many manual types of dispensing devices are useful for transferring a solution into a series of receiving vessels; one which is 

used in this laboratory is the tilting pipet shown in figure 4. It is 

much slower than the pipetting machine but is particularly useful 
APPARATUS AND INSTRUMENTS 

D 

FIGURE 3.-Stopcock pipet device. A, overflow flask; B, exhaust valve; C, squeeze bulbs; D, air-inlet hole. 

7 
8 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

'I:, 

,,,, 

!Ill 

:t,, 

.~/:',: .!·,. .. ·: 

s11 ii 

• 

,;tJJJ!!t~l!iii,\,i~§li!!i'~~!lli\:~~~f} FIGURE 4.-Tilting pipet. 

when only a few solutions are involved. These dispensers are available in a range of capacities and are cheap enough that they can 

be used to store and dispense many reagents. 

SQUEEZE PIPET 

Squeeze pipets (fig. 5) have a rubber bulb into which the upper 

end of the glass pipet is inserted. The bulb is squeezed by a 

plunger activated by finger pressure. Adjustable mechanical limit 

stops allow a fixed displacement of the plunger so that the uptake 

of liquid upon removal of the finger is reproducible. Commonly 

these devices can be used with one percent reproducibility for 

measurements up to about 5 mi. 

HIGH-PRECISION PISTON PIPET 

In recent years high-precision piston pipets have become available for small volumes from 0.001-1.000 mi. Each pipet is made 
APPARATUS AND INSTRUMENTS 9 

FIGURE 5.- Squeeze pipet. 

for a specific volume but is capable of reproducing this volume 

repeatedly within a few tenths of a percent of the amount dispersed. Accurate variable-range piston pipets are also available. 

PIPETTING MACHINE 

Commercial pipetting machines, consisting of motor-driven 

syringes with intake and outlet valves, are available (see page 

18). and can be used with the methods described. The machines 

are used to dispense rapidly a single liquid into a series of containers. Aliquots of a reagent can be dispensed readily into 30 

containers in one minute. The machine can be used for all reagents 

except those that are strongly acidic. 

Use of a pipetting machine has several additional advantages. 

It is advisable to dilute the reagents to be added in such a way 

that the same volume of each may be dispensed, thereby eliminating the need to reset the machine. The dilutions and volumes may 

be arranged so that the final volumes are such that no further 

dilutions need be made. Precision of dispensing a given aliquot is 

high, so that where all reagents are added to a pipetted aliquot of 

sample, volumetric flasks become unnecessary. Furthermore, the 

force of addition of the reagent is sufficient to produce a degree of 

stirring such that no further mixing is required. 
10 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

ONE-LITRE WATER DISPENSER 

One-litre solutions of solution A must be prepared. If many 

solutions of this volume are to be prepared at one time, the physieal effort of measuring into a volumetric flask and mixing by 

inversion can be considerable. The required quantity of water can 

be measured automatically and dispensed easily with the 1-1 water 

dispenser shown in figure 6. The dis·penser is made from two 1-1 

leveling bulbs, two check VJalves, two overflow bulbs, and interconnecting tubing controlled by four pinch clamps. To speed the 

rate of inflow and outflow, the interconnecting tubing is of the 

widest diameter that can be used with the available glassware. In 

setting up the device, the glass outflow tubes are first positioned 

flush with the inside surface of the rubber stopper, and the two 

bulbs are allowed to be filled one at a time by removing one of the 

inflow clamps until the water is checked by the check valve. The 

inflow clamp is replaced, and the other inflow clamp is removed 

allowing the other bulb to be filled. Meanwhile the outflow clamp 

Float valves 

To sink 

Plast1c beakers 

FIGURE 6.-0ne-litre dispenser. 
APPARATUS AND INSTRUMENTS 11 

of the first bulb is removed, and the delivered water is measured, 

preferably by weight. The outflow clamp is replaced, the inflow 

clamp of the second bulb is replaced after it is filled ,and checked, 

and the second outflow clamp is removed to drain the second bulb. 

The delivered water for that bulb is weighed. If the two differ by 

more than 0.5 g or 0.5 ml, the outflow glass tube of the bulb with 

the greater content is pressed a little deeper into the bulb so that 

a little water remains within the bulb after drainage. The process 

of measurement is repeated until the delivery of the two bulbs is 

within 0.5 g. The glass tubes are then cemented in place to prevent accidental shifting. The dispenser can now be depended upon 

to deliver equal amounts of water from both sides. In use, 30 

beakers can be filled with water in about 15 min with little effort, 

as compared to perhaps an hour if volumetric flasks are filled and 

then mixed by hand. 

FUSION EQUIPMENT 

Graphite crucibles approximately 25 mm in diameter and 34 mm 

deep are used for fusions with lithium metaborate-lithium teraborate flux (the single-solution method). They may be used once 

and discarded; they are reusable if soaked in HCl, washed, and 

dried, but the time and effort consumed a~re not worth the small 

savings. Moreover, the surface of a new crucible relea;ses the melt 

better. 

Quartz trays large enough to hold 15 crucibles are used. The 

muffle must be capable of reaching 1,000°C and maintaining that 

temperature for one hour. It is desirable to have a muffle that can 

accommodate several trays at the same time. 

MULTIPLE MAGNETIC STIRRERS 

When the single-solution method is used, fused pellets must be 

dissolved with stirring. Commercial magnetic stirrers are available as banks of six stirrers. When large numbers of samples are 

handled, 5 such banks are us·ed to dissolve 30 samples at the same 

time. 

SEQUENTIAL HEATING DEVICE FOR FeO 

The procedure described for FeO requires that solutions be prepared by boiling samples with HF and H2S0-1 prior to titration 

with standard dichromate solution. Compared to previous heating 

methods, improved reproducibility as well as a reduction in ti.me 

and energy are obtained with the use of a sequential heating device (Shapiro and Rosenbaum, 1962). The advantage of this 
12 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

arrangement is that a reproducible heating time can be obtained 

semiautomatically for a series of samples. Twenty samples per 

hour can be conveniently handled. 

The apparatus accommodates twenty 100-ml platinum crucibles 

at one time. It automatically controls the 10-min boiling time and 

delivers a solution ready for titration every 3.5 min. Additional 

platinum crucibles can be placed into holes as they are vacated, 

so that the number of samples that can be handled in one sequence 

is limited only by the number of crucibles available. 

The device, as shown in figure 7, consists of a circular Transite 

board (A) 48 em in diameter and 12.5 mm thick, mounted on a 

shaft, that is rotated continuously by a small 1lj:!-rpm motor (B) 

through a worm and spur-gear combination having a 100:1 ratio. 

Twenty 50-mm holes are drilled near the periphery of the board 

to carry the platinum crucibles. Two 200-watt heating strips (c), 

18 em long and 32 mm wide, are mounted below five of the holes 

at such a height that they are just cleared by the platinum crucibles as they move past. The heating strips and the motor are controlled by a single-toggle switch (D). A pointer (E) at the end of 

the heating element conveniently indicates the position for removal of the samples. Each sample is boiled for 10-12 min. 

CARBON DIOXIDE SCANNING TEST TUBE 

A borosilicate test tube (10x75 mm) is modified by the addition 

of a bulge, as shown in figure 8. The bulge, about 18 mm from the 

bottom of the tube, is made by heating the test tube over a bunsen 

FIGURE 7.-Sequential heater for FeO. Letters are referred to in text. 
APPARATUS AND INSTRUMENTS 13 

18 mm 

FIGURE 8.-C02 

75 mm 

scanning 

-----+-~J test tube. 

burner and pushing from within with a stiff wire having a small 

hook at one end. 

MIDDLE-RANGE CARBONATE TUBE 

The carbonate tube used for the determination of the middle 

range of C02 is a sim:ple device designed to measure volumetrically 

the C02 evolved from a sample upon treatment with acid. It consists of an 18 x 150-mm test tube with a side arm attached as 

shown in figure 9. 

EVOLUTION-ABSORPTION APPARATUS FOR CO:~ 

The apparatus (fig. 10) for the determination of large amounts 

of C02 is a modification of that of Goldich, Ingamells, and 

Thaemlitz (1959). It consists of a 50-ml decomposition flask connected through a .stopcock to a supply bottle with HCl and also a 

small condenser; the condenser in turn is connected to a waterjacketed gas-measuring buret. A three-way stopcock on the buret 

may be opened either to the decomposition flask or to an absorption pipet containing 40-percent KOH. A saturated solution of 

NaCl in 1 + 3 HCl, to which a few milligrams of methyl red indicator have been added, is used for decomposing the sample and 

for filling the gas buret and leveling bulb. The absorption pipet 

has a bubbler to permit the C02 to bubble through the KOH. Float 

valves are used where liquid levels must be returned to a reproducible position. 

MANUAL SPECTROPHOTOMETER 

The methods are designed for use both with a manually operated 

and an automated spectrophotometer. The Beckman Model B 

spectrophotometer, slightly modified for convenience, is used for 

manual work. This instrument was chosen because .absorbance 

can be measured over the range 0-2 absorbance without reading 

the crowded portion of the scale. Nevertheless, the methods can be 
14 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

OD Test tube 18 X 150 mm 

I 

FIGURE 9.-Middle-range carbonate tube. 

adapted readily to other spectrophotometers. Only one phototube 

is used for the measurements required in our procedures, all of 

which are made in the range 420-650 nm. The stability of the 

instrument is maintained by a voltage regulator in the 115-volt 

a-c line. 

In place of a set of matched absorption tubes, a single fixed tube 

with a drain at the bottom generally was found to be more satisfactory. The simple adapter, shown in figure 11, was built to hold 

the absorption cell in a fixed position in the cell compartment. Care 

must be taken to insure that the adapter and cell compartment are 

rigidly fixed in order to maintain a constant light path. The 

absorption cell should be treated with a water repellent to eliminate the necessity of rinsing between additions of solutions. The 

spectrophotometer must be raised on a platform approximately 

130 mm high to allow access to the absorption tube outlet. 

Readings are taken from the percent transmittance scale, rather 

than from the absorbance scale, as the former is less subject to 
APPARATUS AND INSTRUMENTS 15 

A --

~-~ = "' II , 

FIGURE 10.-Evolution-absorption apparatus for C02. A, decomposition flask; 

B, gas buret; C, absorption pipet; D, leveling bulb; E, float valves; F, 

bubbler; G, HCl supply bottle. 

reading error during routine day to day work because of the uniformity of this scale. 

For those determinations for which the spectrophotometer is 

used, percent-transmittanc,e readings are first converted to absorbance by interpolation of the values in table 1, which was derived 
16 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

'\ 

1 mm brass 

all joints butt-soldered 

Pinch clamp or -'-=;o..;;::::~~~ 

efectrohosecock 

...........__Face plate of 

spectrophotometer 

FIGURE 11.-Cell and cell adapter for the spectrophotometer. 

by the use of the equation, A (absorbance) =2- log T (transmittance). All calculations are made with absorbance values because 

they have a linear rather than logarithmic relationship to concentration, thus simplifying the calculations. 

In recent years spectrophotometers have becom·e available which 

give a digital readout directly in concentration and which are 

equipped with flow-through tubes. Many such spectrophotometers 

are suitable replacements for the above described equipment. 

AUTOMATED SPECTROPHOTOMETER 

An automated spectrophotometer was assembled from commercially avaHable components and coupled with a sample changer 

designed and built in our laboratory (Shapiro and Massoni, 1972). 

The units comprising the entire assembly are: the spectrophotometer equipped with a flow-through cell; a digital converte·r; a 

printer; a pipetter; and ·a sample changer. The apparatus is shown 

in figure 12. It is possible to combine any spectrophotometer of 
APPARATUS AND INSTRUMENTS 17 

TABLE I.-Conversion of percent transmission (T) to absorbance (A) 

T A T A T A T A 

- 30.0 0.523 50.0 0.301 70.0 0.155 90.0 0.046 

30.5 . 516 50.5 .297 70.5 .152 90.5 .043 

31.0 .509 51.0 .292 71.0 .149 91.0 .041 

31.5 .502 51.5 .288 71.5 .146 91.5 .039 

32.0 .495 52.0 .284 72. 0 .143 92.0 .036 32.5 .488 52.5 .280 72.5 .140 92.5 .034 33.0 .482 53.0 .276 73. 0 .137 93.0 .032 

33.5 .475 53.5 .272 73. 5 .134 93.5 .029 34.0 .469 54.0 .268 74.0 .131 94.0 .027 34.5 .462 54.4 .264 74.5 .128 94.5 .025 35.0 .456 55.0 .260 75.0 .125 95.0 .022 35.5 .450 55.5 . 256 75.5 .122 95.5 .020 36.0 .444 56.0 .252 76. 0 .119 96.0 .018 

36.5 .438 56.5 .248 76.5 .116 96.5 .016 

37.0 .432 57.0 .244 77.0 .114 97.0 .013 

37.5 .426 57.5 .240 77.5 .111 97.5 .011 

38.0 .420 58.0 .237 78.0 .108 98.0 .009 38.5 • 415 58.5 .233 78. 5 .105 98.5 .em 

39.0 .409 59.0 .229 79.0 .102 99.0 .004 

39.5 .403 59.5 .226 79.5 .100 99.5 .002 

40.0 .398 60.0 .222~ 80.0 .097 

40.5 .393 60. 5 .218 80.5 .094 41.0 .387 61. 0 .215 81.0 .092 41.5 .382 61.5 .211 81.5 .089 42.(i .377 62. 0 .208 82.0 .086 42.5 .372 62.5 .204 82.5 .084 43.0 .367 63.0 .201 83.0 .081 

43.5 .362 63. 5 .197 83.5 .078 

44.0 . 357 64.0 .194 84.0 . 076 

44.5 .352 64.5 .190 84.5 .073 

45.0 .347 65.0 .187 85.0 .071 

45.5 .342 65.5 .184 85.5 .068 46.0 .337 66.0 .180 86. 0 .066 46.5 .333 66.5 .177 86.5 .063 47.0 .328 67.0 .174 87.0 .060 47.5 .323 67.5 .171 87.5 .058 

48.0 .319 68.0 .168 88. 0 .056 

48.5 .314 68. 5 . 164 88.5 .053 49.0 .310 69.0 .161 89.0 .051 

49.5 .305 69.5 .158 89.5 .049 

good quality, one of several digital converters available from 

chemical supply house catalogues, •and any printer that can be 

coupled to the internal code of the digital converter out:put. Many 

problems are bypassed, however, if the three units are designed 

by the manufacturer to be used together; a number of companies 

make such combinations. 

The spectrophotometer must be stable with no significant drift 

over a 15-min period. Moreover, the output of its photocurrent 

must be accessible to permit connection of a digital converter. 

We selected the Beckman Model DB-G spectrophotometer 1 and 

associated converter and printer. The standard 1-cm sample cell 

is replaced by a commercial 1-cm flow-through cell fitted with 

s·mall glass inlet and outlet tubes at the top, to which plastic tubing is attached. The volume of the inlet side of the cell must be 

kept low to allow adequate flushin_g of one sB~mple by the next one. 

Therefore, 3-mm ID tubing not more than 30 em long is attached 

to the inlet glass tube, while broader gage tubing of any convenient 

size may be attached to the outlet side. 

1 Beekman Instruments, Inc., 2500 Rubor Blvd., Fullerton, Ca!ifomia. 
18 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

FIGURE 12.-Automated spectrophotometer assembly. 

An electrically driven pipet provides the suction by which liquid 

is drawn from the sample beaker, through the flow-through tube 

and out to waste. Pipetters are available from several companies 

at reasonable cost. They generally consist of a motor-driven 

syringe with steel ball valves arranged so liquid can flow into the 

syringe during the downstroke of the syringe piston. Then on the 

upstroke, or discharge stroke, the steel ball valves prevent liquid 

from entering the inlet side but allow it to leave the exhaust side. 

We used the Filamatic Model AB 2 instrument, which we have 

modified by adding two cam-activated microswitches. The normally closed switch is in series with the pipetter motor and is 

positioned to stop the pipetter on the completion of one cycle. The 

normally open switch activates the printer and is positioned to 

close just prior to the completion of the pi petting cycle. The 

pipetter in turn is activated by a switch on the sample changer as 

the dipper arm approaches its lowest position. The rate of pipetting is 5 sec per cycle, and the preset volume is 25 mi. 

Digital converters designed for spectrophotometers all perform 

the same functions. The photocurrent generated in a spectrophotometer is a logarithmic function of the concentration of an 

absorbing solution. The converter electronically changes this 

logarithmic relationship to a linear one and in addition provides 

2 National Instrument Company, Baltimore, Maryland. 
APPARATUS AND INSTRUMENTS 19 

an amplifier so that the resultant voltage can be adjusted to read 

any desired value. It then performs an analog to digital conversion 

which can be read and (or) fed to a printer. With this device the 

reading for a blank solution can be set to zero, after which the 

device can be set to read the numerical concentration value for a 

standard solution. If the colored system being measured follows 

Beer's Law, as is the case in U.S. Geological Survey procedures, 

subsequent solutions will then be read and be printed directly in 

concentration units. 

The sample chang·er is the only instrumental component completely designed and constructed at the U.S. Geological Survey. 

The changer is a modification of an earlier model (Shapiro and 

Massoni, 1968), which is described herein under the section called 

"Automated atomic absorption equipment." The same Geneva 

movement is used to drive and position the placement of each 

beaker, but the motor speed was changed from 1112 rpm to 4 rpm; 

the gear ratio allows 12 sec per sample rather than 45 sec. 

The main changes are designed to accommodate more and 

larger beakers while reducing the dia.meter of the plexiglass carrier plate from a 91-cm disk to a 71-cm disk so the changer could 

fit on a standard table top. The three concentric circles each 

having 24 holes 50 mm in di~meter are spaced uniformly around 

the disk. The centers of the inner and outer holes are aligned on 

the same radii, and the centers of the holes of the middle circle 

are displaced halfway between each pair of radii. These holes are 

cut 11 mm deep into the carrier plate and are intended for 100-ml 

beakers. Holes 59 mm in diameter and 5.5 mm deep are cut at the 

same centers to accommodate 150-ml beakers. A switching arrangement is constructed for the dipper arm so that it can be 

autom·atically shifted from one row of holes to the next at the 

completion of a revolution. After the last beaker a final shift is 

used to activate a latching switch to turn the changer off (see 

figure 13). 

The dipper arm, as seen in figure 13, carries a glass aspirator 

tube connected by a plastic tube to the inlet of the flow-through 

tube in the spectrophotometer. The arm pivots at one end, which 

is mounted on a rotatable shaft. Also mounted on the lower end 

of this shaft are three collars, each having a projecting peg and 

a slot 90° away from the peg. The three collars are positioned so 

that their pegs are offset from each other sufficiently so that a 

projecting metal shifter plate attached to the sample carrier plate 

will press against the upper peg as it passes but will miss the 

second peg, rotating the movable shaft and thereby shifting the 

dipper arm so that the glass aspiration tube is over the second row 
20 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

Latching 

microswitch 

spring 

detent 

FIGURE 13.-Schematic sketch of dipper arm and shifter mechanism of 

sample changer. 

of beakers. A spring detent pressing into a collar slit assures that 

the dipper arm movement is reproducible. After another full 

rotation of the carrier plate, the metal shifter plate now presses 

against the second peg and causes the dipper arm to shift so that 

the glass aspirator tube is over the third row of beakers. At the 

conclusion of the next full rotation, the shifter plate presses 

against the bottom peg causing a cam attached to the rotating 

shaft to press against a latching microswitch that disconnects 

the drive motor and stops the operation of the changer. The latching switch must be manually reset before the next cycle can be 

started. A pilot light connected across this switch serves as a 

reminder to push the reset button. 

The dipper arm is raised and lowered by a cam lifter as in the 

earlier sample changer, but in addition a second cam on the same 

shaft is used to close a microswitch to activate the pipetter at the 

point where the dipper arm is at its lowest point. The sample 

changer has its own on-off switch. 
APPARATUS AND INSTRUMENTS 21 

The solutions to be measured are prepared in 100- or 150-ml 

beakers. The spectrophotometer is turned on to warm up, and 

blank solution is transferred to the reference cell, which is a 

standard 1-cm cell. If fewer than 72 solutions are to be analyzed, 

they are placed on the carrier plate with the last one in the last 

hole so that after the solutions are recorded the changer will be 

turned off automatically. The first beaker in the sequence contains 

a blank solution and the second a known standard. After all of the 

beakers are positioned on the carrier plate it is rotated by hand 

to place the first beaker under the dipper arm, which must be in 

the raised position during rotation. The switches for the spectrophotometer, the converter, and the pipetter must all be on and 

the spectropp.otometer set to the proper wavelength; the printer 

is off at this time. The reset button on the changer is pressed if the 

changer had shut itself off when last used. The toggle switch for 

the changer is switched on, causing the arm to lower, and in so 

doing activates the pipetter, which then transfers 25 ml of blank 

solution through the flow-through tube. When the dipper arm has 

moved to the rai,sed position. the toggle is pushed to the off position. The spectrophotometer is now adjusted manually to read 

100 percent T, the digital converter is s:et to read concentration, 

and the zero adjustment is set so that the displayed number is 000. 

The toggle is then fli'Pped on, the table rotates, and the sequence 

repeats, transferring the known standard solution to the flowthrough tube. When the dipper arm is up again, the toggle is 

turned off and the concentration adjustment knob is adjusted 

manually until the displayed number corresponds to the value of 

the known standard. The printer and the toggle switch of the 

changer are switched on. All the subsequent solutions now are 

measured, and their concentrations printed automatically. A few 

beakers of water are run following the samples to rinse the 

pipetter. The entire measurement and printout procedures require approxi,mately ~5 min if the changer is loaded to capacity. 

AUTOMATED ATOMIC ABSORPTION EQUIPMENT 

Flame-emission techniques previously used by the U.S. Geological Survey for sodium and potassium have been superceded by the 

use of AAS techniques. Automatic EDT A ( disodium ethylenediamine tetra-acetate) titrations for calcium and magnesium have 

also been replaced by automated AAS. 

It is the author's opinion that, if available, excellent spectrophotometric methods that are based simply on the addition of 

reagents to an aliquot and the development and subsequent read-
22 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

ing of a stable color are preferable to other methods. Where satisfactory methods of this type are not available, as is the case for 

the alkalies and alkaline earths, AAS offers an excellent alternative. 

Such equipment is now widely available commercially; it is 

apparently well designed and is capable of achieving good results. 

The atomic absorption unit used in the U.S. Geologic,al Survey 

is the Perkin-Elmer Model 303 3 with digital readout and a printer 

accessory from the same company. To this equipment has been 

added an automatic sample changer designed and built in the U.S. 

Geological Survey (Shapiro and Massoni, 1968). The changer 

allows 64 solutions in 30-ml beakers to be analyzed unattended. 

AUTOMATIC SAMPLE CHANGER 

The automatic sample changer consists of three components: a 

sample carrier, a positioning drive mechanism, and a lifter arm 

to raise and lower the plastic aspirator tube. The whole unit is 

mounted on a small movable table so that it can be attached to the 

atomic absorption equipment or removed as desired. Tolerances 

of construction are nominal. 

The sample carrier is made from a Plexiglas disk, 91 em in 

diameter and 12.5 mm thick Sixty-four holes, each 35 mm in diameter and 6 mm deep, are uniformly spaced near the edge of the 

disk. A 6-mm perforation in the center of each cut provides drainage in case of spillage. A considerable amount of the remaining 

Plexiglas i's cut away to reduce weight, but six strong supporting 

spokes are left. A 12-mm-thick ring of Plexiglas, 63.5 mm ID and 

89 mm OD, is bolted to the bottom. center of the Plexiglas sample 

carrier. Two holes are drilled and tapped into the outer edge of 

the ring from opposite sides; each hole has a steel ball, a spring, 

and a tension-adjusting screw. This ring is mounted around a 

64-tooth gear 13 em in diameter, in such a way that s.pring tension presses the steel balls between the gear teeth and yet permits 

unimpeded rotation of the wheel by hand. The mechanism serves 

as a detent to allow reproducible positioning of the sample. The 

64-tooth gear is fastened to a vertical shaft carrying a 100-tooth 

gear driven by a 25-tooth gear on the positioning drive mechanism. 

Positioning drive mechanism.-Automatic positioning of the 

beakers, sequentially and accurately, is controlled by ,an indexing 

wheel constructed after the Geneva drive design (fig. 14). This 

wheel is a round brass plate 152 mm in diameter with 16 semicircles and 16 radial slots that alternate around the periphery. 

The wheel is moved by a plate-mounted drive pin on a shaft 

3 Perkin-Elmer Corporation, Norwalk, Connecticut 06852. 
APPARATUS AND INSTRUMENTS 23 

Miter gears 

...... ----------. 

Indexing wheel cam 

Motor 

FIGURE 14.-Schematic sketch of pin drive of Geneva mechanism and associated miter gears and cams. 

rotated by a llj2-rpm motor which runs continuously. During the 

part of the rotation of the shaft when the pin is not in the radial 

slot of the index wheel, the wheel is locked in position by a semicircular lug of the Geneva drive design. A 45° miter g·ear on the 

upper part of the same 'Shaft that carries the pin drive engages a 

mated gear on another shaft at right angles to the pin drive shaft. 

One cam on the driven shaft operates a leaf switch that activates 

the readout of the atomic absorption equipment, and a s~econd cam 

causes the lifter to rise and fall. 

Lifter.-The lifter is an aluminum arm about 28 em long whose 

free end rests on a cam attached to the horizontal shaft that also 

carries the cam activating the readout switch. The other end of 

the arm pivots on a fixed pin. This part of the assembly is shown 

in figure 13. The lifter must be raised about 45 mm if 30-ml 

beakers are used. Attached to the movable end of the lifter is a 

3-mm aluminum rod about 38 em long that extends slightly beyond 

the outer edge of the sample carrier. Two bends bring the end of 

the rod about 5 em above the top surface of the sample carrier and 

about 25 mm back toward the center. A tightly fitting polyethylene 

sleeve slipped onto the last several inches of this rod is allowed 
24 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

to extend about 25 mm beyond the end of the rod. A hole large 

enough to accept insertion of the aspirator tube is drilled through 

the free part of this polyethylene sleeve. 

Wiring.-The 1lj2-rpm motor is controlled by an on-off toggle 

switch. The atomic absorption equipment has a pushbutton switch 

used to control the readout equipment when a printer is provided. 

This pushbutton, or readout switch, is connected to the terminals 

of a normally open leaf switch. The leaf switch is positioned underneath the cam made for this purpose in such a way that as the 

cam rotates it closes the switch for 2-3 sec. The exact position of 

the shaft at which this occurs need not be set until attachment is 

made to the aspirator tube. 

Assembly and operation.-The table is positioned so that the 

lifter arm is close to the aspirator tube which is inserted through 

the drilled hole in the plastic sleeve. This is facilitated by passing 

a thin steel wire up through the drilled hole and into the aspirator 

tube for about 25 mm. The aspirator tube may then be brought 

through the hole without being crushed, after which the wire is 

removed. A beaker is plac·ed in the sample-carrier depression just 

below the aspirator tube, and the motor switched on and allowed 

to run until the lifter arm is at its highest point. The aspirator 

tube is then positioned so that its tip is about 3 mm above the top 

of the beaker. 

The cam that activates the readout switch is now loosened, the 

motor is turned on and allowed to run for 3 sec after the time the 

lifter reaches its lowest point, and the motor is then turned off. 

The cam is retightened and is set to close the leaf switch. The 

procedure may have to be repeated a few times to set the exact 

position and degree of closing. The equipment is now ready for 

use, and further adjustment should not be needed. 

In the automatic operation, standards and samples are placed in 

a sequence of 6 standards followed by 15 samples. The flame position and electrical parameters are the same as in manual operation 

of the atomic absorption equipment when the blank and any one 

of the standards are used. When the adjustments are correct, the 

sample carrier is positioned so that the aspirator tube is in the 

beaker preceding that of the blank, and then the toggle is switched 

on. The rest of the operation is fully automatic. 

The a.spirator tube is lifted to its peak height, the table rotates 

to bring the first beaker into position, the aspirator tube is lowered 

into the beaker where it remains for 3 sec, and then the readout is 

activated. The instrument makes four readings and displays the 

average in 12 sec. The result is then printed out, and the arm 

starts up again to repeat the cycie. A slight amount of drift some-
TWO-SOLUTION PROCEDURE 25 

times occurs in the readings of the standards before and after a 

batch of samples. For this reason always average the ~standard 

readings made before and after a given set of samples. The complete cycle of 64 solutions requires 48 min, or 45 sec per sample. 

PREPARATION OF POWDERED SAMPLES 

A jaw crusher, a roll crusher, ·and a ceramic plate pulverizer 

are used to reduce the samples to a powder that will pass a 100-

mesh sieve. 

PROCEDURE 

1. If any of the pieces of the samples are too large for crushing 

with the jaw crusher, break them with a hardened hammer 

to a size ·suitable for the crusher. 

2. Adjust the j;aw crusher so that it will crush the samples to 

an ·average dimension of about 1.25 em. 

3. Clean the crusher thoroughly and pass all of the first of the 

series of samples through it. 

4. Repeat step 3 for each of the samples. 

5. Set the rolls of the roll crusher about 15 mm apart and pass 

the first sample through it. 

6. Set the rolls of the roll crusher so that they ·are touching, and 

again pass the same sample through the crusher. 

7. Repeat steps 5 and 6 with each sample, cleaning the crusher 

between each sample. 

8. Mix thoroughly and split out about 75 g of each sample by 

quartering. 

9. Take the first of the series of samples and pass through the 

pulverizer. 

10. Separate the coarse from the fines with a 100-mesh screen. 

11. Return the coa.rse material to the pulverizer. 

12. Repeat steps 10 and 11 until all the sample passes through the 

100-mesh screen. 

13. Mix thoroughly and transfer to a 60-ml screwcap jar. 

14. Repeat steps 9 through 13 for the remainder of the samples. 

TWO-SOLUTION PROCEDURE 

PREPARATION OF SOLUTION A 

Solution A is used in the determinations of Si02 and Al203. 

Portions of the samples and standards are decomposed by fusion 

with NaOH at a comparatively low temperature for about 5 min 
26 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

in nickel crucibles. After cooling, the melts are leached with water, 

and the solutions are acidified with hydrochloric acid. 

Occasionally, with some fine-grained materials such as clays, 

there is a tendency during fusion for some of the sample powder 

to float on the surface of the molten NaOH and remain unattacked. 

When this occurs, the crucible is swirled, allowed to cool, and then 

reheated so that complete decomposition will result. 

For silicate rocks, 50-mg portions of samples and standard are 

used. This relatively small portion eliminates the necessity for 

double dilutions, which would be required with larger samples. 

Furthermore, silicon is known to be unstable in solution above 

concentrations of 200 ppm: this procedure assures the stability 

of the silicon. For carbonate or phosphate rocks, a 200-mg portion 

of sample powder is used. 

The high percentages of Si02 in silicate rocks (about 40 to 100 

percent) require that the accuracy and precision of this determination be greater than those necessary for most of the other constituents. For this reason, particular care must be given to mixing 

and weighing the samples, to the fusions, and to the handling of 

the relatively concentrated solutions of the melts before dilution. 

REAGENTS 

NaOH solution, 30-percent: Dissolve 450 g of NaOH pellets in 

1,500 ml of water in a stainless steel beaker. Cool and store 

in a plastic bottle. 

HCl, 1 + 1 : Prepare 1 I. 

National Bureau of Standards standard sample No. 70a (feldspar) or equivalent. 

PROCEDURE 

1. Transfer 5-ml portions of 30-percent NaOH solution, measured 

with a plastic graduate, to a series of 75-ml nickel crucibles. 

One crucible will be needed for each sample, two for standards, and one for a blank. The crucibles should be cleaned 

with dilute HCl before use. 

2. Evaporate the solutions to dryness over gas burners or electric 

heaters. Slight spattering can be ignored. 

3. Accurately weigh (to the neBJrest 0.1 mg) 50 mg of each 

sample of silicate-rock powder (200 mg of samples of carbonate and phosphate rocks) and two 50-mg portions of 

National Bureau of Standards sample No. 70a (feldspar). 

As each portion is weighed, transfer to a crucible containing 

the fused N aOH. 
TWO-SOLUTION PROCEDURE 27 

4. Cover and heat the crucibles to dull redness for about 5 min. 

Remove each crucible from the heat and swirl the melt 

around the sides. Allow the melts to cool. 

5. Transfer the crucibles plus contents to a series of 1-1 plastic 

beakers. 

6. Add about 980 ml of water to each beaker using a rapid 1-1 

water dispenser (the exact amount is not important except 

that the same amount must go into each beaker). Allow to 

stand for at least 1 hr. 

7. Stir the contents of each beaker using a phtstic stirring rod. 

8. Use a tilting pipet to add 20 ml of 1 + 1 HCl to each beaker, 

stirring each at the time of addition. 

9. Use nichrome tongs to lift each nickel crucible out and examine 

it; there should be no adhering melt. If any melt remains, 

allow some more time for dissolution with frequent stirring. 

When all the melts are dissolved, remove the crucibles and 

set them aside for washing with HCl before storing them. 

When the crucibles are removed the solutions are ready to 

be used for Si02 and Al203 determinations. 

PREPARATION OF SOLUTION B 

Solution B is used in the determination of total iron, MgO, CaO, 

Ti02, P205, MnO, Na20, and K20. 

The samples are digested on a steam bath overnight with a mixture of HF, H2SOH and HNO:-~. This procedure decomposes almost 

all the minerals that are normally present. 

Organic matter is destroyed by the addition of a few drops of a 

mixture of perchloric and nitric acids after heating to sulfuric 

acid fumes. Any manganese dioxide formed by oxidation is then 

reduced and rendered soluble by the addition of hydrazine sulfate. 

Rarely, appreciable residues do remain. If so, they should be 

removed, identified, and analyzed by suitable methods. 

REAGEANTS 

Solution B acid mixture: Working under a fume hood, transfer 

the contents of a 454-g bottle of HF ( 48 percent) to a 1-1 

polyethylene bottle. Chill the HF in a bath of cold water. 

Keep the polyethylene bottle in the cold-water bath, add 165 

ml of concentrated H2SO.h mix, and allow to cool. Add 40 ml 

of concentrated HN03 and mix. 

HCl04-HN03 mixture: Mix 100 ml of HC104 (72 percent) and 

100 ml of concentrated HN03. Store in a 200-ml glass-stoppered Pyrex bottle. 
28 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

Hydrazine-sulfate solution, 0.2 percent: Dissolve 0.2 g NH2 · NH2 · 

H2S04 in 100 ml of water. Prepare fresh solution for each 

run. 

PROCEDURE 

1. Transfer 0.500 g of each sample to special Teflon beakers 

(see '.i\pparatus and Instrumenrts;'p. 4). 

2. Under a fume hood, add 15 ml of the solution B acid mixture 

to each Teflon beaker and swirl to wet the sample powder. 

3. Cover the Teflon beakers with the Teflon covers, place the 

beakers in the steam bath so that most of each beaker is 

suspended in steam, and heat overnight. 

4. Remove the covers and continue to heat the beakers on the 

steam bath for about 1 hr or until acid fumes are no longer 

emitted. 

5. Completely transfer the contents of the Teflon beakers to 

400-ml Vycor beakers, using a minimum of water and a 

policeman to effect the transfer. 

6. Place the Vycor beakers on a hot plate and heat until S03 

fumes evolve, then remove the beakers~ 

7. When SOs fuming stops, add about 4 drops of the HC104-

HN03 mixture to each beaker from a dropping bottle and 

replace the beakers on the hot plate. Heat each beaker until 

strong fumes are evolved and any color caused by organic 

matter has disappeared. Repeat this step if necessary. 

8. Remove the beakers from the hot plate, allow them to cool 

for a few minutes, and then add about 225 ml of water, 

5 ml of concentrated HN03, and 1 ml of the hydrazine sulfate solution. 

9. Replace the beakers on the hot plate, and heat to boiling. If 

a brown precipitate of Mn02 remains after the solutions 

have boiled for a few minutes, an additional milliliter of 

hydrazine sulfate should be added. If a residue remains 

after the solutions have been boiled for a few minutes, boiling should be continued for an additional half hour. A ·small 

residue can be ignored; an appreciable residue should be 

sep·arated, identified, and analyzed by suitable me·ans. 

10. Cool the .solutions to room temperature, then transfer them 

to 250-ml volumetric flasks. 

11. Dilute to volume, mix, and transfer to polyethylene bottles. 
TWO-SOLUTION PROCEDURE 29 

DETERMINATION OF CONSTITUENTS 

Si02 IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

A ~molybdenum blue method is used for the determination of 

silica (Si02) (Bunting, 1944). As in the preparation of solution 

A, factors that govern precision should be given particular attention in order to avoid errors that might •seriously affect accuracy. 

All of the aliquots of the A solution ~should be precisely ·measured 

with the same 8-ml pipet. Adequately preci,se measurements can 

be made with an ordinary transfer pipet, but better precision is 

assured if a stopcock pipet is used. 

The procedures as written are based on the use of a pipetting 

machine to dispense reagents. Use of this pipet assures a high degree of reproducibility and eliminates the need for the use of volumetric flasks. The force of agitation obtained by its use eliminates 

the need for stirdng, and, be~cause the aliquots and all reagents 

are measured precisely, the final volume is the same for blanks, 

standards, and samples. Even when as few as six samples are being analyzed the mechanical pipet is preferred for the addition of 

reagents for the Si02 and Al203 determinations. 

REAGENTS 

Ammonium molybdate solution: Dissolve 6.0 g of (NH4) 6Mo,024 · 

4H20 in 11 of water. 

Tartaric ~acid solution: Dissolve 16 g of H2C2H.106 in 11 of water. 

Reducing solution: Place 0.28 g of sodium sulphite, 3.6 g of sodium 

bisulphite, and 0.06 g orf 1-amino-2 napthol-4-sulphonic acid 

into a 1-1 bottle, add 11 of water, and stir to dissolve. 

Acidified water: Add 5 ml of 1 + 1 H2S04 to 11 of water. 

Note: Highly consistent results are readily obtained when these 

solutions are prepared fresh, not more than 48 hr before use. To 

avoid the inconvenience of weighing reagents at the time an analysis is underway, a supply of 12 or more containers of each dry 

reagent is preweighed at some convenient time and stored for use 

as required. 

PROCEDURE 

1. With an 8-ml stopcock pipet, transfer 8 ml of the solution A 

reagent blank solution, 8 ml of each standard solution, and 

8 ml of each sample solution to 150-ml beakers. Add the 

following reagents, preferably with ·a pipettting machine. 

2. Add 25 ml of ~acidified water to each beaker. 

3. Add 25 ml of the molybdate reagent to each beaker. Allow to 

stand for 10 min. 
30 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

4. Add 23 ml of the tartaric acid solution to each beaker. 

5. Add 25 ml of the reducing solution to each beaker. Allow to 

stand for at least 45 min. 

6. Determine the percent trans.mission for each solution at 640 

nm using the reagent blank as the reference, or read percent Si02 using an automated spectrophotometer. 

CALCULATIONS 

If the automated spectrophotometer described is used, no further calcuJati<lns are needed. Otherwise: 

1. Convert the readings from percent transmission to arsorbance 

using table 1. 

2. Compute the factor : 

Percent Si02 in the ·standard 

Average of absorbance of standards 

3. Compute the percent Si02 in the samples : 

factor. 

(Factor) X (absorbance orf the sample) = percent Si02. 

4. If carbonate or phosphate rocks have been run using 200-m.g 

samples, multiply the results obtained in step 3 by 0.25 to 

obtain percent Si02. 

Al20 3 IN SILLCATE AND CARBONATE ROCKS 

Alumina (Al20a) is determined by measuring the absorption of 

light at 475 nm by a solution in which aluminum has been converted to a calcium aluminum alizarin red-S complex (Parker 

and Goddard, 1950). 

When ·50-mg samples are used for the preparation of the A solutions and a 15-ml aliquot is used, the determination can be made 

on samples containing as much as 24 percent Al20a. For samples 

containing from 24 percent to 36 percent Al203, a 10-ml aliquot of 

solution A ~hould be used instead of the usual 15 ml. When a 10-

ml aliquot is used, 5 ml of blank solution should also be added to 

the fla:sk to maintain the same pH and concentration of inert salts 

as in the standard solutions. Carbonate-rock samples are run using a 200-mg sample. 

Appreciable quantities of iron and titanium also form colored 

complexes that absorb at 475 nm. Interference from iron is eliminated by the use of potassium ferricyanide and thioglycolic acid 

as complexing agents ; iboth of these reagents are required to completely eliminate this interference. The interference of titanium is 

not eliminated, but an empirically derived table is provided to correct for the effect of titanium. 
TWO-SOLUTION PROCEDURE 31 

REAGENTS 

Complexing solution: To 880 ml of water add 0.3 g potassium 

ferricyanide KsFe(CN) 6; 40 ml of hydroxylamine hydrochloride (NH20H · HCI) solution, 10 percent; and 80 ml of a 

GaCI2 solution (Dissolve 14 g CaC02 in 30 ml concentrated 

HCI and dilute to 1 1). Prepare this solution the same day it 

is to be used. 

Thioglycolic acid solution : Dilute 3 ml of the pure acid to 1 I with 

water. 

Buffer solution: Dissolve 80 g of sodium acetate NaC2Hs02·8H20, 

in 975 ml H20 and add 24 ml glacial acetic acid. 

Alizarin red-S stock solution, 0.05-percent: Dissolve 0.5 g of the 

pure dye in 11 of water and filter. 

Alizarin red-S, 0.01-percent: Dilute 200 ml of the stock solution to 

1 I with water. 

PROCEDURE 

1. Transfer 15 ml each of the solution A reagent blank, the standard solution, and the sample ·solutions to a series of 150-ml 

beakers using a high-precision piston pipet. Add the following reagents to each beaker, preferably with a pipetting machine. 

2. Add 25 ml of the complexing solution. 

3. Add 25 ml of the thioglycolic acid solution. Allow to stand for 

5 min. 

4. Add 25 ml of the buffer solution. Allow to stand 10 min. 

5. Add 25 ml of the 0.01 percent alizarin red-S solution. Allow to 

stand for 45 to 75 min. 

6. Determine the percent transmission at 475 nm for each solution using the blank solution as a reference, or read percent 

constituent with an automated spectrophotometer. 

CALCULATIONS 

If an automated spectrophotometer has been used, correct the 

values for Ti02 where required as indicated below, step 4. If an 

ordinary spectrophotometer has been used, make the following calculations: 

1. Convert the readings for percent transmission to absorbance 

using table 1. 

2. Compute the factor : 

Percent Al20s in standard factor. 

Average absorbance of standards 
32 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

3. Compute the apparent percent Al20a in the samples : 

(Factor) x (absorbance of s~ample) = apparent percent Al20:~· 

4. Correct for Ti02 according to the following: 

Apparent percent Al,O, Correction per percent TiO, 0 ________________________________________________ 0.25 

5------------------------------------------------ .20 

10------------------------------------------------ .15 15 ________________________________________________ .10 

20------------------------------------------------ .05 

(Apparent percent Al20a)- [(Correction per percent Ti02) X 

(percent Ti02)] =percent Al20a. 

If the sample is a carbonate rock and 200-mg samples have been 

used, multiply the result obtained in step 4 by 0.25 to obtain percent Al20a. 

Al20 3 IN PHOSPHATE ROCKS 

Bhosphate rocks normally contain fluorine which tends to lower 

tl!e alumina w.lues when the concentmtions of both constituents 

are about 1 percent or higher. This interference is eliminated by 

volatilization of mos't of the fluorine as fluosilicic acid from the 

aliquots of solution A by evaporation from Vycor or silica vessels. 

Reagents are the same as those for silicat~e rocks (p. 31). 

It should be noted that fluorine does not affect the Al20a determination when the samples are run by the single ... solution procedure. 

PROCEDURE 

1. Transfer 15 ml of solution A reagent blank solution and 15 ml 

of each sample solution to a series of small Vycor beakers 

or evapo~ating dishes. 

2. Evaporate to dryness on a steam bath. 

3. To each beaker or dish add 15 ml of solution A reagent blank 

solution with a pipet and heat for a few minutes on the 

steam bath to dissolve the salts, then remove from the steam 

bath and allow to come to room temperature. 

4. Place each beaker upright into a 150-ml beaker. 

5. Proceed as in stepJ 2 to 6 of the procedure for aluminum in 

·silicate rocks (page 31). 

Calculations are the same as for carbonate r·ocks. 

TOTAL IRON IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

The method for determining total iron (as Fe20.1) is based on 

the orange color developed with orthophenanthroline after the 

~The correction required may vary with each spectrophotometer, so it is advisable to establish a proper correction for each instrument. The corrections shown are for the 

.Model B Beckman spectrophotometer. 
TWO-SOLUTION PROCEDURE 33 

iron is reduced with hydroxylamine hydrochloride and the solution· is buffered with sodium citrate ('Bandemer and Schaible, 

1944). When 5 ·ml of solution B is used, the method is designed to 

accom·modate samples containing from 0 to 15 percent total iron 

(as Fe203). For the occasional,sample in which Fe20:~ exceeds 15 

percent, a smaller aliquot of solution B can be used. 

REAGENTS 

Hydroxylamine hydrochloride, 10 percent. Dissolve 100 g NH20H · 

HCl in 1 I of water. 

Orthophenanthroline solution, 0.1 percent. Dissolve 1 g C12HsN2· 

H20 in 1 I of water. 

Sodium citrate solution, 10 percent. Dissolve 100 g Na3C6H507 · 

2H20 in 1 I of water. 

Standard iron :solution: Add 0.4910 g FeS0-1 · (NH-1) 2S0.1 · 6H20 to 

a 500-ml volumetric flask. Add 16 ml of 1+1 H2SOH 10 ml of 

concentrated HN03, and about 300 ml of water. Stir until 

completely dissolved, then add water to the mark, and mix. 

The Fe203 concentration in thi·s solution is 0.2 mg/ml, which 

is equivalent to that in a solution prepared from a sample 

containing 10 percent Fe203. 

PROCEDURE 

1. With a pipet, transfer two 5-ml portions of the standard iron 

solution and 5 ml of each saimple solution B to a series of 

100-ml volumetric flasks. Use an additional flask for the 

preparation of a reagent blank solution. 

2. With a tilting pipet add 5 ml of the hydroxylamine hydrochloride solution to each fl.ask and allow to stand for 10 

min. 

3. Add 20 ml of the orthophenanthroline solution to each flask, 

preferably with a pipetting machine. 

4. Add 10 ml of the sodium citrate to each flask with a tilting 

pipet and dilute to 100 ml with water. 

5. After 1 hr determine the percent transmission at 555 nm for 

each solution by using the blank solution as the 100 percent 

reference. 

CALCULATIONS 

If the readings are made on an automatic spectrophotometer 

they are already in percent, otherwise : 

1. Convert the readings for percent transmission to absorbance 

using table 1. 
34 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

2. Compute the factor : 

10 

Average of the absorbances of the standard solutions 

3. Compute the percent total iron as Fe20a : 

(F·actor) X (absorbance of sample solution) = 

factor. 

percent total iron as Fe20a. 

Note: Percent Fe20a in the sample is calculated in the usual w·ay 

by subtracting the Fe20a equivalent of FeO (separately determined) from the value for total iron as Fe20a: 

(Percent total iron as Fe20a)- (percent FeO x 1.1) = 

percent Fe20a. 

Ti02 IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

The determination of titania (Ti02) is based on the yellow color 

produced with Tiron ( disodium-1,2-dihydroxybenzene-3,5-disulfonate) (Yoe and Armstrong, 1947). Ferric iron also reacts with 

Tiron to produce a purple solution that can be decolorized by reduction with sodium dithionite. 

Titanium dioxide in the 0- to 3-percent range can be determined 

with 5 ml of solution B. For higher concentrations, a smaller 

aliquot of solution B should be taken. 

REAGENTS 

Tiron (disodium-1,2-dihydroxybenzene-3,5-disulfonate) : Dry reagent powder. 

Buffer solution: Dissolve 80 g of ammonium acetate NH4C2Ha02 

and 30 ml of glacial acetic acid in 2 I of distilled water. 

Sodium dithionite, Na2S204 (sometimes sold as sodium hydrosulfite): Dry reagent powder. 

Standard Ti02 solution: Weigh 0.1013 g of National Bureau of 

Standards standard sample No. 154 (titanium dioxide) or the 

appropriate amount of reagent Ti02 of known composition. 

Transfer to a small platinum crucible. Add 2 g of sodium 

bisulfate, cover, and heat until the sample i'S completely fused, 

allow to cool amd place the crucible into a beaker containing 

125 ml of 1+1 H2S04. Heat and stir until the melt is completely dissolved. Cool, dilute the solution to 250 ml in a 

volumetric fl·ask, and mix well. With a pipet, transfer 50 ml 

of the solution to a 1-l volumetric flask, add 40 ml of 1+1 

HNOa, dilute to the m•ark, and ·mix. Store the solution in a 

Pyrex bottle. The concentration of Ti02 is 0.02 mg/ml which 

is equivalent to that in a solution prepared from a sample containing 1 percent Ti02. 
TWO-SOL UTI ON PROCEDURE 35 

PROCEDURE 

1. With ·a pipet, transfer 5-ml portions of the standard Ti02 solution and 5 ml of each solution B to ·a series of 150-ml beakers. 

2. Add 5 ml of water to another beaker, to be used in the preparation of a reagent blank solution. 

3. Add about 125 m.g of ·Tiron powder to each beaker with a 

measuring scoop. 

4. Add 50 ml of the buffer ·solution to each solution preferably 

with a mechanic·al pipetter. 

5. Set the spectrophotometer to 430 nm. Add ·about 10 to 20 mg 

of sodium dithionite to the solution to be used as the reagent 

blank solution and mix gently ·by rotating the beaker two or 

three times, but not more, as vigorous mixing will cause 

sulfur to precipitate. 

6. Pour the solution into the absorption cell in the spectrophotometer and adjust the slit width to 100 percent transmission. 

7. Add dithionite powder to one of the standard solutions, mix, 

wait for ·a minute, and obtain the percent transmission. Repeat with each sample solution and the second standard 

solution. The dithionite may be added to 6 beakers at a time. 

CALCULATIONS 

If the readings are made on an automated spectrophotometer, 

they are ·already in percent, otherwise: 

1. Convert the readings for percent transmission to absorbance 

using table 1. 

2. Compute the factor : 

1 :factor. 

Average of the absorbances of the standard solutions 

3. Compute percent Ti02: 

(Factor) X (absorbance of sample solution) =percent Ti02. 

MnO IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

The determination of MnO utilizes the intensity of the color of 

the permanganate ion developed by persulfate with silver as a 

c•atalyst (Hillebrand and others, 1953). With a sufficiently high 

concentration of silver, the color can be developed fully in 1 hr 

at room temperature. 

In the determination, 25 ml of solution B is used. This amount 

is suitable for MnO concentrations ranging from 0 to 3 percent. 

For the occasional sample in which MnO exceeds 3 percent, a 

smaller aliquot should be used. 
36 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

REAGENTS 

Silver nitrate :solution, 5 percent: Dissolve 5 g AnNOa in 100 ml 

of water and .store in a dark bottle. 

Phosphoric acid, 1 + 1 : Prepare 100 mi. 

Ammonium persulfate: Dry reagent powder. 

Acid blank solution: Dilute 16 m1 of 1 + 1 H2S04 and 10 ml of concentrated HNOa to 500 ml with water. 

Stand·ard MnO solution: Transfer 0.0531 g of National Bureau 

of Standards standard sample No. 25c (manganese o~e) to 

a 250-ml beaker and add 25 ml of 1+1 HNOa, 64 ml of 1+1 

H2S04, and 2 to 3 ml of 3-percent H202. Heat on a hotplate 

until no bl.ack residue remains. Evaporate and then continue heating until fumes of SOa appear. Cool, add 80 ml of 

1 + 1 HNOa cautiously, and dilute to about 250 .mi. If there is 

a residue, heat until it di·ssolves. Cool Wld dilute to 2 I. The 

MnO content is 0.02 mg/ml, which is equivalent to that in a 

solution prepared from a sample containing 1.0 percent MnO. 

PROCEDURE 

1. Transfer 25 m·l of each of the B ·solutions, ·two 25-ml aliquots 

of the standard MnO .solution, and 25 ml of the acid blank to 

a series of 150-ml beakers. 

Add the following reagents to each beaker and stir after each addition: 

a. 1 ml of 1 + 1 phosphoric acid 

b. 1 ml of 5 percent silver nitrate solution 

c. a dipper of ammonium persulfate powder (about 100 mg) 

2. Allow the solutions to stand for 1 hr. 

3. Add 25 ml of water and mix. 

4. Measure the percent transmission for each solut~on at 525 nm, 

using the reagent blank solution as the reference. 

CALCULATIONS 

If the readingts are made on ·an automated spectrophotometer, 

they are :already in percent. Otherwise : 

1. Convert the reading·s for percent transmission to absorbance 

using table 1. 

2. Compute the factor : 

1.0 

Average of ·ahsorbances of the standards 

3. Galculate percent MnO. 

factor. 

(Factor)+ (absorthance of sample solution)= MnO. 
TWO-SOLUTION PROCEDURE 37 

P20 5 IN SILICATE ROCKS 

The phosphate (P205) content of a .sample i·s determined by 

measuring the light absorbed at 420 nm by a solution containing 

the yellow molybdov·anadophosphoric acid complex (Kitson and 

Mellon, 1944). The wavelength of 420 nm was chosen to provide 

adequate sensitivity and to minimize the effect of iron. At this 

wavelength, each 4 percent total iron as Fe203 in the sample will 

still cau2e the value of P205 to be hi·gh by 0.01 percent unless a 

correction is made. 

REAGENTS 

Molybdova.nadate solution: Dissolve 5.0 g of sodium metavanadate, 

NaV03, in 400 ml of 1+1 HN03. Dissolve 75 g of sodium 

molybdate (Na2Mo0-1·2H20) in 400 ml of water. Mix the two 

~solutions and dilute to 2 I. 

Standard P205 solution: Transfer 0.0950 g of National Bureau of 

Standards sample No. 56b (phosphate rock) to a beaker. 

Add 80 ·ml of 1 + 1 HzS04, dilute to 2 I, and filter. The concentration of P 205 is 0.015 mg/ml, which is equiv·alent to that 

in a ;solution B prepared from ·a sample containing 0.75 percent P205. 

PROCEDURE 

1. Transfer 2·5 ml of water to serve as a reagent blank, two 25--

ml portions of the standard P 205 solution, and 25 ml of each 

B solution to a series of 100-ml beakers. 

2. Add 25 ml of the molybdovanadate solution to each beaker preferably with a mechanical pipet. 

3. Allow the solution to stand for 5 min, then determine the percent transmission at 420 nm for the standard solutions and 

the sample solutions, using the reagent blank solution as the 

reference. 

CALCULATIONS 

If the readings are made on an automatic spectrophotometer 

they are already in percent, otherwise : 

1. Convert the readings for percent transmission to absorbance 

using table 1. 

2. Compute the factor : 

0.75 ---------------------factor. 

Average of the absorbances of the standard solutions 

3. Calculate the apparent percent P 205 : 
38 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

(Factor) X (·absorbance of sample solution) 

=apparent percent P 205. 

4. Correct the apparent percent P 20 5 for the effect of iron : 

(Apparent percent P205)-(percent iron as Fe20aX0.0025) 

=percent P 205. 

P20 6 IN CARBONATE ROCKS 

U·se the method described above for silicate rocks unless the concentration of P205 exceeds 10 percent, in which case use the method described below for phosphate rocks. 

P20 5 IN PHOSPHATE ROCKS 

The method for determining P 205 in phosphate rocks is basically the same as that whioh is used for silioote and carbonate rocks. 

Sample solutions are brought to suitable concentration for color 

development by d()uble dilution, and standard solutions at three 

concentration levels are used to establish a reference line. 

REAGENTS 

Standard P205 solution: Transfer 0.4430 g of National Bureau of 

Standards standard sample No. 56b (phosphate rock) to a 

400-ml beaker. Add about 50 ml of water and 10 ml of concentrat'ed HNOa, cover the beaker, and heat to boiling. Boil 

for about 5 min and allow to cool. Add about 200 ml of water 

and 16 ml of 1+1 H2S04, and filter. Dilute to 500 ml in a 

volumetric flask. 

H2S04, 1+7. 

Molybdovanadate solution: Dissolve 5 g of sodium metavanadate 

(NaVOa) in 400 ml of 1+1 HN03 • Dissolve 75 g of sodium 

molybdate (Na2Mo04 ·2H20) in 400 ml of water. Mix the two 

solutions and dilute to 2 I with water. 

PROCEDURE 

1. Use a pipet to transfer 15 ml of each B solution and two 1,5-ml 

portions of the standard P205 solution to 100-ml volumetric 

flasks. 

2. Dilute the solutions to 100 ml with water and mix. 

3. Transfer 15 ml of the dilute B solutions prepared in step 2 to 

100-ml volumetric flasks. 

4. With the ·same 15-ml pipet that wa-s used above, transfer 15, 

30, and 45 ml of ooch solution prepared from the standard 

P205 solution to additional 100-ml volumetric flasks. 

5. To each of the flasks containing aliquots from the B solutions 

and the standard P 20;; solutions and to one additional flask, 
TWO-SOLUTION PROCEDURE 39 

which will be used for the reagent blank solution, add 5 ml 

of the 1+7 H2S04 solution. 

6. Add 25 ml of the molybdovanadate solution to each flask from 

step 5, preferably using a mechanical pipet. 

7. Dilute to 100 ml and mix. 

8. After 15 'min determine the percent transmission at 420 nm, 

using the reagent blank solution as the reference. 

CALCULATIONS 

1. Convert the readings for percent transmission to absorbance 

using table 1. 

2. Plot the absorbance values for the standard solutions on ordinary coordinate paper. The standards correspond to 14.0, 

28.0, and 42.0 percent P 205. 

3. Use the absorbances for the samples to read the respective 

P 205 percentage from the graph. 

MgO, Na20, AND K20 IN SILICATE, CARBONATE, AND PHOSPHATE 

ROCKS AND CaO IN SILICATE ROCKS 

The alkaline earths, which were determined by automatic photometric titrations with EDTA in Shapiro and Brannock (1962) 

and the alkalies previously det~ermined by flame-emission techniques are now being determined by atomic absorption spectrometry. Lanthanurm is added to all blanks, standards, and samples 

as a releasing ag,ent (Dinnin, 1960), and the four elements are 

determined directly from the same solution. In general, results 

can be expected to be± 1-2 percent of the amount present, which 

is adequate accuracy for these constituents at the levels normally 

encountered. At levels above 15 percent, the maximum concentration readable without further dilution, a better value for CaO or 

MgO can be obtained by visual titration with EDTA. For Na20 

and K20, a dilution is made, and the solutions are reread more 

carefully in duplicate or triplicate by AAS. 

REAGENTS 

Stock standard solution: Transfer 0.8924 g CaC03, 0.9435 g NaCl, 

0.7915 g KCl, and 0.3045 g magnesium ribbon 5 to a 500-ml 

volumetric flask. Add 50 ml of water and 10 ml of concentrated HN03 and heat to dissolve the constituents. Cool, dilute 

to mark with water, and mix. This solution contains the equivalent of 1.000 mg/ml each of GaO, MgO, Na02, and K20. 

s Magnesium ribbon is generally about 99 percent Mg ; therefore, this figure includes a 

1 percent correction. 
40 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

Working standard solutions: To each of six 250-ml volumetric 

fl.31sks add 0, 15, 30, 45, 60, and 75 ml of the stock standard 

solution. Add about 100 ml of water, 6 ml of H2S04 (1 + 1), 

and 5 ml of concentrated HN03. Dilute to mark and mix. 

These solutions represent a blank and 3-, 6-, 9-, 12-, and 15-

percent standards, respectively, for each of the four oxides 

and contain the same acid concentrations as the solution B. 

Lanthanum solution: Carefully di·ssolve 58 g La203 (99.99 percent 

pure or better) in 250 ml of concentrated HCl in a 2-1 beaker. 

When the reaction has :subsided, dilute to 1 1 with water. 

PROCEDURE 

1. Transfer 4 ml of the standard solutions and the solution B of 

each sample, using a transfer pipet, to a series of 200-ml 

plastic bottles or beakers. Plastic 'bottles are preferable if 

the ,solutions are to be stored for some time before analysis. 

2. Add 16 ml of the lanthanum solution to each bottle, preferably 

with a mechanical pipet. 

3. Add 40 ml of water to each bottle, preferably with a mechanical pipet. These solutions are ready to be analyzed, or, if 

they are tightly stoppered, they may be stored as long as a 

month. 

4. Atomic absorption measurements are made following the procedures described by the m'anufacturer for the equipment 

in use. The standard solutions cover the workable range for 

each constituent and encompass conc,entrations derived from 

rocks. Where concentrations exceed 15 percent of the rock, 

the solution B can be diluted appropriately, but accuracy will 

be diminished unless duplicates or triplicates are run to improve the results. 

5. Results are calculated from standard curves made from the 

standard solutions. As the concentration of these solutions 

are already ·stated in terms of percent constituent, no calculations are required once the curve is dr·awn. 

It ·Should be noted that accuracy tends to be better with digital 

readout equipment than with dial readouts. With digital readout 

and the instrument set against a 12- or 15-percent standard, even 

the low values are read sufficiently accurately. With a dial readout, 

solutions having concentrations below 3 percent are reread against 

the 3-percent standard adjusted to give a maximum reading. 

TOTAL Ca0+Mg0 IN CARBONATE AND PHOSPHATE ROCKS 

Because CaO, P205, and C02 are the major constituents of carbonate or phosphate rocks, a higher accuracy is desirable for them 
TWO-SOLUTION PROCEDURE 41 

than for the other components of these rocks. The comparatively 

low level of Al203 and Fe203 in these rocks, and the UJSe of cyanide 

and triethanolamine (Biedermann and Schwarzenback, 1948; 

P.ribil, 1953) to complex them makes a visual EDTA titration for 

CaO plus MgO feasible. Experience has :shown that at the levels 

found in these rocks the visual titration for CaO+MgO is more 

accurate than the AAS techniques and is therefore to be preferred. 

Calcium carbonate of A.C.S. (American Chemical Society) 

grade is used as ,a standard. A measured small quantity of magnesium is added to all solutions prior to titration to ensure the 

presence of sufficient magnesium to convert the dye to its red 

complex so that the end-point change will be sharp. The titration, 

always a large one with this type of rock, can be speeded up by deeanting a small portion of the solution to be titrated into a small 

beaker and then titrating the main portion rapidly, going slightly 

past the end point. This allows a good view of the appearanee of 

the end point. When the decanted portion is restored to the main 

portion, the end point ean then be relocated with more care. A 

mixed indicator is used to provide a small amount of yellow to 

serve as ~an internal filter whieh sharpens the appearance of the 

end point. Without the added yellow, the color changes from red 

to purple and then to blue, making it difficult to determine the 

end point. If the yellow is added, the end point is the complete 

di'Sappearance of red, a distinctly discernible point. 

Results obtained for GaO+MgO can be used, with results obtained for MgO by AAS to ealculate GaO. 

REAGENTS 

Standard solution for GaO+MgO: Transfer 0.500 g of calcium 

carbonate (low in alkalies), A.C.S. grade, to a 150-ml beaker, 

add about 25 ml of w,ater and 5 ml of HN03, cover with a 

watch glass, and bring to a boil on a hot plate. Cool to room 

temperature, add 8 ml of 1 + 1 H2SO.h dilute with water to 

250 ml in a volumetric flask, and mix. 

EDT A solution, 0.3 percent: Dissolve 12 g of disodium ethylenedi,amine tetraacetate in 4 I of w,ater. 

Erichrome blaek T solution, 0.05 percent: Dissolve 50 mg of the 

dye in 100 ml of water. Do not store for more than 3d. 

Hydroxylamine hydrochloride solution, 10 percent: Dissolve 100 

g NH20H·HCI in 11 of water. 

Buffer solution: DiHsolve 66 g of NH!Cl in 500 ml of water, add 

500 ml of concentrated NH.10H and mix. 
42 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

Complexing solution: Dissolve 64 g of KCN in 600 ml of water, 

add 400 ml of triethanolamine, and mix. 

Methyl red solution, 0.02 percent: Dissolve 200 mg of the pure 

dye in 1 I of water. 

Magnesium sulfate solution: Dissolve 6 g of MgS04 ·7H20 in 1 I 

of water. 

PROCEDURE 

1. With a 25-rnl transfer pipet, transfer two aliquots of the 

standard solution for CaO+MgO, an aliquot of the magnesium sulf·ate solution, and aliquots of each B solution to 

a series of 400-ml beakers. 

2. Add 5 ml of the hydroxylamine hydrochloride solution to each 

of the beakers and allow them to stand for 5 min. 

3. Add about 200 ml of water to each beaker. 

4. Add 5 ml of complex.ing solution to each beaker. Use a graduate or pipet that does not require mouth suction. 

5. Add 25 m1 of the buffer solution to each beaker using a g:mduate. 

6. Add with a transfer pipet 2 ml of the magnesium sulfate 

solution to each beaker, including the beaker already containing magnesium sulfate. 

7. To one of the solutions containing the standard Ca0+Mg0, 

·add 1 m1 of the Erichrome black T solution and 1 ml of 

the methyl red solution. 

8. Decant about 10 ml of the solution into a small beaker and 

set aside for the moment. 

9. Adjust the level of EDTA in the buret to zero, then add the 

EDTA rapidly to the ·solution in the 400-ml beaker in slight 

excess of that needed to get complete color change from red 

to blue. 

10. Pour the sol&tion which had been set aside back into the 

400-ml beaker. The color of the solution should change back 

to red. 

11. Titrate carefully until the red color of the solution disappears 

and the solution is dull grey-green in color. This is the end 

point. 

12. Repeat steps 6--11 with each of the sample solutions, the magnesium sulfate solution, and finally with the remaining 

standard GaO+MgO solution. 
SINGLE-SOLUTION PROCEDURE 43 

CALCULATIONS 

1. Divide the volume for the titration of the magnesium sulfate 

solution by 13.5 to obtain the volume of the EDTA equivalent 

to 2 ml of the magnesi urn sol uti on. 

2. Subtract the volume of EDT A equivalent to 2 ml of the magnesium solution from each titration volume to obtain the 

volume of EDT A required for the 25 ml of each solution. 

3. The percent CaO in pure OaC03 is 56.0, therefore to calculate 

a. factor: 

56.0 factor. 

Average of titration volumes for 25 ml of standards 

4. Calculate the percent total CaO+MgO, as CaO in each sample: 

(F·actor) X (titration volume for 25 ml of the 881mple solution) 

=percent Oa:O+MgO calculated as CaO. 

5. The result obtained for Ca.O+MgO as GaO is used in the fol-. 

lowing manner : 

Percent MgO (by atomic absorption) X1.39 

= CaO equivalent of MgO, 

then, 

CaO+MgO (from step 4) -CaO equivalent of MgO=percent CaO. 

SINGLE-SOLUTION PROCEDURE 

PREPARATION OF SOLUTION 

A single solution suitable for determining the same ten constituents described in the two-solution method can be prepared 

rapidly by fusing samples with a lithium metaborate-lithium tetraborate flux. The use of lithium metaborate for thi,s purpose was 

first reported by Ingamells (1964), who later suggested its use 

with atomic absorption spectrophotometry ( Suhr and lngamells, 

1966). Spectrophotometric procedures have also been applied to 

a solution prepared with a metabol'late flux (Ingamells, 1966; 

Shapiro, 1967a). 

Graphite crucibles are preferred in this laboratory for the fusion procedure because the alternative use of platinum crucibles 

gives melts which wet the crucible, adhere to it, and are troublesome to dissolve. Because they do not wet the crucible, melts made 

in gmphite are removed easily in the form of beads. In using 

lithium metaborate fusions with many sa·mples, it was found that 

melts of samples containing high iron concentrations (about 15 

percent Fe203) would often adhere tenaciously to the crucibles. 

Lithium tetratborate, a more acid flux than the metaborate, yielded 
44 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

fusions which in spite of their high iron contents parted readily 

from the graphite isurface. It was found, however, that silica 

values obtained from solutions prepared from tetraborate melts 

were erratic, whereas silica determined in solutions prepared from 

metabomte fusions were excellent. A mixed flux, the proportions 

of which were determined empirically, led to good parting from 

the graphite even at iron-ore levels and also yielded good silica 

results. 

Various facto~s must be considered in deciding sample size, flux 

amount, and sample concentration in order to develop a practical procedure to determine Si02, Al203, GaO, MgO, Na20, K20, 

Fe203, MnO, Ti02, and P205 in a single solut·ion. The flux-sam.ple 

ratio should be kept ·as low as possible because even the purest 

reagents contain impurities that become significant with ·a high 

flux-sample ratio. The :sample size must al·so be as high a.s possible 

to ·assure proper sampling of the mixture of minerals of which a 

rock consists. The quantity of flux plus sample should not be too 

large, however, because it must fit easily within the graphite crucible and also because the bead must dissolve in a reasonably 

short time. The concentration of constituents in solution must be 

sufficiently high to accommodate the least sensitive determinations 

yet low enough to prevent gel·atinous silica .from precipitating. To 

satisfy these qualifications, it was decided to use 200 mg of sample and 1.2 g of flux and to dilute the dissolved melt to 250 ml. This 

provides a solution in which SI02, Al203, total Fe as Fe203, 

Ti02, P205, and MnO can be determined by simple spectrophotometric procedures, and CaO, ·MgO, Na20, and ·K20 can be determined by AAS using procedures which, except for P 205, are very 

similar to those described for the two-solution method. The presence of .silica in the single-solution precludes the use of the vanadomolybdate procedure for small amounts of P 205, so this procedure is in this case replaced by a molybdenum blue procedure 

that c·an be used in the presence of Si02. With c·arbonate and phosphate rocks the high contents of CaO and P 205 are determined 

with better accuracy using the two-solution procedures. 

REAGENTS 

Flux mixture: Thoroughly mix 1 part of reagent-grade powdered 

anhydrous lithium metaborate, LiB02, and 2 parts of anhydrous lithium tetraborate, Li2B407. This mixture can be stored 

-in a tightly closed bottle-so at least several hundred grams 

of the mixture should be prepared. 

HN03, 1+1: Prepare 1 I. 
SINGLE-SOLUTION PROCEDURE 45 

Standard samples: Analyzed ·samples which can be carried through 

the procedure along with the unknown samples serve best as 

standards. In the U.S. Geological Survey a well-analyzed diabase is used as a standard for Si02, Al203, total Fe, CaO, MgO, 

Ti02 and as an internal check on K20, Na20, P205, and MnO. 

The other standards described for the two-.solution procedures 

can be used for all of the constituents. Bureau of Standards 

standard sample Feldspar No. 70a is suitable for Si02 and 

Al203 standards. 

Graphite crucibles: Cylindrical, 25-mm (20 mm ID), of high purity g~aphite. 

PROCEDURE 

1. Transfer approximately 1.2 g of flux mixture to a waxed or 

plastic-coated weighing paper. 

2. Weigh and transfer to the flux mixture 0.2000 g of finely 

ground sample powder. 

3. Mix by rolling successive corners of the paper about 10 times, 

and transfer carefully and completely to an unused graphite 

crucible. 

4. Wei,gh the s~tandard sam·ples along with the unknown samples, 

and also carry along a crucible containing only the flux 

mixture to serve ·as a blank. 

5. Tamp down the powders by tapping each crucible several 

times against the table surface. 

6. Fuse in a muffle at 1,0oooc for 1 hr. 

7. While the fusions are in the muffle, place a 2-1 flask of distilled water on a hot plate and bring to a boil. 

8. Remove the crucibles from the muffle and allow to cool. When 

cool, the beads produced should be easily dislodged from 

the crucibles with the touch of a spatula. 

9. To each of a series of 250-ml plMtic bottles add a Tefloncovered 39-mm magnetic stirring bar. Longer stirring bars 

tend to cause the bottles to walk during stirring. Transfer 

the beads to the bottles. Add 50 ml of boiling water to each 

bottle using a plasic graduate. Transfer to magnetic 

stirrem. 

10. Turn the stirrers on, and, using a tilting pipet, add 5 ml of 

the 1+1 HN03 to each flask. 

11. Stir rapidly for about 60 min. The beads normally dissolve 

in about 20 min, but the additional stirring may be needed 

in some cases, particularly with phosphate and carbonate 

rocks. Stir until the beads dissolve completely. 
46 ANALYSIS OF SILICA'l'E, CARBONATE, PHOSPHATE ROCKS 

12. Remove the bottles from the stirrers, and add about 100 ml 

of water to each. Using a funnel in order to catch the 

stirrer, pour the solutions in turn into a 250-ml volumetric 

flask. Rinse the bottle briefly, then add water to the volumetric flask to mark, mix, and pour the solution back into 

the plastic bottle for storage. Rinse the flask between 

samples. 

DETERMINATION OF CONSTITUENTS 

Si02 IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

Silica (Si02) in concentrations which exceed about 200 ~tg/ml 

forms polymers which will not react with the molybdate reagent. 

It was found however (Shapiro, 1974) that the presence of fluoride ions at the proper concentration leads to depolymerization. 

The procedure for the determination is similar to that described 

with the two-solution method. Plastic beakers must be used because glass beakers are sufficiently attacked by the N,aF solution 

to lead to erratic results. 

REAGENTS 

Ammonium Molybdate solution: Dissolve 6.0 g of (NH4) sMo7024 · 

4H20 in 1 1 of water. 

Tartaric acid solution: Dissolve 16 g of H2C2H40s in 1 I of water. 

Reducing solution: Dissolve 0.28 g of sodium sulphite, 3.6 g of 

sodium bi,sulphite, and 0.06 g of 1-amino-2 naphthol-4-sulfonic 

acid in 11 of water. Prepare within 48 hr of use. 

NaF, 3-percent solution. 

Dilute ~sodium fluoride solution: Dilute 20 ml of the 3-percent Na,F 

solution and 5 ml of 1+1 H2S04 to 1 I with water. 

PROCEDURE 

1. Transfer 0.500-ml aliquots of the blank, the standards, and the 

sample solutions to a series of plastic 100-ml or 150-ml 

beakers. It is best to use a high-precision 0.500-ml pistontype pipet although a w~ater repellant-coated 0.5-ml quantitative pipet is acceptable. If the silica is expected to be low, 

a:s in ca;rbonate and phosphate rocks, use a 2.00-ml aliquot. 

Add the following reagents, preferably with a pipetting machine. 

2. Add 25 ml of the dilute sodium fluoride solution to each beaker. 

Allow to stand 5 min. 

3. Add 25 ml of the molybdate reagent to each beaker. Allow to 

stand for 10 min. 
SINGLE-SOLUTION PROCEDURE 47 

4. Add 25 ml of the tartaric acid solution to each beaker. 

5. Add 25 ml of the reducing solutions to each beaker. Allow to 

stand for at least 45 min. These colors are stable for at least 

the remainder of the day. 

6. Determine the percent transmission for each solution at 640 

nm using the reagent blank as reference, or read percent 

Si02 using an automated spectrophotometer. 

Calculations are the same as those used for the two-solution 

method, but if carbonate or phosphate rocks have been analyzed 

using a 2-ml aliquot for the sample, multiply the result obtained in 

step 3 by 0.25. 

Al20a IN SILICATE, CARBONATE. AND PHOSPHATE ROCKS 

The reagents, procedure, and calculations are identical to those 

described for the two-solution procedure (see the section called 

'~l203 in Silicate and Carbonate Rocks"p. 31) except that a 0. 750-

ml aliquot is transferred instead of a 15-ml one. 

Note that if fluoride is present, as is common with phosphate 

rocks, the aluminum determination is not affected; the two-solution procedure for Al203 requires a separation of fluorine when 

present. 

TOTAL IRON IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

The procedure is similar to that described for the-two-solution 

method, except for aliquot size. The comparison standard used 

may be a known sample or an external standard prepared as described under Reagents. 

REAGENTS 

Hydroxylamine hydrochloride, 10 percent: Dissolve 100 g NH20H · 

HCI in 1 I of water. 

Orthophenanthroline solution, 0.1 percent: Dissolve 1 gm orthophenanthroline in 1 I of water. 

Sodium citrate solution, 10 percent: Dissolve 100 g Na3CsH50r· 

2H20 in 1 I of water. 

Standard iron solution: Transfer 0.1964 g of FeS04 · (NH4) 2S04 · 

6H20 (Mohr's salt) to a 500-ml volumetric flask. Add 10 ml 

of 1 + 1 HN03, about 300 ml of water and 2.4 g of flux mixture. Stir to dissolve. This solution corresponds to 10.00 percent Fe203 for aliquots taken in this procedure. 
48 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

PROCEDURE 

1.. Transfer 5 ml of the blank, standards, and s·ample solutions 

to a series of 150-ml beakers. 

2. Add 5 ml of the hydroxylamine hydrochloride solution, preferably using a tilting pipet or pipetting machine. Allow to 

stand for 10 min. 

3. Add 20 ml of the orthophenanthroline solution to each beaker 

as in step 2. 

4. Add 10 ml of the sodium citmte to each beaker as in step 2. 

5. Add 50 ml of water and mix. Allow to stand for 1 hr. 

6. After 1 ·hr determine the percent transmission at 555 nm for 

each solution using the blank ·solution as the reference, or 

read percent constituent on an automated spectrophotometer. 

CALCULATIONS 

If the automated spectrophotometer is not used, determine percent constituent as follows : 

1. Convert the readings for percent transmission to absorbances 

using table 1. 

2. Compute the factor : 

Percent Fe203 in the standard 

A vemge absorbance of standards 

3. ( F·actor) X (absorbance of samples) 

factor. 

=percent total Fe as Fe203. 

4. (Percent total Fe as Fe203)-(percent FeOx1.1) =Fe203. 

T•i02 IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

The procedure is identical to that described for the two-solution 

procedure under Ti02 in ·silicate carbonate, and phosphate rocks, 

except that 10-1ml aliquots rather than 5-ml aliquots are used. 

REAGENTS 

Tiron (disodium-1,2-dihydroxybenzene-3,5-disulfonate) : Dry reagent powder. 

Buffer solution: Dissolve 80 g of ammonium acetate NH4C2H302 

and 30 ml of glacial acetic acid in 2 I of distilled water. 

Sodium dithionite, Na2S204 (sometimes sold as sodium hydrosulphite) : Dry reagent powder. 

Standard Ti02 solution: Weigh 16.2 mg of N!ational Bureau of 

Standards standard sample No. 154 (titanium dioxide) and 

tmnsfer to 1.2 g of flux mixture. Fuse and prepare as for a 

regular single-solution analysi•s except that 20 ml of 1+1 
SINGLE-SOL UTI ON PROCEDURE 49 

HNOa is used to dissolve_the melt. After solution, dilute to 1 

I in a volumetric flask. This solution corresponds to 2.0-percent Ti02. Alternatively any well-analyzed rock can be used 

to prepare a single solution to serve as a reference standard. 

PROCEDURE 

1. With a pipet transfer 10-ml portions of the blank, standards, 

and samples to ,a series of 150-·ml beakers. 

2. Add approximately 125 mg of T·i'ron powder to each beaker 

with a measuring scoop. 

3. Add 25 ml of the buffer solution to each beaker, preferably 

a pipetting machine, then add 50 ml of water. 

4. Set the .spectrophotometer to 430 nm. Using a spatula add 10-

20 mg of sodium dithionite to the bl'ank solution and mix 

gently by rotating the beaker two or three times ; more vigorous mixing will cause sulfur to precipitate. 

5. Pour the solution into the absorption cell of the spectrophotometer ·and adjust the slit width to give 100 percent transmission. 

6. Add dithionite powder to the next 10 beakers. One at a time, 

rotate each beaker once or twice to decolorize the purple 

color of the iron complex, pour into the absorption cell, and 

measure the percent transmission. 

7. Repeat step 6 with the remainder of the solutions. They are 

taken 10 at a Ume to limit the time between dithionite addition and the reading of the percent transmission. 

CALCULATIONS 

If the automated spectrophotometer is not used, determine percent constituent as follows : 

1. Convert the readings for percent trans,mission to absorbances 

using table 1. 

2. Compute the factor : 

Percent Ti02 in standard factor. 

Average absorbance of the standard solutions 

3. (F~actor) X (absorbance of sample solutions) =percent Ti02 

MnO IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

The MnO determination is ~similar to that described for the twosolution procedure. 
50 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

REAGENTS 

Silver nitrate solution, 5-percent. 

Phosphoric acid, 1+1 with water. 

Arnmoni urn pemulphate : Reagent grade ( NH4) 2S20s. 

Standard MnO solution: Transfer 0.0215 g of National Bureau of 

Standards standard sample No. 25c (manganese ore) to 1.2 g 

of flux mixture, fuse, and prepare as a regular single-solution 

(·see p. 45) except that after the pellet is dissolved -add an 

additional 35 ml of 1+1 HN03 and dilute to 2 1. This solution 

is equivalent to 1.0 percent MnO. 

PROCEDURE 

1. Tmnsfer 50 ml each of the blank, standards, and sample solutions to a series of 100-ml beakers. Add the following reagents to each beaker, mixing after each addition: 

2. Add about 100 mg of ammonium persulfate powder by dipper, 

mix. 

3. Add 1 rnl of 1+1 phosporic acid, mix. 

4. Add 1 ml of 5-percent silver nitrate solution, mix. 

5. Allow the solutions to stand for at least 1 hr. These colors are 

stable for the remainder of the day. 

6. Measure the percent t·ransmission for each solution at 525 nm 

using the blank solution as the reference, or read percent 

constituent on an automated spectrophotometer. 

Calculations are the same as those used for the two-solution 

method (see p. 36) . 

P20 5 IN SILICATE AND CARBONATE ROCKS 

The reagents used in the two-solution method for P 205 react 

with the silicate present in the single-solution, therefore an alternate molybdeum-blue procedure is used for the ·single solution. Absorbance is linear to about 2 percent P 205. If the concentration of 

P 205 is 10 percent or above, the procedure for phosphate rocks is 

used; if it lies in the 2-10 percent range, the procedure described 

for silicate and carbonate rocks is used with a smaller aliquot of 

the single solution. 

REAGENTS 

Ammonium molybdate solution: Dissolve 12.5 g of (NH4) sM01024 

·4Hz0 in 340 •ml of H20, then ·add 160 ml of 1 + 1 H2S04. 

Stannous chloride solution: Transfer 1 g of SnCl2 · 2H20 to a small 

beaker and dissolve completely in 75 ml of concentrated HCI. 
SINGLE-SOLUTION PROCEDURE 51 

Dilute to 3.5 I with water. This solution should be prepared 

within 3-4 hr of the ti•me it i.s to ,be used. 

Standard P205 ·solution: Using 200 mg of the National Bureau of 

Standards standard sample 56b (phosphate rock), prepare a 

single solution after fusing with the flux mixture. This yields 

a standard containing 31.7 percent P205 • Transfer 31.7 ml 

of this solution to ·a 1-l volumetric flask and add 5 ml of 1+1 

HNOa and dilute to volume. This is the 1-percent P 205 working solution. 

PROCEDURE 

1. Transfer 10-ml portions each of the blank, the 1-percent standard, and the sample solutions to a series of 150-ml beakers. 

2. Transfer 50 ml of the stannous chloride solution to a graduate. 

Set a timer for 10 min ; then, to one beaker at a time, add 

0.5 ml of the am.monium molybdate solution (using a squeeze 

pipet); then, promptly add the stannous chloride solution 

and mix. Refill the graduate with stannous chloride before 

proceeding to the next beaker so that the time between addition of molybdate and stannous chloride will be as short 

as possible. 

3. Add the two reagents to as many beakers as the 1 0-min time 

span will allow, then measure the transmittance of each at 

640 nm, using the blank as reference. The blue color that 

develops reaches a peak in about 3 min and remains stable 

for about 20 min. Ten solutions can easily be measured in 

this time. 

4. Repeat steps 2 and 3 with the remainder of the solutions, 

working with groups of ten. 

CALCULATIONS 

1. Convert the reading for percent transmission to 8ibsorbance 

using table 1. 

2. Compute the factor: 

1 factor. 

Absorbance of standard 

3. (Factor) X (absorbance of ·sample) =percent P205. 

P20 5 IN PHOSPHATE ROCKS 

Determine P205 values down to about 10 percent in phosphate 

rocks by the ·molybdovanadate procedure similar to that described 

for the two-solution method; modifications must be made, how-
52 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

ever, to take into account the difference in dilution. For levels between 2-10 percent P205 , use the procedure for silicates, except 

that 2-ml rather than 1 o .... ml aliquots of the sam,ple are taken and 

added to 8 ml of the blank. 

REAGENTS 

H2SO4, 1 + 7 : Prepare 500 mi. 

Standard P20s solution: Prepare a single-~solution of National 

Bureau of Standards standard sa.mple 56b (phosphate rock) 

using 200 mg of sample. This i,s a standard 31.7-percent P20s 

solution. 

Molybdovanadate ~solution: Dissolve 5.0 g of sodium meta vanadate 

(NaVOa) in 400 ml of 1+1 HNOa. Dissolve 75 g of sodium 

molybdate (Na2Mo04 ·2H20) in 400 ml of water. Mix the two 

·solutions and dilute to 2 1 with water. 

PROCEDURE 

1. Prepare a 1+1 dilution of the standard solution by transferring to a small dry beaker 5 ml of the ~standard solution of 

phosphate rock and 5 ml of water; use the same pipet for 

each transferral. Swirl to mix. 

2. Aline a series of 100-ml volumetric fl,asks. U·sing a 3-;ml pipet, 

transfer an aliquot of sample blank solution to the first flask, 

an aliquot of the 1+1 dilution prepared in step 1 to the second, an aliquot of the standard solution to the third, an 

aliquot of the standard solution plus an aliquot of the 1+1 

dilution prepared in step 1 to the fourth, and an aliquot of 

each of the sample solutions to the remainder of the flasks. 

3. With a squeeze pipet transfer 5 ml of 1+7 H2S04 to each of the 

fl·asks. 

4. Pipet 25 ·ml of the molybdovanadate solution into each of the 

flasks. 

5. Dilute to the mark with water and mix. 

6. After 15 min ~measure the percent transmission at 420 nm using the ·blank solution as the reference. 

CALCULATIONS 

1. Covert the readings for percent transmission to absorbances 

using table 1. 

2. Plot the aJbsorbance values for the three standard solutions 

against P205 concentration on ordinary coordinate paper. 

The 1+1 dilution represents 15.8 percent, the standard repsents 31.7 percent, and the standard plus the 1+1 dilution 
SINGLE-SOLUTION PROCEDURE 53 

represents 47.5 percent P20s. The plot should be very nearly 

a straight line. 

3. Use the absorbance for the samples to read the respective 

P 20s percentages from the graph. 

MgO, Na20 AND K20 IN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

AND CaO IN SILICATE ROCKS 

These elements are determined by AAS a~s in the two-solution 

method except for some modifications arising from the differences 

in ~solution composition. In the two-solution method sulfate is present but silica is absent. In the single-solution method the reverse 

is true but borate is also present. As a result, the standard solutions must also be different. Here, as in the two-~solution method, 

with sufficient dilution calcium in limestone·s and phosphate rocks 

can be determined to about 2 relative percent, but a better result 

can be achieved through a visual titration for CaO+MgO using 

EDT A as described in the two-solution procedure. 

REAGENTS 

Lanthanum solution: Transfer 140 g of lanthanum oxide, La203 

(99.997 percent pure) into a 2 I beaker. Add 300 ml of concentrated HCI, slowly, allowing time for the reaction to be 

completed after each addition of acid. The large beaker reduces difficulties from .spatter. After all the acid is added, the 

oxide will have been dissolved and converted to the chloride. 

Add 200 ml of H20. Each 4 ml of this solution contains approximately 1 g of La. 

Stock standard solution: Transfer 0.8924 g CaC03, 0.9435 g NaCl, 

0.7915 g KCl, all of reagent grade, and 0.3045 g magnesium 

ribbon 6 to a 500-~ml volumetric flask. Add 50 ml of water and 

10 ml of concentrated HCI. Boil to dissolve ·the constituents. 

Cool, dilute to volume with water, and mix. This solution contains the equiv·alent of 1.000 mg/ml each of CaO, MgO, Na20 

·and K20. 

Working standard solutions: To each of six 250-ml volumetric 

flasks add 0, 6, 12, 18, 24, and 30 ml of the stock standard 

solution. Add 1.2 g of flux mixture, 5 ml of 1+1 HN03 and 

about 200 ml of water. Mix and dissolve. Dilute to mark with 

water and mix. These represent a blank, 3-, 6-, 9-, 12-, and 

15-percent standard solutions for the single solutions. 

a Magnesium ribbon is generally 99 percent Mg, therefore this figure includes a 1-percent 

correction. 
54 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

PROCEDURE 

1. Transfer 10 ml of the sample solutions and the working standard solutions to a series of small beakers, or plastic bottles 

if the solutions ·are to be stored for later use. 

2. Add 1.5 ml of the lanthanum solution to each beaker and mix. 

Add 25 ml of water and mix. 

3. Use AAS equipment, and follow the procedure recommended 

by the manufacturer. If the instrument is not one of high 

sensitivity, it is advisable to scan all of the solutions quickly 

for each constituent to differentiate those below 3 percent 

from those above. Reread those below 3 percent using the 

3-percent ·standard to set the reading at a maximum. Reread 

thos·e above 3 percent between the appropriate standards. It 

is important to bracket the sample readings between standards, as the responses are not quite linear, but otherwise 

there are no interelement or other interferences to affect the 

results adversely. If CaO or MgO exceeds 15 percent, better 

results are obtained using the total CaO+MgO titration 

described below. If N·~O or K20 exceeds 15 percent, prepare 

more dilute solutions from the .single solution, then proceed 

starting from step 1 above. 

TOTAL Ca0+Mg0 IN CARBONATE AND PHOSPHATE ROCKS 

Total CaO+MgO i·s determined in carbonate and phosphate 

rocks by visual titration with EDT A exactly as is done in the twosolution procedure, except that 50 rni of the single solution is used 

in place of the 25 ml used for the two-sol uti on method. The standard is a sol uti on prepared from 200 mg of CaC03 dis·solved in 

5 ml 1+1 HN03 and diluted to 250 ml with water. The percent 

MgO in the sample, obtained by AAS, is multiplied by 1.39 to 

obtain its equivalent as CaO, which is then subtracted from the 

CaO + MgO to obtaJin percent CaO. See page 42 for the complete 

procedure. 

DETERMINATIONS ON SEPARATE SAMPLES 

H20 IN Sll.ICA TE, CARBONATE, AND PHOSPHATE ROCKS 

Water (H20) in rocks is usually considered to be moisture easily 

driven off at 105°-110°C, designated as H20-, and combined 

water, designated as H20+. The latter is driven off only at much 

higher temperatures. These are arbitrary divisions that provide 

a workable basis for ·analysis, but it ·must be recognized that water 
DETERMINATIONS ON SEPARATE SAMPLES 55 

may ,be present in ·a variety of ways which are obscured by this 

classification. 

In the U.S. Geological Survey, the H20- value is obtained by 

loss of weight of a 1-g sample after it is dried overnight at ll0°C. 

No further description of this procedure need be given. Total 

water is determined in a separate sample using the procedure 

described below. H20+ is obtained by subtracting the H20- value 

from that for total water. 

In the determination of total water, water is driven from the 

sample when it is heated in a Pyrex test tube with sodium tungstate as a flux (Shapiro and Brannock, 1955b). The water is condensed on a piece of preweighed filter paper in the upper part of 

a test tube, which i·s cooled by crushed ice in a polyethylene jacket 

surrounding the test tube during the analysis. 

The cooling jacket is made from a 230-ml polyethylene bottle by 

cutting the top portion off with shears, and cutting holes in the 

sides with a cork borer to permit insertion of the test tube. The 

apparatus is shown in figure 15. 

REAGENT 

Sodium tungstate, anhydrous powder: Heat s·everal hundred 

grams of reagent grade Na2W04 ·2H20 overnight at 180°C in 

a drying oven. Allow to cool, grind to a fine powder, and .store 

in an open container in the drying oven. 

PROCEDURE 

1. Weigh 1.000 g of sample powder, mix with 2 g of anhydrous 

sodium tungstate and transfer, with a funnel, to ·a dry 

18X150-mm Pyrex test tube. 

2. Cover the fingers of one hand with rubber finger stalls or a 

rubber glove, roll a piece of 50 X 50-mm filter paper into a 

cylinder, slip it into an 18X65-mm weighing tube, stopper 

the tube with a solid rubber stopper, and weigh. The fingers 

are covered to avoid transfer of moisture from the fingers to 

the paper. 

3. Quickly transfer the paper cylinder from the weighing tube to 

the upper part of the 18 X 150 ... mm test tube that contains the 

sample powder and stopper with a one-holed stopper. 

4. In·sert the test tube into the polyethylene cooling j-acket as 

shown in figure 15, and put crushed ice into the jacket so 

that the p·art of the tube enclosed by the jacket is well covered. 
56 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

FIGURE 15.-Tube in position for water determination: A, coiled paper water 

absorber; B, polyethylene container with crushed ice. 

5. Place the cooling jacket with the tube in place on a support at 

a height that will allow the tube to get .maxi,mum heat from 

a Fisher burner. 

6. Heat the closed end of the tube, gently at first, then at the full 

heat of the burner for 5 min. Allow to cool for at least 1 min. 

7. Remove the stopper; with the aid of a narrow spatula quickly 

wipe the walls of the tube surrounding the paper by pressing 

and rotating the filter-paper cylinder one full rotation. With 

the fingers covered, quickly transfer the paper from the test 
DETERMINATIONS ON SEPARATE SAMPLES 57 

tube to the weighing tube and replace the stopper in the 

weighing tube. 

8. Weigh the tube containing the filter paper plus the absorbed 

water. 

CALCULATIONS 

1. Subtract the weight for the weighing tube plus the paper strip 

obtained in step 2 from the weight for the weighing tube and 

paper ·strip plus water in step 8 to obtain the weight of 

absorbed water. 

2. Calculate percent water : 

(Absorbed water, in g) X100=percent H20. 

FeO IN SU..ICATE, CARBONAT~ AND PHOSPHATE ROCKS 

Ferrous iron (FeO) is determined by titration with a standard 

dichromate solution using diphenylamine sulfonic acid as the 

indicator (Sarver, 1927). The sample is decomposed by boiling 

with HF-H2S04. A spike consisting of a small known amount of 

ferrous ammonium sulfate solution is added to the sample solution 

just before starting each titration to assure a definitive end-point 

change for samples containing very small amounts of FeO. The 

use of a sequential heating device, figure 7, facilitates the uniform 

boiling of batches of samples. 

REAGENTS 

H2S04, 48 percent HF, K2Cr207 , boric acid crystals, all (A.C.S.) 

American Chemical Society grade. 

H2SO:., 1+3. 

Diphenylamine sulfonate indicator solution: Dissolve 0.2 g of 

sodium diphenylamine sulfonate in 1 1 of 1+1 H2S04. Add 11 

of 85-percent H3P04 and mix. 

Standard dichromate solution: Weigh 2.728 g of dry National Bureau of Standards potassium dichromate (K2Cr20,) or other 

pure dry potassium dichromate and dilute to 2 I in a volumetric flask. The solution equivalent is 2.000 mg/ml of FeO. 

FeO spike solution: Dis·solve approximately 0.5 g of ferrous ammonium sulfate FeS04(NH4)2S04·6H20 in 500 ml of water 

containing a few milliters of H2S04. 

PROCEDURE 

1. Weigh 0.500-g samples and transfer to 100-ml platinum crucibles. 
58 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

2. In a fume hood add 10 ml of 1+3 H2S04, and, with a plastic 

graduate, add 5 ml of HF. 

3. Cover the crucibles and bring to a boil on the sequential heating 

device; boil gently for 10 min. 

4. While the crucibles are heating, add about 15 ml of the indicator solution with a graduate and 5.0 ml of the spike solution with a squeeze pipet to 800-ml beakers containing about 

300 ml of water saturated with boric acid and stir. 

5. After each crucible has been boiling for 10 min, remove it from 

the heating device with tongs and i,mmediately immerse it 

in the solution contained in one of the 800-ml beakers. 

6. With the aid of a stirring rod, remove the crucible from the 

beaker, rinse it with water and set it aside. 

7. Titrate the solution with the standard dichromate solution to 

a purple color that persists for 20 to 30 sec. 

~· For each batch of samples, standardize the spike solution by 

transferring 25.0 ml to a solution containing about 300 ml 

of water saturated with boric acid, about 20 ml of 1+3 

H2SO." and 15 ml of indicator solution and titrate as described in step 7. 

CALCULATIONS 

1. Divide the titration volume obtained for 25 ml of spike solution 

by 5 to obtain the blank. 

2. St=.btract the blank from each of the titration volumes to obtain 

milliliters of dichromate required for the sample. 

3. Compute percent FeO in the sample : 

(ml of dichromate) X0.4 =percent FeO. 

C02 IN Sll..ICATE ROCKS 

The concentration of carbonate ( C02) in rocks can range from 

0 to 47 percent. Silicate rocks rarely contain more than several 

percent C02, ~and approximately 80 percent of those that have 

passed through the U.S. Geological Survey laboratory have contained 0.1 percent or less. A very rapid scanning technique was 

developed to differentiate those samples containing more than 0.1 

percent from those containing less and simultaneously to provide 

a value ± 0.02 percent absolute for those having less than 0.1 percent (Shapiro, 1971). For samples with more than 0.1 percent and 

as much as about 3 percent, a simple method (Shapiro and Brannock, 1955a) is used that employs a glass tube with a side arm 

(fig. 9) for the evolution and measurement of the gas. For those 
DETERMINATIONS ON SEPARATE SAMPLES 59 

samples in which the concentration of C02 exceeds 3 percent, one 

of the methods described for carbonate rocks is used. 

RAPID SCANNING TECHNIQUE FOR LOW LEVELS OF C02 

Carbon dioxide evolved from a 25-mg sample of pulverized rock 

is caught in a small bulge of a modified test tube as shown in figure 

8 and its volume is estimated by comparison with a similarly 

processed standard. The method is restricted to C02 concentrations 

up to 0.10 percent and is especially applicable to most igneous 

rocks and silicate minerals. Samples can be analyzed at the rate of 

2 min each. 

Mercuric chloride is added to the samples to react with metallic 

iron which is frequently introduced during the grinding process. 

Without this precaution, hydrogen gas would be produced along 

with the carbon dioxide. 

APPARATUS AND REAGENTS 

Small borosilicate test tube, 10X75 mm: Modified by addition of 

a bulge about 18 mm from the bottom of the tube; the bulge is 

made by heating the test tube over a bunsen burner flame and 

pressing from within with an opened paper clip having a 

small hook at one end (figure 8). 

Small dipper: Made to hold approximately 25 mg of powdered 

sample. Prepared by drilling a hole 3 mm in diameter and 

3 mm deep into the end of a 6 mm round brass bar. 

Test tube clamp. 

Desk lamp. 

Hotplate. 

Squeeze-type pipet: Suitable for transferring 0.1-0.2 ml of liquid. 

Bunsen burner. 

Wash bottle with water. 

Saturated mercuric chloride solution: Saturate 100 ml of water 

with HgCl2. 

Acidified mercuric ehloride solution: Transfer 20 ml of the saturated solution to a small beaker and add 20 ml of concentrated HCI. 

Standard rock powder containing approximately 0.1 percent C02: 

This laboratory uses a granite which contains 0.08 percent 

C02. 

PROCEDURE 

1. Place the beaker containing acidified mercuric chloride solution 

on a hotplate set to keep the solution nearly boiling. Allow 

enough time for this solution to get hot. 
60 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

2. Transfer approximately 25 mg of the standard powder to the 

modified test tube with a dipper. 

3. Add water to about 6 mm above the bulge. 

4. Bring the contents of the test tube c·autiously to the boiling 

point by passing the tube in and out of the Bunsen burner 

flame. 

5. Hold the tube vertical and tap it gently to displace bubbles 

trapped in the bulge, then tilt the tube about 45° keeping the 

bulge on the upper side. 

6. Use a squeeze pipet to squirt 0.1-0.2 ml of the hot acidified 

mercuric chloride into the tube. It is important, for best 

reproducibility, that both the sample solution and the acid 

be hot. 

7. Observe the accumulation of bubbles in the bulge after about 

10 sec. With a standard rock material containing about 0.08 

percent C02, the C02 bubble should occupy more than half 

the bulge. The amount noted serves as the 0.08-percent 

reference. 

8. Rinse out the contents of the tube. Drying is unnecessary. 

9. Repeat steps 2 through 7 with each sample, and note the 

amount of acoomula.ted gas !in the bulge relative to the 

amount produced by the standard. 

CALCULATIONS 

1. If a .sample yields approximately the sam.e volume of C02 as 

the standard, it is reported as having the same C02 content. 

If it is roughly half it would be called 0.04. Estimates rarely 

vary by more than ± 0.02 percent C02. 

2. If the volume exceeds the capacity of the bulge by more than 

a small amount, the carbon dioxide content is too high to be 

accurately determined by this method; such samples should 

be analyzed by other available procedures. Rougher estimates made using a dipper one-tenth the size of the one 

described, will yield results which can be in error by an 

order of magnitude. 

TECHNIQUE FOR DETERMINING CO, CONTENT ABOVE 0.1 PERCENT 

IN SILICATE ROCKS 

Sample powder, solid mercuric chloride, mercuric chloride solution, paraffin oil, and HCl are added to the middle range carbonate 

tube and the sample is decomposed by heating the lower part of 

the tube in a heating block that is maintained at 145° ±5° C. 

A 1-g portion of sample is used for samples in which the concentration of C02 ranges from 0 to about 1.5 percent and a 0.5-g 
DETERMINATIONS ON SEPARATE SAMPLES 61 

portion of sample is used for samples in the 1.5 to 3.0 percent 

range. 

REAGENTS 

HCl, 1+1. 

HgCl2 : Dry powdered reagent-grade HgCl2. 

HgCl2 solution, saturated: Prepare 500 mi. 

Paraffin oil: Heavy, white. 

C02 standard: A silicate rock sample (100 mesh), containing approximately 1 percent C02, in which the concentration of C02 

has been obtained by analysis of two 5-g portions using the 

evolution procedure described below for C02 in carbonate 

rocks. 

PROCEDURE 

1. Weigh 1.00 g of the C02 standard and transfer it by means of 

a dry funnel to the bottom of the middle-:rtange carbonate 

tube. 

2. Transfer about 1 g of HgCl2 powder, measured with a scoop, 

to the tube. 

3. Add 2 ml of the saturated HgCl2 solution and tap the tube to 

remove entrapped air bubbles. 

4. Add paraffin oil to the mark on the tube, tilt the tube so that 

the oil displaces the air in the side arm and then return the 

tube to a position such that the side arm is vertical. 

5. With the side arm vertical, add 2 rnl of 1+1 HCI. 

6. Place the tube in a clamp so that the side arm is vertical; the 

lower part of the tube is inserted into the heating block 

which is adjusted to 145°±5°C (fig. 16). Heat for 5 min. 

The aqueous phase should begin to boil atbout 2--3 min after 

insertion. 

7. Remove the tube from the heating block, ·allow cool tap water 

to flow down the outside of the side arm for 15 sec, and let 

the tube stand for about 1 min. 

8. Use a millimeter scale to measure the length of the gas column 

in the side arm and record the reading. 

9. Repeat steps 1 through 8 for each sample. 

10. Run a blank in the same manner. 

CALCULATIONS 

1. Calculate a factor after subtracting the blank from each reading. 
62 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

FIGURE l6.-C02 tube in heating block. 

Percent C02 in standard ----------------=factor. 

Gas-column length, in mm, for standard 

This factor is the calibration of the tube and need not be 

repeated. 

2. Calculate percent C02 in the sample: 

(Factor) X (gas-column length of sample) =percent C02 

C02 IN CARBONATE ROCKS BY EVOLUTION 

For samples in which the concentration of C02 exceeds 3 percent, C02 may be determined by an evolution-absorption technique 

in which the volume of C02 is calculated after acid evolution and 

subsequent absorption in a solution of KOH (Goldich and others, 

1959). An alternate titration procedure to be used with limestone 

and dolomite is described later. 
DETERMINATIONS ON SEPARATE SAMPLES 63 

The sample is decomposed by boiling with a small volume of 

HCI. The C02 that is evolved and the extraneous air in the system 

are measured in a gas buret, and the gas is then bubbled through 

a KOH solution several times to absorb the C02. Finally, the 

volume of the remaining air is measured in the gas buret. The 

difference between the original and the final gas volume represents 

the C02 in the sample. Standardization with a sample in which the 

concentration of C02 is known accurately eliminates the need for 

correction for temperature, barometric pressure, and vapor pressure. 

REAGENTS 

NaCl-HCl solution: Saturate a 1+3 HCl solution with NaCl and 

add a few milligrams of methyl red indicator. Prepare 2 1. 

Transfer to the HCl supply bottle (G) of the evolutionabsorption apparatus (fig. 10). 

KOH solution, 50 wt percent: Dissolve 2 kg of KOH pellets in 

4 1 of water. Fill the absorption pipet (C) and store the excess in a plastic bottle with a tight closure. 

Standard CaCOa : Dry reagent-grade CaCOa, low in alkalies. 

PROCEDURE 

1. Turn the stopcock on the buret (B) (fig. 10) and on the 

absorption pipet (C) so that the channel is open between 

them. Bring the level of the KOH in the absorption pipet 

to the check valve by raising or lowering leveling bulb (D). 

Close the stopcock on the absorption pipet. 

2. With the reaction flask removed, turn the stopcock on the 

buret so that the channel is open to the condenser and raise 

the level of the liquid in the buret almost to the stopcock by 

raising the leveling bulb. Close the stopcock on the buret. 

3. Transfer 0.2000 g of the standard CaCOa to the reaction flask 

and push the neck of the flask onto the stopper to which the 

condenser is attached. Make sure that the connection is 

gas-tight. 

4. Lower the leveling bulb until it is at the same level as the 

lower part of the buret, then turn the stopcock to the HCl 

supply so that the channel is open to the reaction flask. 

5. Allow about 10 ml of NaCl-HCl to flow from the supply bottle 

into the reaction flask, and then close the stopcock. 

6. Heat the reaction flask, gently at first to bring the solution to 

a boil. Boil for 3 min. 
64 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

7. Allow NaCl-HCl solution to flow from the supply bottle into 

the reaction flask until the system is filled just to the check 

valve, then close the stopcock. 

8. Hold the leveling bulb beside the cooling jacket so that the 

liquid levels in the buret and the leveling bulb are in the 

same plane. Read the buret and record the reading. 

9. Turn the stopcocks on the buret and on the ·absorption pipet 

so that the channel is open between the buret and the pipet, 

and bubble the gas through the KOH in the absorption 

pipet by raising the leveling bulb until the liquid in the 

buret is almost at the top of the buret. Lower the leveling 

bulb until the KOH in the pipet is about at the check valve. 

Pass the gas to the absorption pipet and back to the buret 

three more times. 

10. Finally, adjust the KOH level to the check valve, close the 

stopcocks on the absorption pipet ,and the buret, bring the 

levels of the liquid in the leveling bulh and the buret to the 

same plane, read the buret, and record the reading. 

11. Remove the reaction flask and rinse with distilled water the 

portions of the condenser and funnel that extend below the 

rubber stopper. 

12. Repeat steps 2 to 11 for each sample and again for the standard CaC03. 

CALCULATIONS 

1. Calculate the volumes of C02 evolved: 

Volume (step 8)-volume (step 10) =ml C02. 

2. Calculate a factor : 

Percent C02 in the standard 

Average volume of C02J in ml, from the standards 

3. Calculate percent C02 in the samples : 

factor. 

(Factor} X ( C02, in ml, from ,sample) =percent C02. 

C02 IN PHOSPHATE ROCKS 

For phosphate rocks in which the C02 concentration does not 

exceed 3 ·percent, the method described for silicate rocks is used. 

Where the concentration exceeds 3 percent, the evolution method 

is used. 

002 IN LIMESTONE AND DOLOMITE BY TITRATION 

An acid-base titration (Grimaldi, Shapiro, and Schnepfe, 1966) 

is used as an alternative for the determination of C02 at high 
DETERMINATIONS ON SEPARATE SAMPLES 65 

levels such as are found in carbonate rocks. This method is simpler 

than the evolution procedure and is particularly advantageous 

when large numbers of samples are to be run. Samples are decomposed overnight under mild conditions in a measured amount of 

standardized acid, thus minimizing difficulties arising from the 

solubility of some ~silicates. The excess acid is back-titrated the 

following day to a pH of about 4. Phosphate, when present, reacts 

as carbonate and can be corrected for, except when P 205 is much 

greater than C02 and the end point becomes obscure. In this case 

C02 is determined by the evolution procedure. 

REAGENTS 

HCl, 0.5 N: Prepare 4 I. 

NaOH solution, 0.35 N: Prepare 2 I. 

Bromphenol blue indicator: Prepare a 0.1 percent .weight/volume 

:aqueous solution from the water soluble bromphenol blue. 

CaCOa A.C.S. grade, to serve as a standard. 

PROCEDURE 

1. Arrange a series of 2·50-ml beakers; leave the first empty in 

order to serve as a blank, add to the second 0.5 g of GaCOa a.s 

'a standard, and to the others add 0.5 g of each sample. 

2. Use a pipet or a pi petting machine to add 25 ml of the 0.5 N 

HCl to each of the beakers, swirl to wet all of the samples, 

cover and allow to sit overnight. 

3. Transfer a few ~millilitres of the ~solution in the first beaker to 

a 30-ml beaker, add 0.5 ml of bromphenol blue indicator to 

the main portion and titflate rapidly with the NaOH solution 

to a blue end point. Return the previously removed small 

portion and again titrate more carefully to the end point. 

Record the volume of NaOH used. 

4. Repeat the titration with each of the beakers. 

CALCULATIONS 

1. Oalculate the acid-alkali ratio: 

25 

Volume NaOH for the blank 

2. Multiply each of the NaOH titration values by the acid...alkali 

ratio. 

3. Subtract each figure obtained in step 2 from 25.00. This gives 

the HCl in millilitres consumed by the carbonate. 
66 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

4. Obtain a factor based on the titration of the standard reagent 

GaCOa (this standard contains 44 percent C02): 

44 

HCl consumed by the standard 

factor. 

5. Multiply each sample value for HCl consumed, as obtained in 

step 3 by the factor to obtain percent C02. 

6. Where P 205 is .greater than a few tenths of a percent, correct 

for its presence by multiplying its value by 0.67 and subtracting this from the value obtained in step 5. 

FIN SILICATE, CARBONATE, AND PHOSPHATE ROCKS 

Fluorine is determined indirectly a:s Si in H2SiF a obtained by 

volatilization from a sample. The fluosilicic acid is subsequently 

converted to silicomolybdate which is then reduced to the more 

stable molybdenum blue. The initial separation is done in a test 

tube and requires 5-25 mg of a sample. Phosphate, which is often 

troublesome in fluoride determinations, does not interfere in this 

method (Shapiro, 1967b). 

REAGENTS 

Sodium molybdate: Dissolve 30 g Na2MoO. · 2H20 in 90 ml of 

water. Add 10 ml of 1+1 H2S04 just before the solution is 

needed, as the acidified solution is stable for only about 24 

hours. 

Oxalic acid: Dissolve 10 g H2C20 4 ·2H20 in 100 ml of water. 

Reducing reagent: Dissolve 30 g of sodium bisulphite (NaHSOa), 

1 g of sodium sulphite (Na2S0a), and 0.5 g of 1-amino-2-

naphthol-4-sulfonic acid in 200 ml of water. 

Sirupy phosphoric acid : Prepare from concentrated phosphoric 

acid by evaporating to fumes in a beaker. Allow to cool. Store 

in a gl•ass-stoppered bottle. 

Bureau of Standards standard sample No. 91, opal glass, 5.7 percent F: When 5.0 mg of standard is used .and 25.0 mg of 

sample is taken this standard is equivalent to 1.17 percent F. 

PROCEDURE 

1. Transfer one or more 5.0-mg portions of the standard and 

25.0-mg portions of each sam·ple to the bottom of a series 

of borosilicate test tubes (18X150 mm) with the aid of a 

long-·stem.med funnel. 

2. Add 3 drops of sirupy phosphoric acid to each tube and to an 

empty tube which serves as the reference blank. 
DETERMINATIONS ON SEPARATE SAMPLES 67 

3. Stopper each tube with a one-hole No. 1 rubber stopper bearing a short piece of thick-walled glass capillary tubing. This 

reduces but does not prevent the free flow of air. 

4. One at a time, wrap a 25 x75-mm slip of wet filter paper 

around the upper part of the tube, then carefully heat the 

bottom of the tube over a low flame. First hold the tube at 

an angle of about 30° from the horizontal, agitate the tube 

so as to wet the entire sample, and move the tube in and 

out of the flame to control the heating rate. After about 

1 min, a coating of silica will have formed. Increase the 

flame temperature, hold the tube horizontally, and heat the 

lower third of the test tube strongly for 30 sec. 

5. Place the test tubes on a sheet of asbestos. 

6. Heat each of the other test tubes in the same manner. 

7. Allow the tubes to cool for at least 5 min but not more than 

45 min before taking them to the next step. 

8. While the tubes are cooling, pipet 50 ml of 5+995 HCl into 

each of a suitable number of 150-ml beakers. Bend a piece 

of glass tubing, 3-mm OD and 46 em long, into a U shape 

with one arm 14 em long and the other 30 em long. The 

outer edge of the two arms should be 12 mm or less apart. 

9. Transfer the 50-ml HCl solution from one of the beakers to a 

100-ml glass graduate having a 25-m·m ID. 

10. Remove the stopper from the first test tube •and insert the 

14-cm side of the U-tube into the inverted test tube. Using 

the long side of the U as a handle, lower the test tube to the 

bottom of the graduate. The U-tube allows air to leave the 

test tube so that its upper part can be washed without wetting the part of the tube containing the s•ample; the 50-ml 

quantity limits the height to which the washing can go. 

Raise and lower the test tube five times. 

11. Transfer the contents of the graduate back to the beaker. 

12. Repeat steps 9-11 with each test tube. 

13. Use a squeeze pipet to transfer 2 ml of the molybdate reagent 

to each beaker while stirring. Allow it to sit 10 min. 

14. Add 5 ml of the oxalic acid solution to each beaker while 

stirring. Allow it to sit at least 1 min. 

15. Add 2 ml of the reducing reagent. Allow it to sit at least 10 

min. 

16. Determine the percent transmission for each solution at 640 

nm, using the blank as a reference. 
68 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

CALCULATIONS 

1. Convert the readings for percent transmission to absorbance 

using table 1. 

2. Compute the factor: 

Percent F in the standard factor. 

Absorbance of the standard solution 

3. Calculate percent F : 

(Absorbance of the sample) X (factor) =percent F 

If the sample contains more than 1.2 percent F the procedure 

should be repeated using a 5.0-mg sample. 

S IN SH.ICATE, CARBONATE, AND PHOSPHATE ROCKS 

Sulfur occurs in most ~silicate rocks at levels rarely exceeding a 

few tenths of a percent; several percent may be present in carbonate and phosphate rocks. In the method described (Shapiro, 

1973) sulfur is determined turbidimetrically as barium sulfate 

after attack with aqua regia. The turbidity is formed in an ammoniacal solution from which R20a has been centrifuged. Reproducibility of turbidity is obtained by adding BaCl2 as a solid rather 

than ·as a solution. Sulfur is determined in the 0.01-4 percent 

range within ± 0.02 percent absolute, or approximately 5 percent 

of the 'amount present, whichever i·s larger. A 100-mg sample is 

used for sulfur concentrations as high :as 0.50 percent. The sample 

is rerun using a 20-mg sample for higher levels. To differentiate 

between sulfate and sulfide the determination is repeated using 

HCl in place of aqua regia. A determination requires about 5 min. 

REAGENTS 

Barium chloride: Crush the crystal·s in a mortar and pestle. 

HNOa, concentrated. 

HCl, concentrated. 

Dilute HNOa. Prepare a 1+3 dilution with water. 

NH40H: Prepare a 1+1 dilution with water. 

Standard sulfate solution: Dissolve 177.5 mg N~S04 in 250 ml of 

water; 1 ml contains 0.16 mg S. 

Calcium chloride solution: Dissolve 5 g CaCl2 · 2H20 in 100 ml 

H20. 

PROCEDURE 

1. Arrange a series of test tubes; add nothing to the first in 

order to have it serve as a blank, add 2 ml of the standard 

sulfate solution to the second, and 100 mg of eac:h sample to 

the additional tubes. 
DETERMINATIONS ON SEPARATE SAMPLES 69 

2. Use a squeeze pipet to add 0.2 rnl of concentrated HNOs to 

each test tube. 

3. Pass each tube containing a santple over a flame for a few 

seconds to bring the solutions to a boil. 

4. Add 0.4 ml of concentrated HCl to each tube. 

5. Pass each tube containing a santple over a flame for a few 

seconds to bring the solutions to a boil. 

6. Add 1 ml of the calcium chloride solution to each tube. 

7. Add 5 ml of water to the tube containing the first sample. 

Bring to a boil over a burner and maintain at or near boiling for 30 sec. 

8. Add 2 ml of 1+1 NHo~OH using a squeeze pipet. Agitate 

briefly. 

9. Add 5 ml of water and agitate briefly. 

10. Repeat steps 7-9 with each tube containing a sample. 

11. Add 2 ml of 1+1 NH4 0H to the blank and standard tubes. 

12. Add 10 ml of water to the blank tube and 8 ml to the standard 

tube. 

13. Centrifuge all of the tubes containing ·samples at about 1,200 

rpm for 1.5 min. 

14. To a second set of dry test tube:~ add 0.1 ml of the standard 

sulfate ~solution to serve as a s:pike, and then transfer 5 ml 

of the supernatant liquids from the first set of test tubes 

using a squeeze pipet. 

15. Use a fresh test tube containing water to set the spectrophotometer to read 100 percent transmission at 650 min. 

Record the readings for all of the second set of test tubes. 

16. Use a dipper to transfer approxim·ately 30 mg of powdered 

BaCl2 to each of the first 10 test tubes without agitation. 

Set the timer for 2 min, then agitate each tube for a few 

seconds to dissolve the BaCl2. It is not necessary to dissolve 

the BaCl2 fully. 

17. When the 2-min time period is completed, read the percent 

transmission for each tube. 

18. Repeat steps 15-17 with each set of 10 tubes or less. 

CALCULATIONS 

1. Convert the readings for percent transmission made for the 

values obtained in step 15 and step 17 to absorbance using 

table 1. 

2. Subtract the absorbance obtained from step 15 from that obtained in step 17. This represents absorbance of BaSO" 

produced. Subtract the blank value from all of these values. 
70 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

3. Compute the factor: 

0.32 

Absorbance of standard solution 

4. Calculate percent S: 

factor. 

(Absorbance of the sample) X (factor) =percentS. 

If the S content is higher than 0.5 percent, the procedure should 

be repeated with a 20-mg sample and a set of ~standards using 0, 1, 

2, 3, 4, and 5 ml of the standard sulfate solution. The S content 

can be determined up to 4 percent. At higher levels of S, s·maller 

samples are taken. 

SULFATES IN Sll...ICATE, CARBONATE, AND PHOSPHATE ROCKS 

When the level of S present justifies the need, discrimination 

between sulfide and sulfate is made using the procedure for S with 

a few modific·ations. No HN03 is used in the attack on the sample 

but 0.4 ml of 1+3 HNOa i.s added along with the spike. The addition of HNOa provides a curve with a steeper slope than is obtained 

in its absence. 

Results obtained using the HCl attack represent the acidsoluble sulfate present. It is calculated as SOa by multiplying the 

value calculated as S hy 2.5. The content of sulfide is obtained by 

subtracting the value of S after HCl attack from the value obtained with the Aqua Regia attack. 

The above procedure fails when alunite is present in substantial 

amounts. Samples containing alunite can he analyzed using the 

following modification of the above scheme: 

Weigh 10 mg of each sam.ple into a series of test tubes. Add 3 

drops of 30 percent NaOH to each from a plastic dropping bottle. 

Grasp each ~test tube at the top between thumb and forefinger and 

flick with a snap of the wrist to ensure a thorough wetting of the 

sample. Heat each tube to dryness, then to a dull red glow for 

about 15 sec. Allow to cool, then proceed as described in the earlier 

procedure adding 0.2 ml HNO, and so forth, except that no CaCl2 

is added. The N·aOH heating step is also carried out with three 

additional test tubes, one to serve as a blank and two as standards. 

After the NH40H has been added, as in the case of the samples, 

add 2 ml of the Na2S04 standard solution to one of the tubes prepared as a standard, and add 3 ml to the other standard tube; then 

add 8 ml and 7 ml of water, respectively, so that the total volumes 

are the rsame as for the sa!nples. 

At the conclusion of the analysis, the tubes used to heat the 

samples with NaOH must be discarded. 
DETERMINATIONS ON SEPARATE SAMPLES 71 

POWDER DENSITY 

Experiments indicate that finely crushed rook ( < 80 mesh) in 

a heavy liquid, if not agitated, sinks or floats as large aggregates 

of the constituent minerals r.ather than as discrete grains (Shapiro, 1969). The average density o:f the clumped crushed material 

closely approximates that of the rock powder ·and ,may be determined by a simple sink-float technique. Small samples of powdered 

rock are centrifuged in heavy liquids in a sequence designed to 

bracket their densities within narrow limits. Only about 100 mg 

ctf sample is required to obtain a density determination within 

± 0.04 g/cm3 • In some cas·es, where minerals of widely divergent 

density are present in substantial a·mounts, the procedure may not 

lead to useable results. In such cases one must use the standard 

pycnometer techniques. 

HEAVY LIQUIDS 

Prepare eight heavy-liquid mixtures from pure carbon tetrachloride (density 1.59 g/cm3 ), bromoform (density 2.89 g/cm3 ), 

and methylene iodide (density 3.32 g/cm3 ) according to the proportions given in table 2. Those :mixtures containing methylene 

iodide should be stored in amber bottles containing a small piece 

of copper wire to minimize light induced decomposition, which 

releases iodine and darkens the liquid. 

Liquid 

designation 

TABLE 2.-H eavy-liquid mixtures 

[Quantities in millilitres 1 

Carbon 

tetrachloride 

(density 

1.59 g/cm 3) 

Bromoform 

(density 

2.89 gjcm 3 ) 

Methylene 

iodide 

(density 

3.32 gjcm 3 ) 

Resultant 

density 

(gjcm3 ) 

A ------------ 25 ------------ 22.5---------------------------- 2.20 

B ------------ 20 ------------ 29 ·--------------------------- 2.36 

c ------------ 14.3 ____________ 35.7.___________________________ 2.52 

D _ _ _ _ _ _ ___ __ _ 8.3_ _ __ _ _ _ _ _ __ _ 43.7 ·- ____________ -------------- 2.68 

E ------------ 2 ------------ 50 ·--------------------------- 2.84 

F ---------------------------- 33~----------- 11.5 ____________ 3.00 

G ----------------------------- 20 ·----------- 33.3 ____________ 3.16 

H -------------------------------------------- 50 ------------ ~32 

EQUIPMENT 

Test tubes, 10X75 mm; several dozen are required. 

Test tube rack. 

Small dipper capable of transferring 30-50 mg of sample powder. 

Squeeze pipet suitable for transferring 0.3 ml of liquid. 

Centrifuge. 
72 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

PROCEDURE 

This procedure is described for one sample, but many can be · 

handled simultaneously. 

1. Transfer approximately 40 mg of sample powder ( 80 mesh 

or finer) by dipper to two test tubes, add 0.3 rn1 of liquid D 

to one, but do net agitate. 

2. Centrifuge for 1 min and examine. 

3. Classify the results into one of four categories : 

(a) The entire sample floats 

(b) The entire sample sinks 

(c) The sample splits to form one layer on top and on~ 

layer on bottom. 

(d) The sample is dispersed throughout the liquid, in which 

oose it has the same density as liquid D. 

In some instances a sm!all part of the sample may go in the 

direction opposite to that of the bulk of the sample. If the 

minor part is roughly 5 percent or less, it may be ignored 

when categorizing the results. 

4. If, in step 3, the sample went to the top of liquid D, add 0.3 ml 

of liquid C to the second tube; if it sank, add 0.3 rn1 of liquid 

E instead. If the sample split into two discrete layers in 

liquid D, add liquid C to the second tube; also add liquid E 

to another dipper of the sample in a third tube. Centrifuge 

the tubes for 1 min and categorize as in step 3. 

5. If required, repeat the procedure with additional dippers of 

sample until two adjoining liquids in table 2 are found in 

which the sample floats in one and sinks in the next. 

6. After the two adjoining liquids are located in step 5, pour the 

contents of these two tubes back and forth to mix them, 

producing a liquid of intermediate density, for example, 

DE. Centrifuge for 1 min and categorize. 

CALCULATIONS 

1. The sample has the density of the liquid (table 2) in which it 

either dispersed or split, a ·substantial part going both up 

and down. For example, if the sample split or was dispersed 

in liquid D and it floated in the next heavier liquid E and it 

also sank in the next lighter liquid C, the sample has the 

density of the liquid (D) in which it split or dispersed, 2.68 

in this case. 

2. If the sample s·ank in one liquid and floated in the adjoining 

liquid the powder density lies between the densities of these 

two liquids. If it was then split in the mixture of these two 
DETERMINATIONS ON SEPARATE SAMPLES 73 

liquids it corresponds to the density of the average of their 

densities, for example, for CD the density would be 2.60. If 

the sample sank in the mixture CD, it is considered to be 

heavier by 0.04 than this average, that is 2.64; if it floated 

in CD it is taken to be less than this average by 0.04, that 

is, 2.56. 

3. If the sa·mple forms two discrete layers in two adjoining 

liquids, as listed in table 2, or if it sinks in the heaviest liquid 

available, the density obviously cannot be deter·mined by 

this procedure. 

BULK DENSITY 

Bulk density is usually determined on a 10- to 40-g rock fragment. The procedure is a minor v·ariation of what might be c·alled 

the eureka method of Archimedes, which is based on buoyancy in 

water. Generally, in such procedures the rock is weighed when dry 

and then again when submerged in water. Most rocks absorb 

water into pores and crevices and, on being submerged, show a 

constantly changing weight. It is common procedure to coat the 

rock fragment with wax to prevent this from occurring; however, 

the process of coating and removal of the wax is time consuming. 

In the procedure described, the rock is saturated with water and 

weighed. A correction is made to take the absorbed water into 

account. When the water-saturated rock is weighed after its 

removal from water, it loses weight at the rate of several milligrams per minute. A weighing can easHy be made however in 

1-2 min and a loss of 10 mg on a 1 O:..g sample represents an error 

of 0.1 percent relative in the density. If the density is obtained to 

within 0.5 percent relative, that is, with ± 0.015 absolute for a 

rock having a density of 3.00, it is satisf·actory for most purposes. 

APPARATUS 

600 ml beaker filled with water. 

Metal bridge consisting of a piece of sheet metal 62 mm wide, bent 

to form a bridge over the balance pan to support the beaker 

so that a sample suspended from the balance hangs submerged 

in the beaker of water. 

Thin piece of wire long enough to hook onto the ·ba.l·ance beam and 

to support the piece of rock when fully submerged. 

PROCEDURE 

1. Submerge each rock sample in a beaker of water for about 

1 hr or longer. 
74 ANALYSIS OF SILICATE, CARBONATE, PHOSPHATE ROCKS 

2. One at a time, hook the thin wire around each rock and hang 

it from the balance beam 'SO that it is fully submerged in the 

beaker of water. When the balance beam is released the 

balance should be able to come to equilibrium. If the rock is 

still absorbing water at a rate in excess of about 2 mg/min, 

the rock ·should be replaced in the original beaker of water 

and allowed additional time to absorb water. Such a rock is 

very porous. If the balance is at or near equilibrium, the 

most common condition, record the weight. 

3. Carefully remove the rock from the water and the wire from 

the rock, so as not to fragment it. Cover the rock com.pletely 

with a dry tissue. Gently press the tissue over the entire 

surface of the rock. Remove the tissue and set the rock near 

the balance. 

4. Remove the beaker ·and bridge from the balance case and place 

the rock on the balance pan. Weigh the wet rock as rapidly 

as possible recording the maximum weight. The rock will 

lose weight constantly at the r·ate of several miligrams or 

even more per minute, depending on its porosity. The time 

between the removal of the tissue and the observed weight 

should be no more than about 1-2 min. 

5. Air-dry the rock overnight at room temperature. 

6. Weigh the dry rock. 

7. Obtain the wire weight when suspended from the balance and 

hanging in the water as when in use. 

CALCULATIONS 

Use the following equation to obtain bulk density : 

Dry weight (after setting overnight) bulk density. (Wet weight) - (underwater weight+ wire weight) 

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