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UNITED STATES DEPARTMENT OF THE INTERIOR

SOME STANDARD

THERMAL DEHYDRATION CURVES

OF MINERALS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 197-E

UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. G. Mendenhall, Director

Professional Paper 197-E

SOME STANDARD THERMAL DEHYDRATION 

CURVES OF MINERALS

BY 

P. G. NUTTING

Shorter contributions to general geology, 1941-42

(Pages 197-217)

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1943

For sate by the Superintendent of Documents, U. S. Government Printing Office 

Washington, D, C. - Price 5 cents
CONTENTS

	Page 

Abstract...---_-__-------__-_-------_---_-__--___-_____-_____-_______-_-___-_---___-_-__--_-__-__.____ 197

Introduction __________________________________________________________________________________________________ 197

Thermal dehydration determinations._-_____.__________________.--__-_-_._-_-______--_-_--___--__-_-_--_---_-_--_ 198

Materials showing slight curvature variation. _________________________________________________________________ 200

Hydrous oxides and zeolites.________________________________________________________________________________ 202

Hydrous oxides and related silicates___-_-__-___-____._______________-___-_-_-__----_-._____-__--___-_-____-___ 204

Kaolin group..___._-___________-___.--_-_______.____._.____-___--__-_-___-____-__--____--__-_-___--_--.___ 206

Montmorillonites-_________________________________________________________________________________________ ' 208

Montmorillonite group _.--___-_-__----_-__-______-_______-__--__-__-_-___-_--_--_--_-__-_---_---_-------__ 210

Chlorites and related silicates________________________________________________________________________________ 212

Micas._________--_____-_-____________________________________-__-_-________--__--_-_____-_-______________ 214

Carbonates:____-______-________-_--_-_-_--____________-___-__-__-_-_--__-----_-_-------_-----_-_---------- 216

ILLUSTRATIONS

FIGURE 14. Curves of slight curvature variation ___________________________________________________________________ 200

15. Curves for hydrous oxides and zeolites.________________________________________________________________ 202

16. Curves for hydrous oxides and related silicates____________________________________________ .._______-____ 204

17. Curves for the kaolin group._________________________________________________________________________ 206

18. Curves for montmorillonites______________________________ JX.___________________________________________ 208

19. Curves for the montmorillonite group__________________________________________________________________ 210

20. Curves for chlorites and related silicates_______________________________________________________________ 212

21. Curves for micas._____________-__._!___.______________-__-___-___________-_-_____-_--___-_-__-_-___- 214

22. Curves for carbonates____--_-___-_-______________-__.--_-_-__-____-________-______--__-_-___-_--___ 216

n
SOME STANDARD THERMAL DEHYDRATION CURVES OF MINERALS

By P. G. NUTTING

ABSTRACT

The curve representing the change in weight of a mineral as its 

temperature is raised is a well-recognized characteristic. Many 

individual curves have been obtained, but no sets of standard 

curves for reference have been published. From about 250 such 

curves obtained by the writer in the past 10 years, 73 of the more 

significant, arranged in 9 groups, are here presented.

Weights were obtained on a 1- or 2-gram sample, crushed to 

150 mesh (0.1 millimeter), in a platinum crucible suspended in a 

vertical electric furnace from one arm of a balance mounted over 

the furnace. After reaching constant weight at one temperature, 

the temperature was advanced about 50° C. The curves refer 

to average room humidity, about 0.6 saturation—that is, to a 

vapor pressure of about 15 millimeters.

Some classification according to material has been attempted, 

but this is incidental to the grouping of curves according to their 

form. The groups of curves are as follows: (1) Curves lacking 

in decided shoulders and referring to such substances as silica 

gel, powdered quartz, volcanic ash, diatoms, and montmorillonite residue (fig. 14). (2) Hydrous oxides, such as opal, allophane, hydrated alumina, and sepiolite, and three zeolites (fig. 

15). (3) Hydrous oxides and related silicates, including gibbsite, diaspore, limonite, and geothite (fig. 16). (4) The kaolin 

group of minerals—anauxite, kaolinite, and dickite (fig. 17). 

(5) Normal pure unweathered montmorillonite, the same material leached and oxidized by weathering, bentonites high in 

alkali, and fuller's earth (fig. 18). (6) The montmorillonite 

group, with halloysite, illite, and saponite (fig. 19). (7) Chlorites and related silicates, including serpentine, picrolite, clinochlore, penninite, paligorskite, vermiculite, and glauconite (fig. 

20). (8) The micas and other minerals characterized by curves 

with rounded high-temperature shoulders, such as pyrophyllite, 

talc, tremolite, and beryl (fig. 21). (9) Five carbonates and 

hydrous carbonates (fig. 22).

Significant features of all these curves are discussed in the text, 

and the numerical data read from the curves are tabulated.

INTRODUCTION

The determination of water in minerals at two different temperatures is routine in chemical analysis. For 

many substances the loss in weight on heating to 110° 

C. is the free water, and the loss between 110° C. and 

ignition temperatures (800°-1000° C.) is the essential 

or crystalline water. Some minerals contain water 

in at least three different forms, and these forms can 

be distinguished and evaluated only when weights are 

determined at a whole series of temperatures so that a 

complete curve representing loss of water may be 

drawn through them. The clay minerals, zeolites, 

and hydrous oxides are conspicuous examples. With 

a series of such curves at hand for reference, the proper 

temperatures for water determination- may often be

chosen. The weight-temperature curve of an unknown 

mineral sometimes serves to identify it when chemical 

and petrographic analyses cannot. The curve for a 

mixture of minerals may help to identify its constituents. With 250 such curves accumulated by the 

writer during the past dozen years, it seems desirab1 ^ 

to select and publish the more significant ones to serve 

as the nucleus of a more comprehensive collection later 

to be available for reference. Many similar observations have been published, but no classified collection 

has been attempted.

In the lower range of temperature, weight curves 

vary with the humidity, an effect which vanishes above 

about 160° C. Determining weight-temperature 

curves (at constant humidity) and weight-humidity 

curves (at constant temperature) below 160° C. 

requires special and much more complicated apparatus 

than that used in this work. The curves here presented 

are for an average room humidity of about 60 percent, 

obtained by heating the specimen in a small open plat:- 

num crucible in an electric furnace. Temperatures 

are read with a platinum, platinum-rhodium thermocouple projecting into the furnace. There may l*j 

some uncertainty in the details of curves that ha^v e 

abrupt breaks downward. To obtain such details 

with a greater degree of accuracy would require temperature control and measurement to within 2°, but 

ordinarily the furnace temperature is sufficiently 

constant.

To obtain a complete dehydration curve requires 

weighings at 10 to 20 points not more than 50° apart. 

Instead of removing the crucible and specimen from 

the furnace, cooling in a desiccator, and weighing, it 

seemed advisable to suspend the crucible permanently 

in the furnace and to make weighings about twice a day 

on a balance mounted over the furnace. The specimen 

is not disturbed during the whole run. The correction 

for air density is roughly 4 milligrams per gram fc Tt 

800° C. A period of about 2 weeks is required to 

complete the average curve, but the actual working 

time required is only 5 minutes twice a day to keep two 

furnaces in operation. Very uniform temperatures 

throughout the furnace are obtained by using a wellfitted covering, with only a small hole for the suspension 

wire. Fairly constant furnace temperatures are obtained on the regular power line by using lamp rhec197
198 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42

stats as ballast, the lamps being replaced by tubes 

containing iron wire in hydrogen, such as are now hi 

commercial use for radio work. The large temperature coefficient of resistance of these tubes so reduces 

the effects of line voltage fluctuations that the furnace 

temperatures seldom vary as much as 2° C.

The sample is ground just fine enough to pass a 150- 

mesh (0.1-mm.) sieve. Coarser grains are slow hi 

reaching constant weight, and much finer material fails 

to give a true curve on account of the increased proportion of broken bonds. Samples fresh from the ground 

are selected and dried just sufficiently to permit grinding. Prolonged room drying, storage in a saturated 

atmosphere, dispersion in water, etc., profoundly affect 

some clay and soil curves below 300° C.; dispersion in 

water or very fine grinding removes the high-temperature shoulder from some clay curves. The effects of 

electrodialysis and of treatment with water, acids, and 

alkalies is a, field of research in itself.

Most clays and soils and many minerals, or at least 

their dehydration products, change little hi weight in 

the range 700°-900° C. This constant weight is 

therefore taken as a base weight rather than the weight 

at 110° C. or at room temperature, where the weights of 

most clays vary widely with both humidity and temperature. Above 900° C. sintering is common and 

reactions may occur. In these curves interest centers 

on the shoulder usually identified with the loss of hydroxyl and of lattice structure and on the sloping portion 

leading up to it, where the slope is usually related to the 

loss of water that was firmly adsorbed as ions.

Oxidation of iron from the ferrous to ferric condition 

(usually at 200°-300° C.) involves a small- negative

correction. Organic matter is oxidized at a low temperature. The decomposition of carbonates must of 

course be allowed for, if they are present. £o far as 

known, most of the specimens studied were free from 

carbonates, but weight-temperature curves for a few 

known carbonate minerals are included in the last 

group (fig. 22) for comparison and reference. No 

extensive study of the thermal decomposition of carbonates has been attempted, as this field has been largely 

covered by other investigators.

THERMAL DEHYDRATION DETERMINATIONS

A number of representative weight-temperature 

determinations are reproduced in the accompanying 

diagrams (figs. 14-22). These are discussed in the 

text, and the numerical data read from the curves are 

given in tables. Some classification according to 

material has been attempted, but this is incidental to 

the grouping of curves according to their form.

Shoulders on a thermal decomposition curve are caused 

by rapid variation (with temperature) of the energy of 

association of some volatile constituents with the residual material. Conversely, the absence of such shoulders 

means either no variation hi energy or variation 

changing only slowly. No mineral has be^n found 

whose thermal dehydration curve shows constant energy 

of association from 20° to 800° C., but many such 

curves are found to consist of only three straight lines 

when log (water) is plotted against the reciprocal of 

the absolute temperature. The slope of the curve thus 

plotted gives the energy of association in calories per 

mole divided by the constant j?= 1,9864,
THERMAL DEHYDRATION DETERMINATIONS

199
200 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

MATERIALS SHOWING SLIGHT CURVATURE VARIATION

IOO C. 2OO 30O tOO 5OO 6OO TOO BOO

TEMPERATURE 

FIGURE 14.—Curves of slight curvature variation.

Q, Powdered quartz (X10).

E, Curve of constant energy.

S, Amorphous silica.

A, Volcanic ash.

M, Nearly amorphous material having the composition of montmorillonite (half scale).

R, Isotropic silica residue remaining after extraction of bases from a pure bentonite.

D, Diatoms.

L, Synthetic magnesium silicate "Magnesol" (half scale).

Curve Q: The smoothest curves found are those given 

by finely powdered anhydrous crystalline minerals 

(quartz, feldspars). Here there is an average adsorption of H and OH on broken surfaces or chemical bonds 

of a wide variety. Curve Q, exaggerated 10 times, is 

for quartz ground to pass a 300-mesh sieve (0.05 mm. 

opening) such that the specific surface is about 560 sq. 

cm. per gram. Water decreases from 0.5 percent at 

80° C. to 0.07 percent at 600° C. Energy of association 

increases steadily, being 38 calories per gram of water 

at 75°, 115 calories at 300°, and 730 calories at 650° C. 

After having been heated to 800° C. the adsorbing power 

of powdered quartz is lost. The adsorption of air, 

carbon dioxide, etc., is probably negligible compared 

with that of water. This curve is thought to show the 

effect of deadsorption of ions from broken bonds. 

Curves for other anhydrous crystals are similar.

Curve E: The variation of weight with temperature 

when the energy of association is 100 calories per gram 

(1,800 cal./mole) through the whole range of temperature from 30° to 800° C. is indicated by curve E. It is 

computed from the equation

where C=q/R and log D is an arbitrary integration 

constant here chosen so that W=l at 600C C. By 

comparing the slope of another curve with that of the 

E curve at the same temperature, the energy of association (heat of vaporization) may be estimated.

Curve 8: Dried silica hydrogel Si (OH) n, obtained by 

evaporation from repeated solution of silica gel in 

distilled water (two liters will dissolve nearly r, gram at 

90° in 24 hours). The dehydration curve is r- straight 

line between 170° and 480° C. At about 300° C. the 

water energy is nearly the same as that of T^ater adsorbed on quartz powder, about 100 cal./gram. In the 

range 50°-100° C., water is more firmly held on the gel 

than on the quartz, as though a hydrate were attempting 

to form; in the higher range, 200°-700° C., the curves 

run nearly parallel. Commercial silica gel and gel 

freshly precipitated from sodium silicate give curves 

that are similar but wavy, probably owing to impurities.

Curve A: A typical volcanic ash from west Texas. 

The amorphous glasses that are the chief constituent of 

volcanic ash might be expected to give smooth, featureless curves, but few do. Even pyrex glass gives a curve 

having a decided shoulder. The ash curve shown is the 

smoothest of many obtained. There is a pronounced
STAlSfDARt) THERMAL t>EfiYt>RATlON CURVES Of MINERALS

hydrate shoulder at low temperatures, which has disappeared at 160° C. Comparatively low-energy water 

is driven off at higher temperatures. Most ashes and 

tuffs show traces of the montmorillonite shoulder at 

500° C., owing to its incipient formation on shard 

surfaces.

Curve M: In strong acids montmorillonite gives up 

strong bases but essentially no silica; in pure water it 

gives up silica but only a small proportion of bases. 

However, in acid solutions of about 0.2 percent concentration bases and silica are removed in the original proportion and may be recovered by evaporation and 

washed free from soluble salts. No crystalline form is 

evident in the residue under a microscope. M is the 

dehydration curve of such a preparation, plotted to 

half the original scale. Slight shoulders at 490° and 

670° C. correspond with those of montmorillonite and 

saponite.

Curve R: The extraction of the bases from montmorillonite with 20 percent acid leaves an isotropic 

residue usually over 99 percent silica. It is of interest 

to compare this residue with silica gel. R is for the

silica residue obtained from a very pure bentonite by 

twice extracting it for 24 hours with 20 percent hydrochloric acid. The residue contains but 0.15 percent 

A12O3. Its dehydration curve is similar to that for pure 

silica hydrogel (fig. 14, S), but the residual shoulder at 

400° C. is still prominent.

Curve D: Diatoms from Ontario, eastern Oregon. 

This curve is for a very pure white diatomite containing 

no visible clay material and consisting of only two 

species of Mdosira. The sample used was freed from 

finer fragments and coarser lumps by washing, settling, 

decanting, and filtering. A12O3 amounted to 2.3 percent, grain density 1.688. The dehydration curve is 

very similar to that for pure silica hydrogel (fig. 14, S), 

being straight in the central part and parallel throughout but slightly higher. The high energy of low-temperature water is suggestive of a hydrate.

Curve L: A commercial synthetic magnesium silicate 

called "Magnesol," extensively used as a bleaching 

agent for certain kinds of oil. It has a remarkably long 

straight central portion and no shoulders. It is plotted 

to half the original scale.

.Weight ratios for materials showing slight variations at different temperatures

Temperature (°C.)

50

100

150

200

300

400

500

600

700

Q

1. 00544

1. 00468

1. 00402

1. 00345

1. 00251

1. 00180

1. 00120

1. 00072

1. 00032

E

1. 0540

1. 0402

1. 0316

1. 0251

1. 0174

1. 0139

1. 0117

1.0000

.9982

S

1. 0560

1.0415

1. 0341

1. 0297

1. 0215

1. 0135

1. 0058

1.0021

1. 0006

A

1. 0408

1. 0375

1.0290

1. 0161

1. 0077

1. 0039

1. 0020

1.0009

1.0003

M

1. 2020

1. 1666

1. 1356

1.1100

1. 0730

1. 0484

1. 0260

1. 0160

1. 0080

R

1. 0832

1. 0458

1. 0388

1. 0333

1. 0283

1. 0242

1. 0122

1.0076

1.0045

D

1. 0825

1.0475

1. 0406

1. 0356

1. 0272

1. 0192

1. 0110

1. 0050

1. 0021

L

1. 1690

1. 1332

1. 1150

1. 1024

1. 0816

1. 0620

1. 0420

1. 0220

1.0116
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

HYDROUS OXIDES AND ZEOLITES

« 140 

111

TV \

3OO 400 500

TEMPERATURE

FIGUEE 15.—Curves for hydrous oxides and zeolites.

Ai and At, Allophanes (half scale).

Hi and Ha, Aluminum hydroxide (half scale).

O, Mexican opal.

S, Sepiolite.

T, Ptilolite.

C, Chabazite (half scale).

P, PhiUipsite.

Curves A! and A2 : Allophanes from Kentucky (A2 

from Morehead). Both curves half scale. Both samples are brownish transparent glasses, so dry that fragments rattle. The curve AI is devoid of shoulders and 

probably represents finer-grained material than A2) which 

carries twice as much water and has a shoulder corresponding to saponite. A sample from Liege, Belgium, has 

the high water of A2 but only a slight shoulder at 550° C. 

Despite its high affinity for water, allophane shows no 

tendency to form hydrates, nor is it at all hygroscopic. 

An isotropic transparent mineral from Iowa, apparently 

allophane, was found by thermal analysis to be halloysite, composed of crystals too small to diffract light and 

lacking definite orientation.

Curves HI and H2 : Aluminum hydroxide. Both 

curves half scale. Alumina precipitated in the ordinary way from a dilute solution gives a curve with a 

shoulder near 350° C., like HI. By using strong solutions it is possible to secure an amorphous hydrate, 

represented by H2 , containing 40 percent water, roomdry, and showing no trace of a shoulder. Curve H2 

was run on such a sample 3 months old, which 

showed no tendency to revert to gibbsite (fig. 16, G).

Curve 0: Mexican opal. From United States Fational 

Museum collection. The sample was an old cut stone, 

highly transparent but with a brownish tint. A12O3 

was 0.3 percent. It has a low-temperature shoulder 

like that of the zeolites (fig. 15, T, G, P) and Texas 

volcanic ash (fig. ^14, A), rather than that of silica gel 

(fig. 14, $), but with twice as much water. A stable 

hydrate under room conditions, the hydrate was 

destroyed at 160° C.

Curve S: Sepiolite, from Asia Minor. From United 

States National Museum collection. A fine-grained 

gray claylike material, probably a mixture of hydrous 

minerals. Sepiolite curves show no definite group 

characteristics. Individual curves differ considerably 

from each other and shade off into those of the saponites. Indications are that a pure mineral wouH give a 

curve with no shoulders. The two slight shoulders 

indicate traces of bauxite and saponite.

Curve T: Ptilolite, from Marysvale, Utah. A very 

pure crystalline sample supplied by W. T. Schaller. 

Freed from calcite by acid. The curve shows the lowtemperature shoulder of the zeolites, analcime, opal 

and some ashes, but no trace of a montmorillonite,
SOME STANDARD THERMAL DEHYDRATION CTJRtES OF MINERALS 203

kaolinite, or saponite shoulder, and is a straight line 

between 370° and 600° C. Energy of water association is high at both high and low temperatures.

Curve C: Chabazite, from Oregon. From United 

States National Museum collection. A pure crystalline material. This is a typical zeolite curve, with lowtemperature shoulder, similar to that for ptilolite but 

higher at low temperatures. Plotted half scale.

Curve P: Phillipsite, from Victoria, Australia. Supplied by United States National Museum. The sample

consists of tiny crystals picked from the inner surface 

of rock cavities. Characterized Jby low-temperature 

shoulders at 20° and 140° C., a straight-line portion 

between 150° and 350° C., and a pronounced shoulder 

at 630° C. corresponding to a HI2O hydrate. The 20° 

C. shoulder is close to 8H2O (1.201). A dozen Atlantic 

and Pacific bottom samples kindly supplied by W. 

H. Bradley and T. W. Vaughan were tested, but none 

showed the characteristics of a philJipsite.

Weight ratios for hydrous oxides and zeolites

Temperature (° C.)

50

100

150

200

300

400

500

600

700

fl

1.365

1. 268

1.203

1. 168

1.103

1. 0610

1. 0242

1. 0150

1. 0052

At

1.575

1.430

1.354

1.303

1.233

1. 167

1. 131

1.025

1.003

Hl

1.277

1.236

1.217

1. 199

1. 163

1.095

1.028

1.015

1.005

Ht

1.530

1.422

1.321

1. 244

1. 150

1.086

1.047

1.023

1.008

o

1. 0871

1. 0813

1. 0615

1. 0393

1. 0130

1. 0058

1. 0039

1. 0027

1.0011

S

-

1. 1790

1. 1535

1. 1424

1. 1332

1. 1030

1. 0700

1. 0507

1. 0370

1. 0166

T

1. 1686

1. 1200

1. 0795

1. 0582

1. 0374

1. 0272

1. 0183

1. 0100

1.0038

1.265

1.226

1. 163

1. 114

1.062

1. 0341

1. 0164

1. 0056

1. 0000

p

1. 1972

1. 1797

1. 1160

1. 0965

1. 0596

1. 0315

1. 0240

1. 0220

1. 0010

463806—42——«
204 SHORTEB CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

HYDROUS OXIDES AND BELATED SILICATES

IOO C. . 2OO

TEMPERATURE

FIGURE 16.—Curves for hydrous oxides and related silicates.

A\ and At, Analcime.

O, Qibbsite (scale, one-third).

D, Diaspore.

Curves AI and A2 : Analcime. AI represents pure 

coarsely crystalline material from Wyoming, supplied 

by W. H. Bradley. It gives a smooth S-shaped curve. 

Structural water is hi the form of a low-temperature 

hydrate, as with the zeolites. A2 represents microscopically crystalline material from Arizona, found by 

C. S. Ross to be altered volcanic ash with a trace of 

montmorillonite. This curve is roughly parallel to AI 

but has a shoulder at 600° C. where about half the 

water has escaped, as though a hemi-hydrate were 

present. Other minerals show a similar difference between coarse-grained and very fine-grained forms.

Curve G: Synthetic gibbsite, A1(OH)3 , precipitated 

by the writer from aluminate solution with CO2 . 

Scale, one-third. Coarse crystals, very pure. Extremely sharp shoulders at 1H2O and 3H2O, possibly 

also at 2H2O. Thermostatic control is required to get 

details between 145° and 240° C. Curves of the 

higher-grade Arkansas bauxites indicate that they are 

composed chiefly of gibbsite.

Curve D: Natural diaspore, A12O3 .H2O, from Rolla, 

Mo., selected by C. S. Ross. The part below 430° C. 

is a straight line, slightly inclined. The shoulder is 

sharp and checks well with the second gibbsite shoulder 

at 480°-490° C.

Curve L, limonite, and curve T, goethite: Limonite, 

from Avery, N. C.; Fe2O3 , 83.4 percent ;_SiO2, 1.4 perL, Limonite. 

T, Ooethite. 

B, Brucite.

J, Inesite.

R, Eectorite (X2).

F, Dufrenite.

cent; A12O3 , 0.6 percent. Goethite, Fe2O3 .H20, rltered 

product from marcasite going over to hematite. Both 

specimens thought to be of high purity. Each curve 

consists chiefly of straight lines with two sharply 

defined shoulders. The limonite contains more water 

below 180° C. but much less than goethite hi the range 

200° to 300° C. Neither shows any evidence of ferrous 

iron and neither shows the theoretical amount of 

hydroxyl at room temperature. Thermal analysis 

should be a useful research tool in differentiating these 

minerals.

Curve B: Brucite, from Nevada. White, crystalline, contains traces of R2O3 , CaO, and CO2 . This 

curve, also, consists chiefly of two straight line? with 

a sharp intersection just under 300° C. Below 250° 

C. it is horizontal and slightly higher than the theoretical curve, owing to the presence of a little magnesite.

Curve /: Inesite, 3CaO.llMnO.15SiO2 .10H2O, from 

Quinault, Wash. Supplied by W. T. Schaller. The 

curve consists of two rounded shoulders, one at about 

280° C., the other at a low temperature as in tts zeolites. Usually classed as a zeolite.

Curve R: Rectorite, from Hot Springs, Ark. Very 

pure Al2O3 .2SiO2 .H2O by analysis. Consists of small 

soft, translucent flakes having high luster. Orc'inates 

are magnified X2. The shoulder between 30C° and 

400° C. is similar to those of beidellite and halloysite
SOME STANDARD THERMAL DEHYDRATION CURVES OF MINERALS 205

but.the intermediate part of the curve, 160°-320° C., 

is horizontal instead of inclined.

Curve F: Dufrenite, from Cherokee County, Ala.

A hydrous iron phosphate, 2Fe2O3 ,P2O5 .3H2O. It sh ows 

a definite hydrate, stable at 300° C., and 'resembles 

the curve for brucite.

Weight ratios for hydrous oxides and related silicates

Temperature 

(T)

50 

100

150

200 

250

300

350 

400

450

500

600

Ai

1.089 

1.086

1.083

1.077 

1.069

1.055

1.031 

1.014

1.006

1.003

1.001

G

1. 545 

1.529

1.525

1. 345 

1. 150

1. 105

1.090 

1.084

1,079

1.027

1.003

D

1. 167

1. 165

1. 163 

1. 162

1. 160

1.015

1.005

L

1. 134 

1. 128

1. 120

1.096 

1.040

1.014

1.008

1.004

1.002

T

1. 122

1. 115

1. 112 

I. 100

1.050

1.015 

1.010

1.004

£

1.465

1.460 

1.458

1.430

1.080 

1.030

1.010

I

1. 0765 

1. 0552

1. 0413

1. 0354 

1. 0317

1. 0275

1. 0080 

1. 0042 ,

1. 0038

1.0030

1. 0019

R

1. 1010 

1. 0736

1. 0684

1. 0674 

1.0684

1. 0684

1. 0660 

1. 0514

1. 0250

1. 0064

1.0002

F

1. 1035 

1. 1035

1. 1035

1. 1035 

1. 1035

1. 1010

1. 0712 

1. 0408

1. 0245

1. 0142

1. 0033
206 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

KAOLIN GROUP

3OO 40O 500

TEMPERATURE

FIGUKE 17.—Curves for the kaolin group.

A, At, and At, Anauxite. 

K\ and K?, Kaolinite. 

D\ and Dt, Dickite.

The minerals of the kaolin group—anauxite (?), 

kaolinite, dickite, and nacrite—have been classified 

and critically discussed by Ross and Kerr. 1 On the 

evidence of their thermal analysis curves alone, the 

anauxites form a group quite distinct from the kaolinites. The nacrite curves differ widely from each other 

and cross those of both dickite and kaolinite. The 

data on the kaolin groups used in this report, except 

those for the kaolinites from Bishop, Calif. (K2), 

Stiiiwater, Mont., and Gordon, Ga., and for the dickite 

from Pennsylvania (A), are taken from 1927 determinations by J. G. Fairchild.

Curves A, AI, and Az:- Anauxite group. AI refers to 

a sample from the Mokemmne River,, Calif. It has a 

considerable range (0.1 to 3 mm.) of grain sizes and 

shows no indication of a shoulder near the half-weight 

ratio (1.08). The intermediate curve, A, fully drawn, 

is for a sample from the Newman pit near lone, Calif., 

with grain sizes from 1.5 mm. down. Curve A2 is for 

a fine-grained (0.002 to 0.02 mm.) sample from 

Franklin, N. C. These anauxite curves have a decided 

slope, whereas the kaolinite curves drop vertically. 

They break at a temperature 50° C. lower than the 

kaolinite curves. In the lower range of temperature 

(100°-350° C.), all anauxite curves available exhibit 

a definite slope and are lower than they should be

1 Ross, C. S., and Kerr, P. P., The kaolin minerals: U, 8, Geol. Survey Prof. 

Paper 165, pp. 151-180, 1931,

theoretically (2H2O—1.1628), whereas the kaolinite 

curves are full height and horizontal in this region.

Curves K\ and K2 : Typical kaolinites. K\ in from 

lone, Calif., K2 from Bishop, Calif. The curves show 

horizontal tops close to the theoretical 1.162, and the 

parts between 485° and 540° C. (chiefly 500°-510° C.) 

are vertical. The finer-grained materials (colloids?) 

show a small shoulder halfway down. On thermal 

evidence alone, the Mexican dickite, D2 , might be a 

pure kaolinite. Curves for other samples from Pontiac, 

S. C., Jerome, Ariz., Stiiiwater Range, Mont,, and 

Gordon, Ga., all fall within the range shown by KI 

and K2.

Curves Dl and D2 : Dickite. DI refers to material 

from Pennsylvania. The large lustrous platy crystals 

(0.3 mm.) are very pure and without a visible trace of 

foreign substances. The curve is horizontal as far ffs 

580° C. The vertical part is at 600° C. The lowtemperature part indicates a 1.1482 weight ratio 

corresponding to 1.83 H2O instead of 2H2O. The toe 

of the curve is horizontal at 800° C., indicating that the 

base weight is correct. D2 refers to a clay from 

Cusihuiriachic, Mexico (crystals 0.03 mm.). The 

curve is horizontal (1.162) as far as 480° C., then drops 

straight down at 540°.

Nacrite was available only in small sample? from 

Brand, Saxony, and from Pikes Peak, Colo. The 

thermal curves of these differ widely." Each shows
SOME STANDARD THERMAL DEHYDRATION CURVES OF MINERALS 207

two shoulders: the material from Brand at 450° and 

620° C.; that from Pikes Peak at 550° and 650° C.

A Russian sample, said to be^nacrite, has the thermal 

curve of anauxite.

Weight ratios for the kaolin group

Temperature rc)

100

200

300

400

450

475

500

525

550

575

600

650

700

Anauxite

Ai

1. 140

1. 137

1. 134

1. 127

1.078

1.033

1.012

1.008

1.005

1.003

1.001

1.000

1.000

A

1. 150

1. 146

1. 144

1. 136

1.096

1.050

1.020

1.012

1.008

1.005

1.003

1.000

1.000

A3

1.163

1. 163

1. 162

1. 153

1.105

1.083

1.046

1.029

1.019

1.014

1.011

1.007

1.002

Kaolinite

Ki

1.164

1. 164

1. 164

1. 160

1. 155

1. 152

1. 147

1.018

1.013

1.009 '

1.007

1.004

1.001

Ki

1.163

1. 163

1. 163

1. 160

1. 157

1. 154

1. 149

1.085

1.021

1.014

• 1. 012

1.009

1.006

Pontiac,

N. C.

1. 169

1. 168

1. 167

1.062

1.056

1.051

1.041

1.020

1.016

1.014

1.012

1.009

1.006

Jerome, 

Ariz.

1. 165

1.165

1. 165

1. 161

1. 159

1.156

1.087

( 1 f\^G. j 1. 076 

I 1.032

1.023

1.019

1.015

1.009

1.004

Stillwater, 

Mont.

1.161

1. 160

1. 159

1. 158

1. 157

1. 156

/ 1. 152 

•\I 1.086

} 1.052

1.031

1.021

1.015

1.006

1.002

Dickite

Vi

1. 149

1. 149

1. 149

1. 149

1. 149

1.148

} 1. 147

1. 143

1. 135

1. 119

1.070

1.012

1.003

Nacrite

Brand,

Saxony

1. 173

1.173

1. 167

1. 167

1. 163

1.157

1. 134

1.096

1.075

1.062

1.050

1.016

1.009

Pikes F-ak, 

Cole.

1. 149

1. 149

1. 149

1.147

1. 146

1. 145

1.144

1.143

/ 1. 138 

\ 1. 106

1.075

1.066

/ 1. 057 

\ 1.028

1.007
208 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

MONTMORILLONITES

2 1.05

3OO 4OO 5OO

TEMPERATURE

FIGUEE 18.—Curves for montmorillonites.

S, Pure bentonitic montmorillonite. 

W, Bentonitic montmorillonite, slightly weathered. 

C, High-potash bentonite from Suifu, China. 

B, High-potash bentonite from High Bridge, Ky. 

Y, High-sodium bentonite from TJpton, Wyo.

The curves shown in figure 18 represent investigations of pure or nearly pure montmorillonite as determined by petrographic and X-ray analysis. Water and 

alkalies and alumina-silica ratios vary considerably. 

The various members of the montmorillonite group are 

considered in another section, represented on figure 19.

Thermal curves indicate several classes- of montmorillonites: (1) Pure normal fresh bentonite, collected 

well below the zone pf weathering, and modifications 

resulting from weathering and storage in dry and in 

saturated atmospheres; (2) bentonites high in alkalies, 

whose curves have a less inclined central part than 

those of group (1); and (3) very waxy hygroscopic 

bentonites, much higher in water than normal, as though 

in colloidal condition. All these curves are characterized by (a) a very steep initial part from room temperature to about 160° C., (6) a uniform slope in the middle 

range, 200°-450° C., and (c) a final steep drop in the 

region 500°-600° C. The beidellites (fig. 19, B, R, G) 

drop at lower temperatures (400°-500° C.) and the 

saponites (fig. 19, T) at higher temperatures (630°-680° 

C.).

The montmorillonite clays are used industrially for 

bleaching oils and fats, for bonding molding sands, for 

stiffening drilling muds, and in many other ways. Five 

types and several subtypes are clearly recognized.

U, High-sodium bentonite from TJpton, Utah. 

T, Bentonite from Whitewright, Tex. 

O, Fuller's earth from Olmstead, 111. 

F, Fuller's earth from Florida.

These types are (1) sodium-saturated swelling bentonites of the Wyoming class, useful for their gelling 

and detergent properties but useless for decolorizing,

(2) pure normal bentonites capable of high activation 

as bleaching agents but inactive until acid treated,

(3) clays that are active untreated (like the fuller's 

earths) and also capable of high activation, (4) clays 

such as the Porters Creek, which are active ur treated 

and indifferent to acid treatment, and (5) the normal 

fuller's earths from Georgia and Florida, which are 

considerably deactivated by acid treatment. Only 

the high-sodium clays are in class 1, whereas tl °t highpotash bentonites are in class 3. The property of 

bonding molding sands is dependent upon hygroscopicity, and selected deposits of clay of all five classes are 

used for the purpose. All classes except (1) are^used for 

bleaching oil.

Curve S: Normal pure bentonitic montmorillonite. 

This standard curve is identical with that for montmorillonite from the Vicksburg (of Oligocene age) at 

Polkville, Miss. It is very near those for clays from 

Mainburg, Bavaria; Tatatila, Mexico ;Kosedale, Alberta; 

Chito, Ariz. ;Mojave and Otay,Calif.;Bogalusa, La.; and 

many others. Aside from the three general characteristics 

mentioned above, the curve for a pure normal bentonite 

may be quicky identified by the value 1.05 (1H2O) at
SOME STANDARD THERMAL DEHYDRATION CURVES OF MINIBALS

400° C. and by its slope in a straight line from 1.061 

at 200° to 1.048 at 520° C. When plotted as log 

(water) versus 1/T, this curve breaks up into three 

straight lines, the slopes of which—d log W/d (1/T)—indicate energies of association of 173 calories per gram 

from 30° to 150° C., 37.6 from 150° to 500° C., and 

1,068 from 500° to 650° C., with a fairly sharp transition 

(less than 50° C.). The loss in weight at high temperatures is due to loss of hydrogen and hydroxyl in 

the crystal lattice; that at intermediate temperatures 

represents the escape of water adsorbed on or between 

lattice planes; and that at low temperatures represents 

the escape of water that is present as simple moisture, 

although its energy of association is four times that of 

the adsorbed water.

Curve W (dashed): Bentonitic montmorillonite, 

slightly weathered. This curve represents bentonite 

taken from a place so near the surface that atmospheric 

agencies have weakened its lattice structure. Bentonite stored for a few months at room temperature, either 

in a dry or in a water-saturated atmosphere, shows a 

similar alteration of curve S toward >W. Anhydrous 

impurities (quartz, feldspar), of course, lower the whole 

curve, in proportion to their quantity, to the position 

of curve B or lower. Many montmorillonite curves 

show the results of impurities and weathering.

Curves C, B, Y, and U: Bentonite. C, from Suifu, 

China; high potash (5.16 percent); known as "soap 

clay." B, from High Bridge, Ky.; of Ordovician age; 

high potash (5.72 percent); contains feldspar. Y, from 

Upton, Wyo., sodium saturated. U, from Upton, 

Utah; Na2O, 2.75 percent; low slope, high breaking 

temperature. These curves are all of decreased slopecompared with that for normal bentonite, indicating 

that the interlattice adsorbed water is much less firmly 

held when considerable alkali is present. If sodium 

causes an effect different from potassium, these curves 

do not show it. These curves are normal in their hightemperature shoulders.

Curve 1: Bentonite from the Austin chalk (of 

Upper Cretaceous age), near Whitewright, Tex. Contains many biotite grains; very waxy; hygroscopic like 

a dried colloid. Although normal in appearance, 

texture, fracture, etc., it suggests a bentonite abnormally high in water. Similar elevated curves are common 

for the beidellites and for other clays whose curves 

break around 450° C., but they are rare in the montmorillonites. Although high in water retention, this 

clay also holds its water firmly, as shown by the 600° C. 

break.

Curves 0 and F: Fuller's earth. 0 is from Porters 

Creek clay (of Pal eocene age), Olmstead, 111.; nearly 

normal montmorillonite (curve is lower and breaks at 

500° instead of 580° C., indicating that material is 

slightly weathered and impure). Porters Creek clay 

from Pinson Hill, Tenn., gives curves like 0. F is 

from the Hawthorn (of middle Miocene age) of Florida; 

slight humps at 190° and 310° C. are not shown in 

figure 18. Although curve F shows no shoulders, most 

curves of this type show near 200° and 300° C. minor 

shoulders that probably indicate hydrates that become 

unstable at those temperatures. Clay from Attapulgus, Ga., gives curves with humps at 150° and 310° C., 

similar to F. Some samples from the same pit ar 0, 

Olmstead, HI., gave curves like F, having no hightemperature shoulder but one or two shoulders at lower 

temperatures. The fuller's earths, whether from the 

Porters Creek clay of Tennessee and Illinois or from, the 

Hawthorn of Georgia and Florida, all exhibit two widely 

different types of thermal curves, even in samples from 

the same or neighboring pits. One type, 0, evidently 

represents nearly normal montmorillonite; the other 

type, F} shows no lattice shoulder and resembles the 

featureless curves of an amorphous hydrogel. The 

thermal evidence is that either two different minerals, 

montmorillonite and "attapulgite" (?) are present or 

that the same substance occurs in two widely different 

forms, one a hydrogel.

• Weight ratios for montmorillonites

Temperature (° O.)

100 

200 

300 

400

500 

550 

600 

700

8

1. 0860 

1. 0610 

1. 0555 

1. 0500

1. 0444 

1. 0395 

1. 0225 

1. 0014

C

1. 0483 

1. 0449 

1.0437 

1. 0426

1. 0411 

1. 0340 

1. 0122 

1. 0015

B

1. 0552 

1. 0408 

1. 0380 

1. 0356

1. 0326 

1. 0243 

1. 0122 

1. 0020

Y

1. 0492 

1. 0449 

1. 0424 

1. 0398

1. 0372 

1. 0320 

1. 0130 

t 0013

T

1. 1090 

1. 0815 

1. 0784 

1. 0753

1. 0715 

1. 0650 

1.0400 

1. 0030

u

1. 0635 

1. 0585 

1. 0551 

1. 0518

1. 0484 

1. 0462 

1. 0316 

1. 0020

0

1. 0555 

1. 0470 

1. 0408 

1. 0348

1. 0234 

1. 0108 

1. 0058 

1.0020

F

1. 1290 

1. 0870 

1. 0555 

1. 0316

1. 0154 

1.0118 

1.0092 

1. 0045
210 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

MONTMORILLONITE GROUP

IOO C. 2OO 3OO 4OO 5OO 6OO 7OO 

TEMPERATURE

FIGUEE 19.—Curves for tbe montmorillonite group.

M, Standard curve for normal montmorillonite (see also fig. 18, S).

B, Beidellite.

G, TMn bed of dried gel (beidelllte ?).

R, "Racewinite."

N, Nontronite.

H, Halloysite.

J, Illite.

T, Saponite.

In figure 19 are assembled typical dehydration curves 

for the various members of the montmorillonite group 

including beidellite, nontronite, and saponite, and the 

related halloysite and illite, as well as montmorillonite 

itself. The type material from Montmorillon, France, 

does not give the curve for normal bentonitic clay but 

a curve that has two shoulders, at about 480° and 640° 

C., one or both of which appear to represent minerals 

other than that generally accepted as montmorillonite. 

Pyrophyllite, Al2O3 .4SiO2 .H2O (fig. 21, P), has a constant weight of 1.05 up to 500° C., then follows the 

normal montmorillonite shoulder and toe very closely.

Curves B, R, and G: Beidellite. B, a very pure clay 

mineral replacing calcareous shells and excluding all 

calcium, from Pontotoc, Miss. R, "racewinite," from 

a seam in a Utah copper deposit, described by Winchell; 2 very close to beidellite in composition but 

optically positive. G, Sample from thin bed of dried 

colloidal material apparently intermediate between 

beidellite and montmorillonite; from West Chester, Pa. 

At 400° C. water is slightly higher than normal in B, 

and considerably higher in R. These three curves are

' Wincnell, A. N., Racewinite, a peculiar mineral from ore deposits in Utah: Boon. 

Geology, vol. 13, pp. 611-613,1918.

typical of many beidellites and well show the ir,nge of 

variation found in a dozen clays of this class ftudied 

by the writer.

Curve N: Nontronite, from Santa Kita, N. Mex. 

This curve is typical of three nontronites studied. 

Samples from Sandy Kidge, N. C.; Spokane, Wash.; 

Warthen, Ga.; and others give nearly identical curves. 

These curves differ little from those for weathered 

beidellites. Water appears low chiefly on account of 

the heavier iron constituent.

Curve H: Halloysite, from Liege, Belgium. Curves 

of samples from Adams County, Ohio; Home, Ga.; and 

Anamosa, Iowa, are almost identical with th«s type 

material plotted in H. All halloysite curves are characterized by their height (1.145 at 400° C.), by their 

straight line slope between 250° and 450° C., r.nd by 

their dip in the range between 50° and 250° C. The 

steep drop occurs between 450° and 500° C. There 

has been some discussion of a change in halloysite near 

50° C., but according to these curves such a change 

should be sought at 250° C. Several glauconite curves 

(fig. 20, 60 also show a similar dip followed by a straight 

line but at 1.05 instead of 1.15 in weight ratio.

Curve /: Illite. The curve is plotted from -data
SOME STANDARD THERMAL DEHYDRATION CURVES OF MINERALS 211

obtained by R. E. Grim 8 in 1937 on selected material 

and is given here to show its resemblance to that of 

nontronite. It differs widely from the thermal curves 

for such minerals as sericite and muscovite.

Curve T: Saponite, from Hector, Calif. This white 

swelling clay is normal saponite except for small

< Grim, R. E., personal communication.

amounts of Na and Li. The dried hydrogel ga^e a 

curve having shoulders at 500°, 600°, and 960° C. and 

was still losing weight at 1000° C., the limit of the 

apparatus. The curve is tentative and is the smoothed 

mean of five curves, which differ considerably in detail 

and are uncertain as to base weight. Three minor 

shoulders are omitted. '

Weight ratios for the montmorillonite group

Temperature 

CC.)

50

100

150

200

300

350

400

450

500

550

600

700

M

1.182

1.086

1.065

1.061

1.055

1.033

1.050

1.047

1.044

1.039

1.020

1.001

B

1. 148

1. 120

1.107

1.100

1.093

1.089

1.084

1.067

1.036

1.020

1.011

1.003

G

1.145

1. 134

1.126

1. 119

1.105

1.097

1.090

1.083

1.064

1.034

1.018

1.004

It •

1.182

1.139

1.123

1.117

1. 109

1. 104

1.096

1.072

1.029

1.017

1.011

1.003

N

1.186

1.080

1.059

1.053

1.048

1.046

1.039

1.017

1.009

1.006

1.003

1.000

H

1. 188

1. 174

1. 169

L 166

1. 157

1. 151

1. 145

1. 138

1.029

1.017

1.011

1.003

J

1.083

1.069

1.065

1.062

1.056

1.050

1.034

1.019

1.010

1.005

1.002

1.000

T

1.113

1.085

1.077

1.074

1.070

1.069

1.067

1.065

1.061

1.055

1.045

1.011
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

CHLOBITES AND BELATED SILICATES

3OO 4OO 500 

TEMPERATURE

FIGURE 20.—Curves for chlorites and related silicates.

8, Serpentine. 

P, Picrolite. 

. C, Chlorite.

L, Clinochlore.

This group comprises mainly chlorites and related 

hydrous magnesium alumino-silicates. They have not 

been studied sufficiently to permit many general conclusions to be drawn, but the nature of the dehydration 

curves to be expected in further work is indicated.

Curve S: Fibrous serpentine, supplied Hy W. T. 

Schaller. Very pure, Fe2O3 but 0.3 percent. The 

curve of this sample shows the characteristic rather 

sharp shoulder and steep drop between 600° and 650° C. 

No points were obtained within that range but the 

alinement of the branches suggests a shoulder half 

way down. Granular serpentines containing nickel 

and 5 to 7 percent of FeO show a lower curve having-a 

greater slope below 500° C. and a more rounded shoulder 

but a break at the same temperature.

Curves L, N, C, and P are similar to that for serpentine (S), but all except picrolite have a second 

shoulder at a higher temperature. C, chlorite, from 

Danville, Va.; base weight at 930°, 1.006 at 830° C. 

L, Clinochlore, from Chester County, Pa.; approximately 5MgO.Al2O3 .3SiO2 .4H2O. N, Penninite, from 

Paradise Range, Nev.; content of (Na, K) 2O less than 

0.1 percent; base weight at 870° C. P, Picrolite, from 

Baltimore County, Md.; dark-green fibrous serpentine.

N, Penninite. 

K, Paligorskite. 

V, Vermieulite. 

0, Glauconite.

Curve K: Paligorskite ("Mountain leather"), from 

Montana. A gray fibrous, extremely tough spec;men. 

The curve has three shoulders, at 170°, 340°, and 620° 

C., with intervening straight-line parts.

Curve V: Vermieulite, from North Carolina. The 

base weight (anhydrous) is uncertain and has been 

arbitrarily chosen as 1.050 at 340°-405° C. AT free 

moisture has been driven off below 340° C. The characteristic swelling occurs above 400° C. Curves for 

vermiculites from other localities are similar in general 

character but differ in detail.

Curve O: Glauconite, from Lyons Wharf, Md. 

Light-green, opaque " greensand." Of the four glauconites studied—from Huntington, Oreg., Bonne Terre, 

Mo., Sewell, N. J., and Lyons Wharf, Md.—the Forme 

Terre sample contained the most water, 1.051 at 4CO° C. 

On account of admixed quartz grains, the sample from 

Lyons Wharf contained the least water, but it fhows 

most clearly the characteristic shape of the curve, 

which is concave from 50° to 320° C. and a straight 

line approaching the shoulder at 470° C. A correction 

of only 1.016 in base weight would bring the highest 

and lowest curves together at 400° C.
SOME STANDARD THERMAL DEHYDRATION CURVES OF MINERALS 

Weight ratios for chlorites and related silicates

213

Temperature (° C.)

50

100

150

200

.300

400

500

550

600

650

700

800

8

1.196

1.188

1.184

1. 181

1.178

1.175

1. 172

1. 170

1. 161

1.080

1.015

1.000

L

1. 168

L 165

1.164

1. 163

1. 161

1. 159

1. 158

1. 152

1.074

1.064

1.058

1.043

N

1. 151

1.150

1. 149

1.148

1.146

1. 145

1.140

1.066

1.033

1.030

1.027

1.013

C

1. 142

1. 139

1. 138

1.137

1. 136

1.134

1.132

1.080

1.069

1.059

1.047

1.017

P

1. 133

1.130

1. 128

1. 127

1. 125

1. 122

1. 120

1. 119

1.060

1.012

1.003

1.000

K

1.141

1. 121

1. 114

1.085

1.077

1.054

1.025

1.020

1.015

1.006

1.001

1.000

v •

1.107

1.074

1.062

1.058

1.052

1.050

1.039

1.033

1.027

1.022

1.020

1.010

G

1.068

1.053

1.048

1.045

1.042

1.033

1.015

1.009

1.005

1.002

1.000

1.000
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42

MICAS

O 4OO SOO 6OO 

TEMPERATURE

FIGURE 21.—Curves for micas.

ipoo

P, Pyrophyllite. 

L, Lepidolite. 

N, Taenlolite. 

T, Talc.

Figure 21 shows dehydration curves of misceUaneous 

minerals, chiefly micas, in which there is a small but 

characteristic weight decrease, usually at a high temperature. Several specimens of sericite and of pyrophyllite but only single specimens of other minerals 

were investigated. These minerals hold no water as 

such under room conditions but all weight losses appear 

to be caused by the driving out of H and OH from the 

crystal lattice; hence base weights are chosen at lower 

temperatures as representative of the mineral. All 

weights are corrected for air density.

Curve P: Pyrophyllite, Al2O3.4SiO2.H2O, transparent 

radial crystals of a losette that contained no more than 

traces of impurities. Swells like vermiculite on heating 

above 500° C. The material mined at Hemp, N. C., 

contains some potash, piobably in sericite. Pyrophyllite mined near Staley, N. C., is partly hi rosette form 

and very pure. Base weight between 300° and 400° C. 

The shoulder and toe of this curve are closely similar 

to those foi pure montmorillonite, as would be expected 

in two minerals having the same internal lattice structure.

Curve L: Lepidolite, from Manitoba. The 3 percent water in this sample is lost very gradually, the 

structure breaking down only at 690° C. As none of 

the curve is horizontal, the base weight was taken at 

the usual 110° C.

Curve N: Taeniolite, from Magnet Cove, Ark. 

Collected by H. D. Miser. Pure KLiMg2Si4OioF2 , as 

determined by R. E. Stevens, in 0.5 mm. crystals. 

Base weight between 100° and 425° C. The weight is 

constant up to 420° C.; between 420° and 490° C., 

0.80 percent of water is driven off, and the anhydrous

R, Tremolite asbestos. 

S, Sericite. 

M, Biotite (X3). 

B, Beryl.

mineral is stable up to 810° C., above which the fluorine 

is gradually driven off. At the last observed point 

(1110° C.), the fluorine was not completely removed. 

An ignition loss for water should not be determined 

above 800° C., and preferably not above 600° C.

Curve T: Talc. Transparent, colorless. R2O8, 0.36 

percent; Fe and Ca absent. The curve shows no weight 

loss as far as 730° C., and then a rather steep drop 

which becomes complete at 930° C.

Curve R: Tremolite asbestos, from near Baltimore, 

Md. This sample contained 0.3 percent water at 100° 

C. This was lost at a uniform rate up to 350° C. The 

weight remained constant hi the range 350° to 780° C., 

then fell off in a rounded shoulder. The last obs^ivation was at 990° C.

Curve S: Sericite, from near Prince Rupert, British 

Columbia. Lustrous massive material. Its lo^s of 

weight, beginning at about 500° C. amounted to 3.7 

percent. Of the other two samples tested, that from 

Franklin, N. C., lost 3.0 percent, and that from near 

Baltimore 4.2 percent/with the break occurring between 

500° and 550° C.

Curve M: Biotite, from near Philadelphia, Pa. The 

ordinates have been increased three times to show I stter 

the three shoulders at about 200°, 500°, and 90C° C., 

with weight losses of 0.2, 0.5, and 0.2 percent, indicating 

different associations of hydroxyl and/or fluorine in the 

structure.

Curve B: Beryl, from Warren Depot, Me. Weight 

constant up to 630° C., followed by a rounded shoulder 

extending above 800° C. Constant weight is approached very slowly even at 950° C.

Phlogopite showed no change in weight up to 1025° C.
SOME STANDAED THEEMAL DEHYDEATION CUEVES OF MINERALS 

Weight ratios for micas

215

Temperature (° C.)

100

200

300

400 500-

600

700

800

900

1000

p

1. 0014

1. 0014

1.0000

1. 0000

.9980

.9850

.9595

.9496

.9486

.9486

L

1. 0000

.9989

.9978

.9969

.9955

.9912

.9739

.9705

.9705

.9705

N

1.0000

1. 0000

1.0000

1. 0000

.9921

.9921

.9921

.9921

.9800

.9460

T

1.0000

1. 0000

1. 0000

1.0000

1.0000

1. 0000

1. 0000

.9925

.9572

.9552

R

1. 0030

1. 0018

1. 0007

1.0000

1.0000

1.0000

1. 0000

.9997

. 9936 '

.9788

s

1.0000

1. 0000

1. 0000

1. 0000

1.0000

.9980

.9880

.,9675

.9630

.9630

M

1. 0000

.9998

.9945

.9935

.9908

.9846

.9805

.9805

.9800

. 9750

B

1.0000

1.0000

1. 0000

1. 0000

1. 0000

1. 0000

.9983

.9885

.9787

.97^5
216 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1941-42 

CARBONATES

:oo 300 AOO soo e 

TEMPERATURE

FIGURE 22.—Curves for carbonates.

C, Calcite. 

D, Dolomite. 

M, Magnesite.

The curves of figure 22 illustrate the behavior of 

certain carbonates and hydrocarbonates on heating. 

The thermal decomposition of the carbonates is an 

extremely slow and steady process, particularly at 

temperatures just above the first break; however, the 

attainment of a definite weight with CO2 only partly 

removed is beyond question. Diffusion of CO2 out 

of the crystal fragments limits the rate of approach 

to this constant weight. With crystals ground to 150 

mesh (0.1 mm.), the rate for calcite is about 1 mg. in 

24 hours per gram of samples.

Curve C: Calcite, a perfect crystal; CaO, 56.88 

percent. Ground to 0.1 mm. fragments. The main 

break is between 615° and 620° C. with a tailing off of 

less than 30° C. The point at 618° C. required 44 

days to reach constant weight; at 642°, 31 days. 

Coarsely crystalline marble gave an identical curve. 

The aragonite form of CaCO3 has not yet been investigated by the writer.

Curve D: Dolomite, from Cockeysville, Md. Extracted with 2 percent HC1 by R. C. Wells. A double 

break at 505° and 553° C. is indicated by all samples.

H, Hydromagnesite. 

N, Nesquehonite.

The details of the transition between these two drops 

remains to be worked out. Another crystalline sample 

selected by C. S. Ross broke at 515° and 565°. Another from Joplin, Mo., gave a curve like a mixture of 

calcite and magnesite.

Curve M: Magnesite, from Cote St. Paul, Quebec. 

Optically puie crystal; traces of calcium and iron, no 

silica. The first break is at about 420° C. Between 

450° and 600° C. the curve is uncertain, as the three 

runs made did not agree.

Curve H: Hydromagnesite, 3MgCO3 .Mg(OH)2 . 

3H2O. Large natural crystals from Luna, N. Mex., 

supplied by Eugene Callaghan. The curve shows two 

shoulders at 220° and 355° C., apparently corresponding to H2O and CO2 alone, but gas tests by R. C. Wells 

indicate that the gases are evolved as a mixture through 

the intermediate range. The runs agree very well. 

Two runs took 9 months.

Curve N: Nesquehonite, MgCO3 .3H2O; pure, crystalline. Synthetic, prepared by R. C. Wells. Shows 

shoulders at 90° and 375° C.

Weight ratios for carbonates

Temperatare (° C.)

50

100

150

200

250

300

400

500

600

700

C

1.000

1.000

1.000

1.000

1.000

1.000

1.000

1.000

.995

.552

D

1.000

1.000

1.000

1.000

1.000

1.000

1.000

.956

.528

.528

M

1.000

1.000

1.000

1.000

1.000

1.000

.975

. 540

.508

.508

H

1.000

1.000

1.000

.998

.900

.837

.467

.445

.435

.433

N

1.000

. 825

.736

.691

. 661

.643

.330

.310

.300

.300
INDEX

Abstract—————————.—-...—................____............ 197

Allophane._____——_____-_____________________ 202 

Aluminum hydroxide—_____..._________..__________ 202 

Analcime___———-——___._______________________ 204 

Analcite. See Analcime. 

Anauxite.. .............................^..................................... 206

Anhydrous crystalline minerals-_. _ ____._ ______.. ___ _ _ 200 

Ash, volcanic____ ______.________ -_____________ 200-201 

"Attapulgite," possible occurrence of..____---.._...._______... 209

Austin chalk, bentonite from,__..-.________...._...._...._ 208,209

Base weight of minerals, temperature selected for_.___ ________ 198,214 

Bauxite-——————_.._ ................^........................ 204

Beidellite..................................................................... 210

Bentonite, curves of _________.______.. _____________ 208-209

silica residue from..____________._____________ 200,201

Beryl.——.——-—.——— — .--..——..—....................... 214

Biotite.__.——— ——— ————————__ ................................. 214

Bruclte.—————————————————.————_.................. 204

Calcite.-..——---—-—- ———----—— — ----- — .— .—-—---- 216

Carbonates-_____._______.______._____._______ 216 

Chabaiite..------—.———.—............——-....—...__-----_... 202,203

Chlorite.—-———————————— —— —— --——————————— 212

Clay from—

Adams County, Ohio————— ————————————— ————__ 210

Anamosa, Iowa..._.________.__._____.______ 210 

Attapulgus, Ga-———————————-————„-.--_____.____ 209 

Bishop, Calif————————— ——————— — — ———————— 206

Bogalusa, La..---———————— ———...——..————————— 208

Brand, Saxony.-----——------------_—...——..._________ 208-207

Chito, Ariz———-————-——— ———————— — ———— _ 208

Cusihuiriachic, Mexico.______.________.__________ 206 

Florida——-——--—-—-—— — - ————-——————. 208,209

Franklin, N. C—————————————— — __—————————_ 206

Gordon, Ga.-..-.—_—-_——___..--._——————— ——— 206

Hector, Calif____......___..-..--._.-----..-.__.......__... 210,211

High Bridge, Ky._ —.-——— — — .——————————— ——— 208,209

lone, Calif._.....—...-....—.—.„.-..——_-----.---.---......... 206

Jerome, Ariz__——- — _——-.- —————————..._____...__.. 206 

Liege, Belgium,———.———————— ——————— -_...._________ 210

Mainburg, Bavaria-—...———------_————-._.______.__ 208

Mojave, Calif—------------------------------------__............. 208

Mokelumne River, Calif—.—....... — ........—...................... 206

MontmorDlon, France__.______________. ________ ___ 210 

Omstead, 111-———————- ——---,——————————— 208,209

Otay, Calif. —-—-- — ————-——-———--————--. 208

Pennsylvania——————— ——————.——————________._ 206 

PikesPeak, Colo.———— —————————————— ———— 206-207

Pinson Hill, Tenn_—...______.________.______.__ 209 

Polkville, Miss.... ——-- —— — - — — -—————......——......... 208

Pontiac, S. C............................................................. 206

Pontotoe, Miss. — ——— ——— — —— — — ——— -----.__.__---._ 210

Rome, Ga....___——_________________________... 210

Rosedale, Alberta... ————. ————. —.——————.._________ 208 

Russia____._———---——____--——.-.--._________ 207 

Sandy Ridge, N. C....................................................... 210

Santa Rita, N. Mex........——.......................................... 210

Spokane, Wash_ ——— ___.___ —......__.-._.______... 210

StDlwater Range, Mont..___.______.____________... 206 

Suifu, China-—-.._--....-.-...-.---.-.--..--.--.---.....-...-...... 208,209

Tatatila, Mexico_——_________.._____.______... 208 

Upton, Utah.....-—---._..........------.---_.--.-..........-...._ 208,209

~TJpton, Wyo-.—-— ——— ——— —— —— ———— ———— ——— 208,209

Warthen.Ga----—- — .—----•—— — — --—.........-—-— 210

West Chester, Pa.....................................................__ 210

WWtewright,Tex...—................................................ 208,209

Clay minerals-.——.—...——........ — ..——....——................. 206-211

Clinochlore—-——- ————— ————.——-——— — .————. 212

Dehydration, procedure in______.___.....————_________ 197-198

Diaspore___——————____________—.____.______ 204 

Diatomite....—.———————————————————____...... 200,201

Dickite.....—.———.—.———- — —--- — ——.—...-.-.-...- 206

Page 

Dolomite--.-..-...—----_-----_...............——..———..—...— 216

Dufrenite—————— ———————-—--——————————— 204,205

Energy of association with water, constant.—————— ——————————— 200

Feldspar.———— ——————————— ———————————-i. 200

Fuller'searth———--.—---— —— —————-—,- — -—-——— 208,209

Gibbsite————————————-- —————————- — — —— -— 204

Glass....__..._.———--———————_-_-.——— — — -.—__ 200

aiauconlte...——— — ————--————————————— 210,212

Goethite————————-—— — - ———————————————— 204

Halloysite. ———————————————— ————————— 210

Hawthorn formation, fuller's earth from-_..—....——————————— 208,209

Hydromagnesite, curve of.... ——— — — - —— - — — — -————————— 216

• Hydrous oxides..——.——————-———————————-'——————————— 202-203

Elite, curve of.———-- —————-— ———- ———————— 210-211

Inesite, curve of-———.————————- —————— -————————————— 204

Eaolinite, curves of—-——-.-—...............——— — ..-—.....—.— 206

Laboratory methods and equipment..————————————————————. 177-198 

Lepidolite. ————— —————— —— — ——--—'———————— 214

Limonite———————————— ———— — —————————— 204

Magnesite———— — — ——— — - — ---- —— ——— ——— ——— — ----——,- 216

Magnesium silicate, synthetic...-—........—————.— — .————— 200,201

"Magnesol"——————— ———— — ——— — —————————— 200,201

Marble.————————— — — ———————— — —————— 216

Micas--..-.---__----------------------_——————————— 214-210

Montmorillonite, curves of.-..————————————————— 208-209,210

residuefrom———.————————"———————————— 200, 201

Mountain leather. See Paligorskite.

Nacrite- ————-———- —— — —- — -—————— ————— — — 206-207

Nesquehonite__—... ——— ———_. —— — —— —— — - — —. — — —— 216

Nontronite——————————————————————————— 210

Opal-----—————————————————-———————— 202

Oxides, hydrous.... ——— —————— ---——————————————————— 202-205

Paligorskite.-———————— —————————————————— 212

Penninite—.— -- — -——- — --—- — -———-—————— — -——- 212

Phillipsite———————— —— ——————————— ———-— 202,203

Phlogopite_—————— - —————————————————————————————— 214 

Pierolite——————— ———————— — —— —— ————— 212

Porters Creek clay, fuller's earth from...——. ——— ——————.———— 208,209 

Ptilolite-- ———— ——————————————— — —— ————————— 202-203

Pyrophyllite—...————— —————————————————————— 210,214

Quartz, powdered.------- ———...... —— — —— ——— ——————————— 200

"Racewinite".... —————————————————————————— 210

Rectorite————————————————————————————— 204-205

Saponite—— ——— ——————————— ———————————..,— 210,211

Sepiolite..———————— ——— —————————————- 202

Sericite. —— — —— —— ——————————————————————— 214

Serpentine—----------------------------------------------------—-—— 212

Silica——————————————————————————— — — — 200,201

Taeniolite.- —— ————— —— —— —— — —————————————— 214

Talc— ——————————— ——— — —— — — — —— —— — ————— 214

Temperature, control of, in thermal dehydration.— — — — — — — — — -. 197-198

Tremolite. — -- ————————————— -——-———————————————-- 214

Vermiculite———————-—- ——-——— — ---———--——-—- 212

Vicksburg group, montmorillonite from.—. — ——...... ———— ———— — 208

Volcanic ash——————————— — — —— —— — — ——————— 200-201

Zeolites...—— ———— — — --——— — —— — -——-— — --—— 202-203,204

o

217