Assessment of the Geoavailability of Trace 

Elements from Selected Zinc Minerals

By Rhonda L. Driscoll, Phillip L. Hageman, William M. Benzel, Sharon F. Diehl, Suzette Morman, 

LaDonna M. Choate, and Heather Lowers

Open-File Report 2013–1309

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Driscoll, R.L., Hageman, P.L., Benzel, W.M., Diehl, S.F., Morman, Suzette, Choate, L.M., and 

Lowers, Heather, 2014, Assessment of the geoavailability of trace elements from selected 

zinc minerals: U.S. Geological Survey Open-File Report 2013–1309, 78 p., 

http://dx.doi.org/10.3133/ofr20131309.

ISSN 2331-1258 (online)

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must be secured from the copyright owner.
iii

Contents

Abstract.............................................................................................................................................1

Introduction .......................................................................................................................................1

Sample Preparation ..........................................................................................................................3

Mineral Descriptions and a Brief Discussion of the Franz Magnetic Separator .............................3

Introduction................................................................................................................................3

Sample Preparation ...................................................................................................................3

Mineralogical Analyses .....................................................................................................................6

Introduction....................................................................................................................................6

Methods.........................................................................................................................................7

Discussion and Results .................................................................................................................9

Sulfide Mineral—Sphalerite (ZnS)..............................................................................................9

Sphalerite Specimen—Creede, Colorado................................................................................10

Carbonate Minerals—Smithsonite (ZnCO3) and Hydrozincite (Zn5(CO3)2(OH)6) .....................18

Silicate Minerals—Hemimorphite (Zn4Si2O7(OH)2·H2O) and Willemite (Zn2SiO4) ....................25

Summary .....................................................................................................................................29

Bulk Chemistry, Leachate Chemistry, and Acid-Base Accounting Geochemical Results for Five 

Zinc-Bearing Minerals .....................................................................................................................31

Introduction—Bulk Sample Chemistry .........................................................................................31

Bulk Chemistry Summary ............................................................................................................31

Comparison of Bulk Chemistry for Two Sphalerite Samples .......................................................33

Introduction—Leaching Studies...................................................................................................34

U.S. Geological Survey Field Leach Test of Hageman (2007a) ..................................................34

U.S. Environmental Protection Agency Method 1312 (Synthetic Precipitation Leaching 

Procedure)...................................................................................................................................34

U.S. Environmental Protection Agency Method 1311 (Toxicity Characteristic Leaching 

Procedure)...................................................................................................................................35

Materials, Methods, and Sample Analysis...................................................................................35

Leachate Geochemistry...........................................................................................................35

Summary of Leaching Studies.....................................................................................................42

Acid-Base Accounting Study .......................................................................................................43

Summary of Acid-Base Accounting Study ...................................................................................44

In Vitro Bioaccessibility Extractions.................................................................................................45

Introduction..................................................................................................................................45

Methods.......................................................................................................................................45

Ingestion Pathway .......................................................................................................................45

Inhalation Pathway ......................................................................................................................46

Results ........................................................................................................................................46

Discussion ...................................................................................................................................58

Limitations of Results ..................................................................................................................58

Summary .....................................................................................................................................59

Procedure and Results of Metal Toxicity for Five Zinc Mineral Concentrates Using MetPLATETM..60

Introduction..................................................................................................................................60

General Method...........................................................................................................................61

Summary .....................................................................................................................................62

Conclusion ......................................................................................................................................69
iv

References Cited ............................................................................................................................70

Appendix A......................................................................................................................................76

Panalytical X’PERT X-ray Diffractometer Instrument Set-Up...................................................76

Data Processing .......................................................................................................................76

Limitations of the data ..............................................................................................................77

Appendix B......................................................................................................................................77

Appendix C ................................................................................................................................... link

Appendix D ................................................................................................................................... link

Figures

Figure 1. Sphalerite unit cell showing zinc and sulfur sites. Metal substitutions occur 

at the zinc sites ...............................................................................................................................10

Figure 2. A. Photograph of a sphalerite hand sample from Creede, Colorado. Sphalerite

is a vein-filling cement in brecciated host rock. B. Scanning electron image showing sphalerite 

intergrown with quartz and potassium feldspar. Galena is common as micron-size inclusions. 

Kspar, potassium feldspar. C. Scanning electron micrograph showing intergrown sphalerite, 

pyrite, and galena in veins. Galena (PbS) has partially oxidized to anglesite (PbSO4). 

D. Scanning electron micrograph of dissolution-etched pyrite partially altered to oxidation 

minerals. Galena is altered to lead sulfate (anglesite). μm, micrometer; cm, centimeter................11

Figure 3. X-ray diffraction pattern of the Creede, Colorado, sphalerite identifies the 

minerals present. The center region of the scan is expanded to show trace phases. 

S, sphalerite; G, galena; Q, quartz; O, orthoclase; P, pyrite; C, chlorite; deg, degree.....................12

Figure 4. A. Photograph of a massive sphalerite specimen from the Balmat, New York, 

sedimentary exhalative deposit (SEDEX). B. Backscatter scanning electron image showing 

dark-gray oxidation-alteration minerals formed along sphalerite grain boundaries and fractures. 

C. Scanning electron image showing accessory sulfide minerals galena and pyrite in sphalerite. 

Qtz, quartz; μm, micrometer; cm, centimeter..................................................................................16

Figure 5. X-ray diffraction pattern of the Balmat, New York, sphalerite. Sphalerite and 

quartz are observed in the pattern as well as a 7Å (angstroms) clay that is most likely 

kaolinite but may be chlorite. Three other low-intensity reflections are indicated in the 

figure by their d-spacing (Å) and represent one or more trace phases that are not identified. 

These reflections were evaluated by scanning electron microscope analysis, but no certain 

mineral identifications can be made. Q, quartz; S, sphalerite; deg, degree.....................................17

Figure 6. Smithsonite unit cell. Smithsonite forms a limited solid solution with rhodochrosite 

(MnCO3) and siderite (FeCO3). A wide variety of metals can substitute for zinc in the structure.....19

Figure 7. Smithsonite: A. Transmitted-light micrograph of radiating smithsonite crystals and 

accessory iron oxide minerals. B. Backscatter scanning electron image displaying intergrown 

smithsonite (medium gray) and hemimorphite (light gray) with accessory lead carbonate (brightwhite areas) preferentially hosted in hemimorphite. C. Scanning electron micrograph showing 

element compositional zonation in smithsonite. Light-gray bands are cadmium-rich; darker bands 

are calcium-rich. D. Cadmium-element distribution map of area depicted in C. Light-purple bands 

highlight cadmium enrichment, which is concentrated in the outermost areas of late-stage 

crystallization. E. Zinc distribution map of C and D indicating the homogeneous distribution of 

zinc, which contrasts with the zoned distribution of cadmium and calcium. F. Backscatter 

scanning electron image of lead zoning in zinc silicate. Of the three intergrown zinc phases 

present, lead is preferentially hosted in growth zones in willemite. μm, micrometer.......................20
v

Figure 8. X-ray diffraction pattern of the smithsonite sample. Several other minerals are 

identified in the pattern including hemimorphite, quartz, willemite, and chlorite. The center 

region of the scan is expanded to show trace minerals. S, smithsonite; H, hemimorphite; 

Q, quartz; W, willemite; C, chlorite; deg, degree .............................................................................22

Figure 9. Hydrozincite: A. Transmitted-light photomicrograph showing a cross section of 

radiating hydrozincite crystals in botryoidal masses. B. Scanning electron image showing 

porous masses of radiating hydrozincite crystals. Hydrozincite is accompanied by calcite and 

accessory lead, silver, antimony, and copper minerals (bright-white spots). μm, micrometer ........23

Figure 10. X-ray diffraction pattern of the hydrozincite sample. The center region of the scan is 

expanded to show trace phases. C, calcite; W, willemite; H, hematite; deg, degree.......................24

Figure 11. Hydrozincite unit cell is shown without the hydrogen atoms. The cell is monoclinic; 

zinc occurs along planes through the crystal separated by carbonate groups ................................25

Figure 12. Hemimorphite unit cell; water molecules consist of cross-hatched oxygen and 

blue hydrogen atoms. The cell contains channels that are sufficiently large to allow metal 

atoms and water inside. These channel-fill metals are exchangeable ............................................26

Figure 13. Willemite unit cell showing the rhombohedral form. The channels formed in the 

structure are not large enough to allow metal atoms.......................................................................27

Figure 14. Hemimorphite/willemite: A. Transmitted-light photomicrograph of radiating 

hemimorphite crystals in a cross section of a botryoidal mass. B. Backscatter electron

image of intergrown hemimorphite and willemite. Iron and manganese oxides occur along 

grain boundaries. C. Amorphous iron and manganese alteration/oxidation minerals are host 

to lead and other metals. μm, micrometer ......................................................................................28

Figure 15. X-ray diffraction pattern for hemimorphite/willemite specimen. Calcite and hematite 

are also identified in the pattern. The center region of the scan is expanded to show trace 

phases. H (red), hemimorphite; W, willemite; C, calcite; H (green), hematite; deg, degree ............29

Figure 16. (Log) Bulk zinc concentration using inductively coupled plasma–mass 

spectrometry (ICP–MS) for five mineral samples. Mg/kg, milligrams per kilogram..........................32

Figure 17. Bulk major-element profile—five zinc minerals. Mg/kg, milligrams per kilogram.........32

Figure 18. Bulk trace-element profile—five zinc samples. Mg/kg, milligrams per kilogram..........33

Figure 19. Comparison of selected bulk chemistry results for the Creede, Colorado, and 

Balmat, New York, sphalerite samples. Mg/kg, milligrams per kilogram .........................................33

Figure 20. Leachate pH values for five zinc minerals using five leach tests. FLT, 

field leach test; SPLP, synthetic precipitation leaching procedure; TCLP, toxicity 

characteristic leaching procedure....................................................................................................36

Figure 21. Leachate specific conductance (SC) values for five zinc mineral samples using 

five leach tests. FLT, field leach test; SPLP, synthetic precipitation leaching procedure; TCLP, 

toxicity characteristic leaching procedure; μS/cm, microSiemens per centimeter...........................37

Figure 22. Zinc results for sphalerite (Balmat, New York). ICP–MS, inductively coupled 

plasma–mass spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, 

synthetic precipitation leaching procedure; TCLP, toxicity characteristic leaching procedure.........39

Figure 23. Zinc results for sphalerite (Creede). ICP–MS, inductively coupled plasma–mass 

spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic precipitation 

leaching procedure; TCLP, toxicity characteristic leaching procedure ............................................39

Figure 24. Zinc results for hemimorphite. ICP–MS, inductively coupled plasma–mass 

spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic precipitation 

leaching procedure; TCLP, toxicity characteristic leaching procedure ............................................40
vi

Figure 25. Zinc results for smithsonite. ICP–MS, inductively coupled plasma–mass 

spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic 

precipitation leaching procedure; TCLP, toxicity characteristic leaching procedure ........................40

Figure 26. Zinc results for hydrozincite. ICP–MS, inductively coupled plasma–mass 

spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic 

precipitation leaching procedure; TCLP, toxicity characteristic leaching procedure ........................41

Figure 27. Field leach test (FLT) leachate plot A: Graph plot of selected elements and 

their concentration in leachates produced using the 5-minute field-leach test (USGS FLT) 

versus the zinc mineral. SEDEX, sedimentary exhalative deposit; μg/L, micrograms per liter........42

Figure 28. Acid-base accounting (ABA) test results for five zinc-bearing mineral 

samples. Kg, kilograms ...................................................................................................................44

Figure 29. Bioaccessibility of zinc (Zn) for Balmat, New York, and Creede, Colorado, 

sphalerite samples measured in simulated gastric fluid (SGF), simulated intestinal fluid 

(SIF), simulated lung fluid (SLF), and simulated phagolysosomal fluid (SPF) presented 

as the percent of the solid soluble in the fluid..................................................................................51

Figure 30. Bioaccessibility of zinc (Zn) for hemimorphite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid ....................52

Figure 31. Bioaccessibility of zinc (Zn) for hydrozincite/willemite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid. ...................52

Figure 32. Bioaccessibility of zinc (Zn) for smithsonite sample measured in simulated gastric 

fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid ....................53

Figure 33. Bioaccessibility of arsenic (As) for Balmat (Bal) and Creede sphalerite samples 

measured in simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid 

(SLF), and simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble 

in the fluid. RL, reporting level.........................................................................................................53

Figure 34. Bioaccessibility of arsenic (As) for hemimorphite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid ....................54

Figure 35. Bioaccessibility of arsenic (As) for hydrozincite/willemite sample measured in 

simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and 

simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid....54

Figure 36. Bioaccessibility of arsenic (As) for smithsonite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid ....................55

Figure 37. Bioaccessibility of cadmium (Cd) for Balmat (Bal) and Creede sphalerite samples 

measured in simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid 

(SLF), and simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble 

in the fluid. RL, reporting level.........................................................................................................56

Figure 38. Bioaccessibility of cadmium (Cd) for hemimorphite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid ....................56

Figure 39. Bioaccessibility of cadmium (Cd) for hydrozincite/willemite sample measured in 

simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and 

simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid....57
vii

Figure 40. Bioaccessibility of cadmium (Cd) for smithsonite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid ....................57

Figure 41. ß-galactosidase catalyzed hydrolysis of chlorophenol red-ß-galactopyranoside 

(Bitton and Koopman, 1997; Rossel and others, 1997; and Ward and others, 2005) .....................60

Figure 42. A developed 96-well MetPLATETM bioassay showing negative (toxic) and positive 

(nontoxic) controls and dilution series for several water samples ...................................................61

Figure 43. Percent inhibition of serial dilutions of the U.S. Geological Survey-field leach test 

solution compared to the zinc concentrations (in micrograms per liter) for the mineral 

hemimorphite. EC50, half-maximal effective concentration..............................................................63

Figure 44. Percent inhibition of serial dilutions of the U.S. Geological Survey-field leach

test solution compared to the zinc concentrations (in micrograms per liter) for the mineral 

hydrozincite. EC50, half-maximal effective concentration ................................................................64

Figure 45. Percent inhibition of serial dilutions of the U.S. Geological Survey-field leach test 

solutions compared to the zinc concentrations (in micrograms per liter) for the minerals 

smithsonite, sphalerite (Balmat, New York), and sphalerite (Creede, Colorado). EC50, 

half-maximal effective concentration...............................................................................................65

Figure 46. Percent inhibition of serial dilutions of the U.S. Geological Survey-field leach test 

solution compared to the zinc concentrations (in micrograms per liter) for duplicate leaches of 

sphalerite (Balmat, New York). EC50, half-maximal effective concentration ....................................66

Figure 47. Percent inhibition of serial dilutions of the U.S. Geological Survey-field leach test 

solution compared to the zinc concentrations (in micrograms per liter) for the mineral 

hydrozincite; the five dilutions nearest 50-percent inhibition with the fitted logarithmic curve. 

EC50, half-maximal effective concentration .....................................................................................67

Figure 48. Percent inhibition of serial dilutions of the U.S. Geological Survey-field leach test 

solution compared to the zinc concentrations (in micrograms per liter) for the minerals 

smithsonite, sphalerite (Balmat, New York), and sphalerite (Creede, Colorado); the seven 

(smithsonite), six (sphalerite (Creede)), or ten (sphalerite (Balmat)) dilutions nearest 50-percent 

inhibition with the fitted logarithmic curve. EC50, half-maximal effective concentration ...................68

Tables

Table 1. Microprobe data for zinc minerals ................................................................................... 8

Table 2. Semiquantitative mineralogy using whole-pattern-fit method. Mineral percentages 

reported as percentage of crystalline components. Amorphous content reported as 

percentage of bulk specimen ..........................................................................................................14

Table 3. Total concentration, leachate concentration, and bioaccessibility of 

selected trace metals ......................................................................................................................47

Table 4. Summary of the EC50 (half-maximal effective concentration) values and 100-percent 

leachate concentrations for zinc, cadmium, lead, calcium, magnesium, sulfate chlorine, and pH 

(from P. Hageman, this volume) for five zinc-bearing minerals and diluent.....................................69
viii

Initialisms and Acronyms

ASTM American Society for Testing and Materials International

CVAFS cold-vapor atomic fluorescence

DI deionized

EC50 half-maximal effective concentration

EDS energy-dispersive spectroscopy

EMPA electron microprobe analysis

FLT field leach test

HDPE high-density polyethylene

IC ion chromatography

ICP–AES inductively coupled plasma–atomic emission spectrometry

ICP–MS inductively coupled plasma–mass spectrometry

IVBA in vitro bioaccessibility

NAP net acid production

RL reported level

SC specific conductance

SEDEX sedimentary exhalative deposit

SEM scanning electron microscope

SGF simulated gastric fluid

SIF simulated intestinal fluid

SLF simulated lung fluid

SPF simulated phagolysosomal fluid

SPLP synthetic precipitation leaching procedure

TCLP toxicity characteristic leaching procedure

USEPA United States Environmental Protection Agency

XRD X-ray diffraction
1

Assessment of the Geoavailability of Trace 

Elements from Selected Zinc Minerals

By Rhonda L. Driscoll, Phillip L. Hageman, William M. Benzel, Sharon F. Diehl, Suzette Morman, 

LaDonna M. Choate, and Heather Lowers

Abstract

This assessment focused on five zinc-bearing minerals. The minerals were 

subjected to a number of analyses including quantitative X-ray diffraction, optical 

microscopy, leaching tests, and bioaccessibility and toxicity studies. Like a previous 

comprehensive assessment of five copper-bearing minerals, the purpose of this 

assessment was to obtain structural and chemical information and to characterize the 

reactivity of each mineral to various simulated environmental and biological conditions. 

As in the copper minerals study, analyses were conducted consistent with widely 

accepted methods. Unless otherwise noted, analytical methods used for this study were 

identical to those described in the investigation of copper-bearing minerals.

Two sphalerite specimens were included in the zinc-minerals set. One sphalerite 

was recovered from a mine in Balmat, New York; the second came from a mine in 

Creede, Colorado. The location and conditions of origin are significant because, as 

analyses confirmed, the two sphalerite specimens are quite different. For example, data

acquired from a simulated gastric fluid (SGF) study indicate that the hydrothermally 

formed Creede sphalerite contains orders of magnitude higher arsenic, cadmium, 

manganese, and lead than the much older metamorphic Balmat sphalerite. The SGF and 

other experimental results contained in this report suggest that crystallizing conditions 

such as temperature, pressure, fluidization, or alteration processes significantly affect 

mineral properties—properties that, in turn, influence reactivity, solubility, and toxicity.

The three remaining minerals analyzed for this report—smithsonite,

hemimorphite, and hydrozincite—are all secondary minerals or alteration products of 

zinc-ore deposits. In addition, all share physical characteristics such as tenacity, density, 

streak, and cleavage. Similarities end there. The chemical composition, unit-cell 

parameters, acid-neutralizing potential, and other observable and quantifiable properties 

indicate very different minerals. Only one of each of these minerals was studied. Had this 

assessment included multiples of these minerals, geochemical and mineralogical 

distinctions would have emerged, similar to the results for the two sphalerite specimens.

Introduction

Curiosity about minerals dates back to the earliest humans. With only eyes, 

tongues, and scratching tools, Paleolithic and indigenous peoples assessed the importance 

of minerals and discovered practical uses for them. Could a mineral be fashioned into a 
2

weapon, a tool, a talisman, or an ornament? The science of today is just as curious about 

minerals and just as interested in their importance. Does a mineral generate acid? As an 

airborne particle, does it have the potential to impair the human respiratory system? How 

soluble is it? How is it structured at the microscopic level? This report is one in a series 

that attempts to answer these questions using contemporary experimental techniques and 

technologies. 

Five zinc-bearing minerals were selected for this study. To determine the purity of 

the selected minerals, we relied on X-ray diffraction analysis (XRD) to define and 

quantify the composition of each mineral. Electron microscopy was used for threedimensional imaging of the zinc samples. Various leach tests were performed to obtain 

information about solubility, alkalinity, pH, specific conductance, and element 

mobilization. Experiments to determine the absorption potential of a suite of elements 

and the effect of liberated elements on microorganism reproduction were also included. 

These laboratory tests and results are discussed in subsequent separate sections of this 

report.

In general, data collected from this study indicates that zinc, as a discrete element, 

may not be harmful to terrestrial life. The potential health risks associated with zincbearing minerals are more likely linked to other elements such as lead, cadmium, and 

arsenic present in the zinc minerals selected for this study. Our findings suggest that zincbearing minerals from ore deposits or weathered sites warrant comprehensive analyses to 

determine if elements injurious to plant and animal life are present.
3

Sample Preparation

By Rhonda L. Driscoll

Mineral Descriptions and a Brief Discussion of the Franz Magnetic Separator

Introduction

Five zinc-bearing minerals were purchased from a commercial supplier, and each 

was reduced to grains and powder for distribution to a team of analysts. The sample 

preparation process is a critical component of materials characterization. Samples must 

be reduced to specified size fractions and must be free of contamination to ensure that 

subsequent analyses yield reliable data.

Sample Preparation

A zinc-bearing set of mineral specimens was prepared for distribution and 

analyses. Preparation details can be found in Driscoll and others (2012). A general 

overview of the preparation method and details not included in Driscoll and others (2012)

are described in this report. The zinc-bearing minerals were selected based upon 

chemistry, variations in type, representative location, monomineralic appearance, and 

gross weight. The selected specimens included the following:

• Smithsonite (ZnCO3, zinc carbonate)—Chihuahua, Mexico; San Antonio Mine, 

replacement deposit.

• Sphalerite ((Zn, Fe)S, zinc sulfide)—Creede, Colorado; Commodore Mine, vein 

deposit.

• Sphalerite ((Zn, Fe)S, zinc sulfide)—Balmat, New York; St. Joe No. 3 Mine, buried 

sulfide ore body or sedimentary system. 

• Hemimorphite (Zn4Si2O7(OH)2·H2O, zinc silicate)—Chihuahua, Mexico; Potosí 

Mine, mineralized vein system.

• Hydrozincite (Zn5(OH)3(CO3)2, hydrated zinc carbonate)—Dragoon, Arizona; TexasArizona Mine, complex polymetallic zone.

In order to separate the zinc phase from matrix or secondary minerals, some 

special processing was required. An early project decision to avoid use of organic 

reagents to achieve separation prevented use of a heavy liquid such as methylene iodide. 

Consequently, the Frantz magnetic separator was identified as the best means of phase 

separation/concentration. 

After hand specimens were crushed to ≤2 mm (millimeter) using a porcelain 

mortar and pestle, a one-half-gram split of each was powdered and X-rayed for phase 

confirmation only (complete qualitative and quantitative X-ray diffraction data for each 

mineral is detailed in the section that follows). Phases were identified using a Shimadzu 

6000 X-ray diffractometer. XRD analyses revealed the presence of associated, secondary, 

or trace phases in each zinc-bearing specimen:
4

• Smithsonite/Chihuahua—minor hemimorphite, minor to trace smectite (hydrous 

aluminum phyllosilicate with variable amounts of iron, magnesium, alkali metals, 

alkaline earths, and other cations), and clinochlore (Mg5Al)(AlSi3)O10(OH)

• Sphalerite/Creede—minor smithsonite, hemimorphite, quartz (SiO2), and clinochlore 

• Sphalerite/Balmat—trace quartz

• Hemimorphite/Chihuahua—major willemite ((Zn, Co)2SiO4) and hematite (Fe2O3))

• Hydrozincite/Dragoon—major willemite

Frantz Magnetic Separator

The Frantz magnetic separator is most effective when used to separate small (250 

to 180 microns (0.25 to 0.18 mm, 60 to 80 mesh ASTM [American Society for Testing 

and Materials International] E specification)) rounded or equidimensional particles of 

near- or uniform size into magnetic and nonmagnetic mineral fractions. Mineral 

concentrates in this size range are preferred and are usually intended for chemical 

analyses. Coarser grained metalliferous ores—250 to 500 microns (0.50 mm, 35 mesh 

ASTM E specification)—have been effectively processed using the Frantz (R. Tripp, 

USGS, oral commun., 2009). Particles greater than 500 microns are rarely processed on a 

Frantz separator because they tend to be compound, angular, and generally behave 

unpredictably on the Frantz. Therefore, magnetically separating grains of approximately 

2 mm (2,000 microns, 10 mesh ASTM E specification)—a size requirement of some 

specific analyses—required modification of the standard magnetic separation method.

Ordinarily, the Frantz is setup with the magnet positioned at 15° on the horizontal 

axis, the bent-stem thistle funnel (also called the hopper) tilted to 15° on the vertical axis,

adjusted to a particle feed rate of between 50 and 70 (minimum 10, maximum 100), and 

track vibration initially set at about 50 (minimum 10, maximum 100). This ordinary setup 

was incompatible with the very coarse ≥2-mm material. The coarse material jammed in 

the narrow funnel stem, flooded onto the separation tracks when the jams were loosened, 

and overflowed, bounced, or stalled on the tracks. The Frantz was modified to produce 

steady, regulated delivery of the coarse material and steady, measured movement of 

material down the length of the tracks in the following manner. The bent-stem thistle 

funnel was replaced by a slightly wider straight-stem thistle funnel positioned at 35°

perpendicular to the tracks. The wider, straight stem prevented particle jamming, and the 

flatter angle slowed the gravitational movement of the coarse grains. This modification 

improved grain mobility; material flowed steadily from the hopper, and magnetic and 

nonmagnetic particles separated and moved down their respective tracks. Nonmagnetic 

zinc-bearing grain separates, or concentrates, were examined with a microscope; zincmineral grains appeared to dominate the concentrates by about a 4:1 ratio or greater. 

Nonmagnetic compound or quartz grains were seen most often in the zinc concentrates.

Frantz Magnetic Separator Results

As reported by Rosenblum and Brownfield (1999), the zinc-bearing minerals 

smithsonite and hemimorphite display paramagnetism1 at a magnetic field strength 

greater than 1.7 amperes (A). The Frantz used for separation in this study has a maximum 

 1

The property of being weakly attracted to either pole of a magnet. Paramagnetic materials become slightly 

magnetized when exposed to a magnetic field but lose their magnetism when the field is removed.
5

magnetic field strength of 1.5 A. The maximum setting was applied to the Chihuahuan 

smithsonite effectively segregating the paramagnetic smectite and clinochlore from the 

essentially nonmagnetic zinc-bearing phases. Splits of the nonmagnetic phases were 

submitted for analyses as a combination smithsonite-hemimorphite.

The Creede sphalerite specimen was also processed on the Frantz to separate the 

zinc-bearing phases from clinochlore. At a setting of 0.8 A, paramagnetic clinochlore was 

removed from the sample leaving concentrated zinc minerals and quartz. The 

nonmagnetic zinc minerals mixture, consisting of major sphalerite and minor smithsonite 

and hemimorphite, was distributed to analysts with no attempt made to separate quartz. 

Quartz is a minor constituent, and its removal was not considered critical. 

The Balmat sphalerite was nearly pure; it contained insignificant quartz. It was 

crushed, sieved, and distributed without additional mechanical processing.

The Chihuahuan hemimorphite sample consisted of a strongly magnetic phase 

(hematite) and an uncertain magnetic phase (willemite). The magnetic susceptibility of 

willemite is reported in Rosenblum and Brownfield (1999) as either weakly magnetic 

(0.30 A) or nonmagnetic (>1.70 A)

2

. In this particular sample, willemite displayed no 

magnetic susceptibility. The hematite, however, was easily separated from the 

hemimorphite-willemite at a Frantz setting of 0.10 A. Analysts were given splits of a 

mixture containing about 70 percent hemimorphite and 30 percent willemite. Percentages 

were visually estimated with a binocular microscope. 

The last mineral in this set, hydrozincite, was not processed beyond crushing, 

grinding, and sieving. XRD analysis indicated major hydrozincite-minor willemite. Splits 

of this mineral mixture were distributed to analysts with a note regarding composition. 

The post-Frantz zinc-bearing concentrates were split and distributed according to 

analytical needs. Analyses included X-ray diffractometry, optical microscopy and 

scanning electron microscopy (SEM) examinations, bulk and leach chemistry, and 

bioaccessibility and toxicity investigations. Analytical methods are briefly described in 

this report, including any modifications. Detailed descriptions of analytical methods are 

reported in Driscoll and others (2012). Complete analytical data for the zinc-minerals set 

is reported in appendixes 2 through 4.

 2

If Zn2+ is replaced by Mn2+ and Fe2+ in the willemite structure, willemite becomes weakly magnetic. The 

Mn-Fe willemite variation is called troostite. Because the sample willemite was not susceptible to the 

magnet, minimal to no cation replacement was assumed. 
6

Mineralogical Analyses

By William B. Benzel, Sharon F. Diehl, and Heather Lowers

X-ray Powder Diffraction and Microanalytical Studies: Thin Section, Scanning Electron Microscopy, 

and Electron Microprobe Analyses Micrographs

X-ray Diffraction Instrument Setup and Specifics: Appendix 1

Introduction

Mining companies and regulatory agencies have long relied on net carbonate 

calculations to determine the amount of carbonate overburden necessary to prevent acidrain leaching of waste piles. More recently, work has been completed to measure specific 

acid-generating reactions from bulk materials that lead to release of metals into the 

environment (Smith, 2007). The next step necessary to understand metal mobilization is 

to look at the individual minerals in the waste pile and determine which mineral phases 

are the major contributors. Studies within these sections mineralogically and chemically 

characterize a suite of zinc minerals with the intent to gain insights into the relative 

mobility of zinc and associated elements by various pathways that occur in the 

environment or the human body.

The bioaccessibility of elements depends upon weathering factors and the specific 

mineral source. For example, the United States Environmental Protection Agency 

(USEPA) solid-species studies of lead in monitored blood levels in juvenile swine show 

widely varying bioaccessibility of lead between cerrusite (PbCO3), anglesite (PbSO4),

and galena (PbS). Although lead has been extensively studied, very little is known about 

other metal-bearing minerals of industrial importance. The bulk composition of ore 

minerals and the resident sites of minor and trace elements within minerals are essential 

to understanding trace-element mobility under various weathering conditions and oredeposit types.

A previous report on the characterization of concentrated copper “monominerals” 

showed there is a disparity in metal and anion release from similar minerals due to 

different structural settings, ore-deposit types, trace-element content, or associated 

mineral assemblages microscopically intergrown in the primary mineral phase (Diehl and 

others, 2012; Driscoll and others, 2012). 

Knowledge of the speciation of elements is important, especially for elements of 

concern for regulatory standards. Ore minerals are complex with most minerals hosting 

many individual elements. The bioaccessibility of a specific metal or element is 

dependent on the mineralogic source. Minerals in different ore-deposit types host 

different elements; therefore, sphalerite was examined from two different ore-deposit 

types—a vein system and a sedimentary system—in order to contrast trace-element 

chemistry in different geological settings that underwent different formation conditions 

and processes. Leachate and bioaccessibility studies (later sections) record different 

chemical behavior between the two sphalerite samples; this section on mineralogy 

explains the chemical residence and content of trace metals that contribute to those 

differences.
7

The zinc-bearing minerals, sphalerite (ZnS), smithsonite (ZnCO3), hydrozincite 

(Zn5(CO3)2(OH)6), and hemimorphite/willemite mixture (Zn4Si2O7(OH)2·H2O)/(Zn2SiO4) 

were selected in order to understand weatherability and potential to release constituents 

into the environment. Sphalerite is a primary zinc ore that forms under a wide 

temperature range by hydrothermal processes in sedimentary rocks such as sandstone or 

limestone, in veins cross-cutting coals, and in igneous and metamorphic rocks. Sphalerite 

forms as euhedral crystals to botryoidal, fibrous, or massive morphologies. 

Hemimorphite, hydrozincite, smithsonite, and willemite are all alteration products that 

form in the oxidation zone of zinc ore deposits. These alteration minerals typically occur 

as botryoids, which are agglomerated spheres composed of radiating crystals. 

Precipitation of botryoidal shapes is a more rapid formation process than precipitation of 

well-ordered euhedral crystals, and therefore, botryoids may incorporate elevated 

concentrations of trace elements such as copper and (or) iron or other elements. 

The minerals were examined by X-ray powder diffraction, petrographic 

microscope, scanning electron microscope (SEM), and electron microprobe analysis

(EMPA) to (1) identify basic mineralogy and associated structural and textural features, 

(2) calculate the unit cell of the zinc mineral and estimate the amorphous content of the 

sample, (3) quantify trace-element content associated with the main mineral phase, and 

(4) determine the residence, or mode of occurrence, of trace elements, that is, whether 

trace metals are homogeneously distributed within the mineral or associated with mineral 

inclusions. By utilizing microanalytical techniques, the residence sites of major, minor, 

and trace elements can be determined but only at the detection limits of each of the 

instruments. The microanalytical study complements both the XRD data by validating 

major mineralogy results as well as showing the presence of accessory minerals that are 

below XRD detection limits, and the leachate studies by showing the exact mineralogic 

residence sites of minor to trace elements that are released into solution.

Methods

An effort was made to pick a clean sample after crushing, but minor amounts of 

accessory minerals are present that conventional methods of mineral separation do not 

remove (Driscoll, this volume). Detailed mineral (X-ray and SEM) and chemical (EPMA 

and leachate) studies were used to identify micro-textural and structural characteristics 

and elemental distribution within the zinc minerals.

Polished standard thin sections of the five zinc-bearing mineral samples were 

examined in reflected-light microscopy and with a JEOL 5800LV scanning electron 

microscope equipped with a Thermo NORAN silicon-drift energy-dispersive 

spectroscopy (EDS) detector to determine basic mineralogy, identify textures or any 

structural defects, analyze qualitatively and semiquantitatively trace- and minor-element 

content, and relate trace-metal distributions to mineral phases. The SEM has an element 

detection limit of approximately 500–1,000 parts per million (ppm). Backscatter scanning 

electron imagery was used to help identify and separate the intergrown zinc phases. More 

dense phases, as well as heavy metals such as lead, appear bright white against less dense 

phases, which appear in shades of gray. 

Concentrations of trace metals in the primary mineral phases were determined 

with a JEOL JXA-8900 EMPA (table 1). The results reported in table 1 are from select 

areas of a magnified image where nearly pure mineral phases were analyzed.
8

Table 1. Microprobe data for zinc minerals.

[Wt%, weight percent, No, number] Sulfide

No Analyses Zn WT% Cu WT% Fe WT% Ni WT% Co WT% Mn WT% Cr WT% Mg WT% Na WT% Ca WT% Sb WT% Cd WT% Ag WT% Pb WT% S WT% Sr WT% Total

Sphalerite Creede 3 66.58 0.04 0.38 0.01 BDL 0.03 0.01 0.04 BDL BDL BDL 0.45 BDL BDL 32.85 0.08 100.29

ideal stoichiometry 67.10 32.90 100.00

No Analyses Zn WT% Cu WT% Fe WT% Ni WT% Co WT% Mn WT% Cr WT% Mg WT% Na WT% Ca WT% Sb WT% Cd WT% Ag WT% Pb WT% S WT% Sr WT% Total

Sphalerite Balmat 3 59.60 BDL 6.67 0.05 0.01 0.12 BDL 0.03 BDL BDL BDL 0.10 BDL BDL 33.09 0.07 99.66

ideal stoichiometry 67.10 32.90 100.00

No Analyses Zn WT% Ca WT% Cd WT% Co WT% Pb WT% Ag WT% Fe WT% Ni WT% Cu WT% As WT% Mn WT% C WT% O WT% H WT% Total

Smithsonite (bright) 36 51.55 0.12 NM 0.02 0.11 BDL 0.00 0.00 0.02 BDL 0.08 9.58 38.22 0.00 99.70

Smithsonite (medium) 50 50.04 1.37 NM 0.02 0.05 BDL 0.00 0.00 BDL BDL 0.08 9.58 38.34 0.00 99.47

Smithsonite (dark) 48 48.92 1.32 NM 0.12 BDL BDL 0.00 0.00 BDL BDL 0.08 9.58 38.07 0.00 98.09

Smithsonite (all) all (134) 50.05 1.02 NM 0.05 0.03 BDL 0.00 0.00 BDL BDL 0.08 9.58 38.21 0.00 99.02

ideal stoichiometry 52.15 9.58 38.28 100.01

No Analyses Si WT% Zn WT% Cd WT% Pb WT% Al WT% Fe WT% Ni WT% Cu WT% Mn WT% Mg WT% Ca WT% Sr WT% Na WT% S WT% O WT% H WT% C% Total

Hydrozincite 7 0.11 42.35 0.00 1.50 0.01 0.01 0.00 0.43 0.01 0.19 0.32 0.00 0.74 0.02 11.27 0.00 NM 56.96

ideal stoichiometry 59.55 34.97 1.1 4.38 100

No Analyses Zn WT% Si WT% Cd WT% Pb WT% Al WT% Fe WT% Ni WT% Cu WT% Mn WT% Mg WT% Ca WT% Sr WT% Na WT% S WT% O WT% H WT% Total

Hemimorphite 3 60.56 13.27 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.03 0.00 0.00 0.00 29.98 0.00 103.87

ideal stoichiometry 54.29 11.66 33.21 0.84 100.00

No Analyses Zn WT% Si WT% Cd WT% Pb WT% Al WT% Fe WT% Ni WT% Cu WT% Mn WT% Mg WT% Ca WT% Sr WT% Na WT% S WT% O WT% H WT% Total

Willemite 3 52.01 13.08 0.00 0.00 0.00 0.02 0.01 0 4.50 0.12 0.01 0.00 0.00 0.00 29.04 0.00 98.79

ideal stoichiometry 58.68 12.6 28.72 100.00

Silicate Carbonate Sulfide
9

The average chemical composition represents the mineral of interest, not the bulk 

specimen chemistry. There has been an increasing awareness that element associations and 

residence sites of trace metals in minerals are important in determining weathering behavior, 

not just the concentration of a trace element within a mineral (Diehl and others, 2006, 2007); 

therefore, a digital element map, showing the spatial distribution of zinc and cadmium in 

smithsonite, was generated on the electron probe to demonstrate the common zoning of trace 

metals in ore minerals. 

Powder XRD analysis was employed to make two measurements on each specimen. 

First, the scan was interpreted to identify mineral phases present in the sample. The second 

measurement determined the unit-cell parameters of the zinc-bearing mineral. The unit cell of 

each specimen is compared to the calculated unit cell of an idealized pure end-member 

mineral (a chemically pure mineral with no substitutions). The difference between the 

calculated and actual mineral unit cells reflects chemical substitutions and strains and stresses 

in the crystal structure. These factors influence the stability, reactivity, and solubility of the 

mineral.

A less than 75-micrometer (μm)-size separate of each mineral was provided for 

analysis. This material was further reduced in size to approximately 50 μm using an agate 

mortar and pestle. Two grams (if available) of each hand-ground powder was mixed with 

20 mL (milliliter) of propanol and micronized in a McCrone mill for 4 minutes. The milled 

particles were less than 5 μm in size. After drying in air, the micronized powder was 

disaggregated by lightly grinding in a mortar and pestle. Next, an appropriate amount of 

corundum (NOAH Technologies Corporation 1-micron alumina) was added to the sample to 

yield a mixture with 10-percent-by-weight corundum as an internal standard. The mixture was 

transferred to a plastic vial containing two polystyrene balls; the vial was shaken for 

10 minutes in a Spex Mixer mill. The blended powder was then sieved through a 250-μm 

screen yielding a fluffy powder made up of aggregates of the fine particles. To minimize 

preferred orientation, the sieved material was side packed into a Scintag XRD sample holder.

X-ray diffraction scans were collected on a Panalytical X’Pert diffractometer with 

Bragg Brentano geometry, theta-theta motion with a strip detector set to collect only Kα

(strongest X-ray spectral line) radiation. The instrument set-up parameters and calibration are 

listed in appendix 1. 

Discussion and Results

Sulfide Mineral—Sphalerite (ZnS)

Sphalerite has a zinc ion surrounded by six sulfur atoms arranged in tetrahedral 

coordination (fig. 1).
10

Figure 1. Sphalerite unit cell showing zinc and sulfur sites. Metal substitutions occur at the zinc sites.

Sphalerite almost always contains some iron as well as other metal substitutions for 

the zinc in the structure, so much so that sphalerite is a major ore for Zn as well as Cd, In, Ge, 

and Ga (Benedetto, 2005). Natural sphalerite has been reported with metal substitutions 

including individual substitutions of up to 26 percent Fe, 5.8 percent Mn, or 1.7 percent Cd as 

well as numerous other elements including Pb, Cu, Hg, In, Ga, Tl, Au, Ag, Sb, and Sn 

(Palache and others, 1961). Lead and copper are observed in natural sphalerite, both as 

intergrowths of chalcopyrite and galena and as substitutions in the sphalerite structure.

In addition to the secondary zinc minerals studied in this suite, alteration of sphalerite 

by oxidation also yields goslarite (ZnSO4·7H2O) and (or) gunningite (Zn(SO4)(H2O)). 

Sphalerite can be zoned, and recent studies have shown that some metal substitutions are 

coupled, such as iron and manganese. Zoned sphalerite may generate cyclic metal releases 

during weathering reflecting each zone’s chemistry. An example of cyclic release can be 

found in Benedetto (2005) who reported high iron–low manganese followed by low iron–high 

manganese releases.

Sphalerite Specimen—Creede, Colorado

Sphalerite from Creede, Colorado, is hosted in a vein system and acts as cement 

around breccia fragments in the sample studied (fig. 2A). X-ray diffraction shows the host 

rock is quartz and orthoclase (fig. 2B). Accessory minerals include silver-bearing galena and Sphalerite (ZnS)

Space Group

Cubic

‒

F43M

a, b, and c Alpha, Beta,

and Gamma Cell Volume

ideal 5.415 90.000 158.74

observed range 5.318‒5.687 synthetic

5.401‒5.687 natural 150.4‒183.9

Creede 5.409 90.000 158.25

Balmat 5.410 90.000 158.34
11

pyrite (FeS2) as well as oxidation products of these accessory sulfide minerals such as 

anglesite (PbSO4) and amorphous iron oxides, respectively (fig. 2C). Pyrite is deeply etched 

and altered to amorphous iron sulfate and oxides, which are sinks for adsorbed metals; here 

the amorphous alteration by-products of oxidized pyrite are host to metals such as lead and 

arsenic, which were detected by SEM/EDS (fig. 2D). Anglesite is not considered to be a very 

soluble mineral, but the anglesite is porous and fine grained (figs. 2C and 2D), which offers a 

greater surface area for further alteration and dissolution.

Figure 2. A. Photograph of a sphalerite hand sample from Creede, Colorado. Sphalerite is a vein-filling 

cement in brecciated host rock. B. Scanning electron image showing sphalerite intergrown with 

quartz and potassium feldspar (kspar). Galena is common as micron-size inclusions. C. Scanning 

electron micrograph showing intergrown sphalerite, pyrite, and galena in veins. Galena (PbS) has 

partially oxidized to anglesite (PbSO4). D. Scanning electron micrograph of dissolution-etched pyrite 

partially altered to oxidation minerals. Galena is altered to lead sulfate (anglesite). μm, micrometer; 

cm, centimeter.

The XRD scan of the specimen with the phases identified is presented in figure 3.
12

Figure 3. X-ray diffraction pattern of the Creede, Colorado, sphalerite identifies the minerals present. 

The center region of the scan is expanded to show trace phases. S, sphalerite; G, galena; Q,

quartz; O, orthoclase; P, pyrite; C, chlorite; deg, degree.
13

The refinement of the mineralogy shows the bulk sample is composed of 85

percent sphalerite, 7 percent quartz, 4 percent orthoclase, 3 percent pyrite, 1 percent

chlorite and less than 1 percent galena. In addition, the refinement indicates the sample 

contains 7-percent amorphous component not included in the semiquantitative 

mineralogy (table 2).
14

Table 2. Semiquantitative mineralogy using whole-pattern-fit method. Mineral percentages reported as percentage of crystalline components. 

Amorphous content reported as percentage of bulk specimen.

[wt%, weight percent]

values in wt%

CaCO3CaFPbSFeO(OH)Fe2O3Zn4Si2O7(OH) 2·H2OZn5(CO3)2(OH)6KAlSi3O8FeS2ZnCO3ZnS

Zn2SiO4SiO2

Specimen calcite

clay (kaolinte 

or chlorite) fluorite galena goethite hematite hemimorphite hydrozincite orthoclase pyrite smithsonite sphalerite willemite quartz total

amorphous 

content

Sphalerite Creede 1 <1 4 3 85 7 100 7

Sphalerite Balmat 3 80 17 100 11

Smithsonite <1 7 92 <1 1 100 10

Hydrozincite 1 <1 97 1 99 11

Hemimorphite-Willem 5 <1 4 3 72 <1 16 100 13

Semi-quantitative mineralogy calculated using Whole Pattern Fit method. Mineral percentages reported as 

percentage of crystalline components. Amorphous content report as percentage of bulk specimen.
15

The scan was tested for anglesite, which was observed in the SEM work, but it would 

not refine in the whole-pattern-fit model. There are interferences between the other mineral 

phases and anglesite’s primary reflections making the identification of trace anglesite by XRD 

in this sample dubious. 

The unit cell of the Creede sphalerite refined to be 5.409Å (angstrom) for the cubic-cell 

axial length with 90o axial angles (fig. 1). The calculated unit-cell dimensions for an ideal 

sphalerite are 5.415Å and axial angle of 90°. Natural sphalerite has been reported with axial 

dimensions ranging between 5.401 and 5.687Å with axial angles of 90°. Synthetic sphalerite has 

extended the range from 5.318 to 5.687Å. The Creede sphalerite has a unit cell close to ideal 

suggesting that the sphalerite has limited substitution in the zinc sites of the crystal, and this is 

supported by the microprobe data (table 1) that shows minor to trace substitutions total less than 

1 percent in the zinc sites of the mineral. 

Sphalerite Specimen—Balmat, New York

Sphalerite from Balmat, New York, is from a sedimentary exhalative deposit (SEDEX). 

SEM studies found the minor accessory minerals are quartz, pyrite, ilmenite, and secondary 

alteration oxidation minerals along fractures within the sphalerite (figs. 4A–4C) and along grain 

boundaries.
16

Figure 4. A. Photograph of a massive sphalerite specimen from the Balmat, New York, sedimentary 

exhalative deposit (SEDEX). B. Backscatter scanning electron image showing dark-gray oxidationalteration minerals formed along sphalerite grain boundaries and fractures. C. Scanning electron 

image showing accessory sulfide minerals galena and pyrite in sphalerite. Qtz, quartz; μm, 

micrometer; cm, centimeter.
17

Bulk XRD analysis shows that the specimen is composed of sphalerite and quartz 

(fig. 5). There are two minor reflections not assigned in the XRD scan that possibly could be 

assigned to a nickel pyrite; however, there is not sufficient nickel to support nearly a weight 

percent of nickel pyrite in the sample. There is at least one more phase present that does not 

match any of the accessory minerals found by SEM. The refinement of the XRD scan shows the 

sample is composed of 80 percent sphalerite, 17 percent quartz, and 3 percent clay (a 7Å phase), 

most likely kaolinite, but possibly chlorite (table 2). The amorphous content is calculated to be 

11 percent of the bulk mineralogy exclusive of the semiquantitative mineral estimates.

Figure 5. X-ray diffraction pattern of the Balmat, New York, sphalerite. Sphalerite and quartz are 

observed in the pattern as well as a 7Å (angstroms) clay that is most likely kaolinite but may be 

chlorite. Three other low-intensity reflections are indicated in the figure by their d-spacing (Å) and 

represent one or more trace phases that are not identified. These reflections were evaluated by 

scanning electron microscope analysis, but no certain mineral identifications can be made. Q, quartz;

S, sphalerite; deg, degree.
18

The microprobe results for Balmat show that more than 11 percent of the zinc 

sites in the unit cell are substituted primarily by iron followed by trace substitutions of 

manganese and cadmium (table 1). The unit cell is very similar to Creede, with an axial 

length of 5.410Å and an axial angle of 90°. The 10-percent iron substitution in the zinc 

sites is not sufficient to cause a change in the unit-axial length to shrink the crystal 

volume. Studies on natural sphalerite from the International Centre for Diffraction Data 

and International Centre for Structure Data databases report minimum axial lengths of 

5.401Å. Substitutions of metals with ionic radii larger than zinc cause expansion of the 

cell to larger volumes reported as high as 5.687Å in both natural and synthetic sphalerite.

Carbonate Minerals—Smithsonite (ZnCO3) and Hydrozincite (Zn5(CO3)2(OH)6) 

Smithsonite was originally named calamine and then renamed in the 1800s to 

distinguish it from hemimorphite. Smithsonite forms in the oxidized zone of weathered 

zinc deposits and sometimes as a replacement of other carbonates. Metal impurities in 

smithsonite include Fe, Co, Cu, Mn, Ca, Cd, Mg, and In. Observed zinc substitutions

have been reported for the following: up to 4 percent iron, 1.5 percent calcium (but up to 

9 percent has been reported from Marien Mine, Silesia, Poland), 4.8 percent copper, 

7 percent manganese, 1 percent cadmium, or 1 percent magnesium. A limited solid 

solution exists between smithsonite and rhodochrosite (manganese end-member) and 

between smithsonite and siderite (iron end-member).

Smithsonite Specimen

The unit cell for smithsonite is hexagonal with carbonate groups alternating with 

zinc atoms (fig. 6).
19

Smithsonite ZnCO3

Space Group

Trigonal -

Hexagonal 

‒

R3c

a b c Alpha Beta Gamma Cell

Volume

ideal 4.653 4.653 15.026 90.00 90.00 120.00 325.32

observed

range

solid

solution

solid

solution

solid

solution 90 90 120

measured 4.667 4.667 15.118 90.00 90.00 120.00 329.28

Figure 6. Smithsonite unit cell. Smithsonite forms a limited solid solution with rhodochrosite 

(MnCO3) and siderite (FeCO3). A wide variety of metals can substitute for zinc in the structure.

The unit cell is close to ideal, even though there is substantial calcium and 

cadmium substitution. The ideal unit cell for calcite is 4.99Å for a- (and b-) and 17.06Å 

for c-axial lengths and a cell volume of 367.8 Å. The carbonate framework can 

accommodate atoms as large as calcium with atomic radius of 0.99Å; thus zinc (0.74Å), 

manganese (0.46Å), and cadmium (0.97Å) fit readily into the structure.

Frost and others (2008) found from Raman spectroscopy studies of smithsonite

that ions smaller than calcium—including cadmium, cobalt, copper, iron, magnesium, 

manganese, and nickel—can substitute for zinc in the structure, whereas ions with radii 

larger than calcium when substituted into the cell forced a transformation to aragonite 

group orthorhombic structure. Ions of interest that are larger than calcium include lead as 

well as strontium and barium.

Figure 7A is a transmitted-light photomicrograph of a cross section across 

botryoids demonstrating the radiating crystals that form these spheroidal masses. This 

texture is also demonstrated in the other secondary oxidation zinc minerals. Secondary 

alteration zinc minerals are commonly intergrown (fig. 7B), and here smithsonite occurs 
20

with hemimorphite. Other accessory minerals are clinochlore and quartz as well as 

manganese oxides hosted along the radiating crystals (fig. 7A).

SEM studies show that the growth zones in smithsonite alternate between 

calcium-rich and cadmium-rich banding (figs. 7C and 7D). Element-distribution maps, 

also known as element-intensity maps, show that zinc is homogeneously distributed 

(fig. 7E), but the trace metals are not. Lead mineralization is preferentially hosted in an 

intergrown zinc silicate, likely willemite (fig. 7F), in which crystals show lead 

mineralization and lead growth banding.

Figure 7. Smithsonite: A. Transmitted-light micrograph of radiating smithsonite crystals and 

accessory iron oxide minerals. B. Backscatter scanning electron image displaying intergrown 

smithsonite (medium gray) and hemimorphite (light gray) with accessory lead carbonate 
21

(bright-white areas) preferentially hosted in hemimorphite. C. Scanning electron micrograph 

showing element compositional zonation in smithsonite. Light-gray bands are cadmium-rich; 

darker bands are calcium-rich. D. Cadmium-element distribution map of area depicted in C. 

Light-purple bands highlight cadmium enrichment, which is concentrated in the outermost 

areas of late-stage crystallization. E. Zinc distribution map of C and D indicating the 

homogeneous distribution of zinc, which contrasts with the zoned distribution of cadmium and 

calcium. F. Backscatter scanning electron image of lead zoning in zinc silicate. Of the three 

intergrown zinc phases present, lead is preferentially hosted in growth zones in willemite. μm, 

micrometer.

EPMA analysis of the specimen identified smithsonite with three levels of 

substitution deemed bright, medium, and dark based on backscatter brightness on the 

microprobe screen (table 1). The change in brightness (from bright to dark) is directly 

correlated to the zinc content in the smithsonite ranging from 51.55 percent zinc for the 

light, 50.04 percent for the medium, and 48.92 percent for the dark (table 1). Cobalt and 

calcium increase from light to dark, whereas lead and cadmium decrease. Based on the 

elemental maps above, calcium and cadmium have an inverse content relation.

An X-ray diffraction scan of the specimen shows that this specimen is primarily 

smithsonite (92 percent) (fig. 8). Accessory minerals include hemimorphite (7 percent), 

quartz (1 percent), willemite (<1 percent), and chlorite (<1 percent). The amorphous 

content calculated from the internal standard is 10 percent (table 2), which likely includes 

the manganese oxides noted in the SEM work.
22

Figure 8. X-ray diffraction pattern of the smithsonite sample. Several other minerals are identified 

in the pattern including hemimorphite, quartz, willemite, and chlorite. The center region of the 

scan is expanded to show trace minerals. S, smithsonite; H, hemimorphite; Q, quartz; W,

willemite; C, chlorite; deg, degree.

Hydrozincite Specimen

Hydrozincite forms in the oxidized zones of zinc deposits, typically an alteration 

of sphalerite or smithsonite. Impurities include iron, copper, lead, and cadmium. Most 

commonly hydrozincite occurs as massive, botryoidal, and (or) reniform incrustation on 

exposed zinc ore. There is a closely related mineral, brianyoungite (Zn3(CO3,SO4)(OH)4),

which has (SO4)

2- groups substituting for (CO3)2-

.

Recent studies have shown that hydrozincite precipitation is promoted by 

biological photosynthetic communities (algae and cyanobacterium) and that heavy metals 

are incorporated into the hydrozincite (Podda, 2000; Lattanzi, 2010a). Further studies 

show that the metals in the hydrozincite are bound in two ways—by direct substitution 

for zinc in the lattice and by forming weakly bound amorphous metal-carbonate 

precipitates on the surface of the mineral (Lattanzi, 2010b). Lead was found in both 
23

binding sites, whereas cadmium was observed only in amorphous surface precipitates on 

hydrozincite. These surface precipitates can easily be leached from the material and will 

thus release first, followed by dissolution and release of metals associated with the 

hydrozincite mineral.

Figure 9A shows the typical radiating crystal pattern of botryoidal secondary 

mineralization, which commonly forms crusts on oxidized surfaces of zinc deposits. 

Intergrown with the hydrozincite are willemite and a cadmium-rich calcite. All three 

intergrown minerals were detected by XRD and SEM. These accessory minerals are not 

insignificant phases, and weathering of these minerals, especially the calcite, will add 

metals and other elements to effluent.

In addition to calcite and willemite, SEM studies revealed the presence of micronsize lead, copper, silver, and antimony phases (fig. 9B). These minerals are hosted within 

the porous fibrous hydrozincite, a texture that is open to fluids and oxygen. The small 

grain size of the accessory minerals offers a large surface area for oxidation and 

alteration.

Figure 9. Hydrozincite: A. Transmitted-light photomicrograph showing a cross section of radiating 

hydrozincite crystals in botryoidal masses. B. Scanning electron image showing porous 

masses of radiating hydrozincite crystals. Hydrozincite is accompanied by calcite and 

accessory lead, silver, antimony, and copper minerals (bright-white spots). μm, micrometer.

The XRD scan and whole-pattern fit refinement show this is another nearly pure 

specimen consisting of 97 percent hydrozincite, 1 percent each calcite and willemite, and 

<1 percent hematite (fig. 10).
24

Figure 10. X-ray diffraction pattern of the hydrozincite sample. The center region of the scan is 

expanded to show trace phases. C, calcite; W, willemite; H, hematite; deg, degree.

The bulk specimen may contain as much as 11 percent amorphous material, 

which is most likely related to the accessory minerals and alteration products of the 

fibrous hydrozincite (table 2). The calculated unit cell (fig. 11) is close to ideal and falls 

within the reported range of axial length and interaxial angles found in the mineral 

structure databases.
25

Hydrozincite 

Zn5(CO3)2(OH)6

Space Group

Monoclinic

C2/m

a b c Alpha Beta Gamma Cell

Volume

ideal 13.620 6.300 5.420 90.00 95.80 90.00 459.20

reported

range

13.45

13.62

6.30

6.31

5.36

5.42 90.00 95.30

95.88 90.00 454.3

462.7

measured 13.578 6.298 5.406 90.00 95.89 90.00 462.29

Figure 11. Hydrozincite unit cell is shown without the hydrogen atoms. The cell is monoclinic; zinc 

occurs along planes through the crystal separated by carbonate groups.

The microprobe results were inconclusive due to the fibrous open nature of the 

specimen. Numerous analysis lines were measured; however, the variation from line to 

line was substantial. Looking at the results in table 1 for hydrozincite, a group of seven 

analysis lines were selected and averaged. The zinc and oxygen values are extremely low 

compared to ideal and suggest that other minerals are present but not included in the 

analysis. The lead (1.5 percent) and copper (0.4 percent) noted in the SEM discussion 

above are present in this analysis (table 1). 

Silicate Minerals—Hemimorphite (Zn4Si2O7(OH)2·H2O) and Willemite (Zn2SiO4)

Hemimorphite is a zinc silicate hydrate typically associated with smithsonite. It 

forms in the oxidizing zone during the weathering of zinc ores. The mineral has two 

distinct habits: the more common being botryoidal crusts on zinc ore and a second form 

of bladed crystals with doubly terminated ends.

Hemimorphite is classified as a sorosilicate, meaning it has double silica 

tetrahedrons, that is, two (SiO4)

4- that share one oxygen making the (Si2O7)6- radical 

forming the backbone of the chain (fig. 12).
26

Hemimorphite

Zn4(SiO7)(OH)2(H O) 2

Space Group

Orthorhombic

Imm2

a b c Alpha Beta Gamma Cell

Volume

ideal 8.370 10.710 5.110 90.00 90.00 90.00 458.30

observed range 8.366 

8.380

10.700

10.728

5.110

5.116 90.00 90.00 90.00 458.2

459.3

measured 8.370 10.722 5.116 90.00 90.00 90.00 459.13

Figure 12. Hemimorphite unit cell; water molecules consist of cross-hatched oxygen and blue 

hydrogen atoms. The cell contains channels that are sufficiently large to allow metal atoms and 

water inside. These channel-fill metals are exchangeable.

Zinc and hydroxyl groups bond with the silicate chain to balance the charge. The 

silicate tetrahedra are linked by (ZnO3(OH))

5-

to form a network that has channels 

running through it (Kolesov, 2006). Water and metals such as cadmium and lead can fill 

the channels and have been shown to be exchangeable (Mao, 2012). Weathering of 

hemimorphite may have an initial release of metals absorbed in the channels followed by 

breakdown of the network along cleavage planes (silicate tetrahedrons) releasing the 

metal oxides.

An anhydrous zinc silicate mineral, willemite (Zn2SiO4), forms at slightly higher 

temperatures and pressures. Experimentally the dehydration from hemimorphite to 

willemite + H2O occurs near 210 °C (Roy, 1956); however, the reaction may take place at 

lower temperature (near-surface conditions) in natural systems (Markham, 1960).

Zinc substitutions in the willemite structure include major to minor amounts of

manganese, magnesium, iron, and copper. Zinc occupies two symmetrically distinct 

tetrahedra (fig. 13) that differ slightly in size. Metals, especially manganese, prefer to 

substitute in the larger tetrahedron (Harper and Griffen, 2005).
27

Willemite Zn2SiO4

Space Group

Trigonal -

Rhombohedral

‒

R3

a b c Alpha Beta Gamma Cell

Volume

ideal 13.971 13.971 9.334 90.00 90.00 120.00 1577.76

measured 13.926 13.926 9.301 90.00 90.00 120.00 1562.07

Figure 13. Willemite unit cell showing the rhombohedral form. The channels formed in the 

structure are not large enough to allow metal atoms.

Hemimorphite/Willemite Specimen

Figure 14A is a transmitted-light photomicrograph showing radiating crystal 

morphology, red iron oxide phases in growth zones within hemimorphite, and amorphous 

yellow iron phases, probably goethite, at intergrown grain contacts. Hemimorphite and 

willemite are intricately intergrown zinc phases (fig. 14B). Iron, manganese, and lead 

oxides preferentially form along partially dissolved grain boundaries (figs. 14B and 14C); 

these alteration minerals may also form as layers of botryoids. Calcite is also interstitial 

at grain boundaries.
28

Figure 14. Hemimorphite/willemite: A. Transmitted-light photomicrograph of radiating

hemimorphite crystals in a cross section of a botryoidal mass. B. Backscatter electron image of 

intergrown hemimorphite and willemite. Iron and manganese oxides occur along grain 

boundaries. C. Amorphous iron and manganese alteration/oxidation minerals are host to lead

and other metals. μm, micrometer.

Hemimorphite and willemite are orthorhombic and rhombohedral, respectively 

and have XRD patterns with many sets of repeating planes; that is, the patterns are busy 

and identification of trace components can be difficult due to interferences from the 

primary minerals (fig. 15). The refinement of the pattern included the following 

mineralogy: hemimorphite (72 percent), willemite (16 percent), calcite (5 percent), 

geothite (4 percent), hematite (3 percent), smithsonite (<1 percent), and fluorite 

(<1 percent).
29

Figure 15. X-ray diffraction pattern for hemimorphite/willemite specimen. Calcite and hematite are 

also identified in the pattern. The center region of the scan is expanded to show trace phases. 

H (red), hemimorphite; W, willemite; C, calcite; H (green), hematite; deg, degree.

The bulk sample may contain as much as 13 percent amorphous component

(table 2), which most likely relates to the iron, manganese, and lead oxides observed by 

SEM. The unit cells of both hemimorphite and willemite are close to ideal.

The microprobe results indicate that the hemimorphite is fairly pure with little 

substitution for the zinc in its structure; apparently the channels in this specimen are not 

loaded with metals or they were exchanged during the sample preparation and polishing 

for the analysis. The willemite in the specimen has significant manganese and minor 

magnesium substitution for the zinc. 

Summary

Petrographic, SEM, and EMPA studies of the mode of occurrence of trace metals 

are important because trace metals affect the chemical and physical behavior of a 

mineral, especially its solubility under weathering conditions in a mine-waste pile (Diehl 
30

and others, 2007). Microanalytical techniques are tools for (1) identifying mineral 

inclusions; (2) measuring the grain size of minerals and identifying the degree of 

crystallinity of individual minerals, both of which are important physical properties that 

influence solubility; (3) recognizing preferred groupings of minerals (that is, mineral 

assemblages); and (4) locating mineralogic residence of minor-to-trace elements.

Scanning electron microscope and XRD studies complement one another to 

produce a comprehensive summary of mineralogy in a sample. Microanalytical data aids 

in explaining geochemical results such as element occurrence and concentration in 

leachate solutions. Each zinc-bearing mineral sample in this study hosts trace to minor 

elements; the SEM micrographs and EMPA-element maps reveal whether the trace 

elements are due to mineral inclusions, or whether the trace elements are distributed in 

the lattice structure of the mineral. Elemental chemistry detected by SEM analysis was 

corroborated by the bulk chemistry inductively coupled plasma–mass spectrometry

analysis (discussed in the next section). XRD studies revealed strains on the crystal 

lattice, which may lead to increased solubility. 

Pyrite was not observed by XRD in the Balmat sphalerite sample due to 

interferences between sphalerite and pyrite signals, and the galena is below the detection 

limit of the XRD instrument. Therefore, a variety of techniques are needed to identify all 

minerals, especially pyrite, which is a prime source of acid-rock drainage, and its 

presence should be recorded at any concentration.
31

Bulk Chemistry, Leachate Chemistry, and Acid-Base Accounting 

Geochemical Results for Five Zinc-Bearing Minerals

By Philip L. Hageman

Bulk geochemistry for five zinc-bearing minerals: Appendix 2 

Leachate chemistry composition: Appendix 3

Introduction—Bulk Sample Chemistry

Total element chemistry was determined for five prepared zinc-bearing mineral 

samples using inductively coupled plasma–mass spectrometry (ICP–MS) after a four-acid 

digestion method (Briggs and Meier, 2002). Mercury was determined using cold-vapor 

atomic fluorescence (CVAFS) (Hageman, 2007b). The zinc carbonate minerals, 

smithsonite and hydrozincite, were analyzed for forms of carbon; total carbon was 

determined by combustion using an automated carbon analyzer (Brown and Curry, 

2002a); carbonate carbon was determined using coulometric titration of CO2 produced

after treatment of the sample with hot 2N perchloric acid (Brown and others, 2002); and 

organic carbon was calculated from the difference between total carbon and carbonate 

carbon. Total sulfur was determined using an automated sulfur analyzer. The two 

sphalerite samples were analyzed for total sulfur by induction furnace following the 

method of Brown and Curry (2002b). Complete details and references for all the methods 

used in this study are provided in Driscoll and others (2012). Complete bulk geochemical 

results are in appendix 2.

Bulk Chemistry Summary

After digestion and analysis, bulk ICP–MS results show that total zinc

concentrations ranged from a high of 640,000 milligrams per kilogram (mg kg-1

) in the 

Balmat sphalerite to a low of 380,000 (mg kg-1) in the hemimorphite. A graph comparing 

total zinc concentration for the five samples is provided in figure 16.
32

Figure 16. (Log) Bulk zinc concentration using inductively coupled plasma–mass spectrometry 

(ICP–MS) for five mineral samples. Mg/kg, milligrams per kilogram.

Total bulk zinc concentration fell in the following order for the five concentrated 

mineral samples: sphalerite (Balmat) > sphalerite (Creede) > hydrozincite > smithsonite > 

hemimorphite. Overall bulk geochemistry varied widely in the five samples as seen in 

figures 17–19, which show plots of selected major- and trace-element concentrations.

Figure 17. Bulk major-element profile—five zinc minerals. Mg/kg, milligrams per kilogram.
33

Figure 18. Bulk trace-element profile—five zinc samples. Mg/kg, milligrams per kilogram.

Figure 19. Comparison of selected bulk chemistry results for the Creede, Colorado, and Balmat, 

New York, sphalerite samples. Mg/kg, milligrams per kilogram.

Comparison of Bulk Chemistry for Two Sphalerite Samples 

The two sphalerite samples have significantly different bulk geochemical 

composition. For many elements such as cadmium, cobalt, manganese, and lead, the 

sphalerite from a vein deposit at Creede, Colorado, generated higher leachate 

concentrations than the sphalerite from Balmat, New York, which is a sedimentary 
34

exhalative deposit (fig 19). As explained in the preceding section “Mineralogical 

Analyses” by Benzel and others, minerals from different ore-deposit types have distinct 

characteristics that strongly influence their geoenvironmental reactivity and metal-release 

potential.

Although the bulk zinc concentrations of the two sphalerite samples are nearly 

equivalent, abundances of some constituents vary by an order of magnitude or more

(appendix 2). Documenting these differences is important because the higher bulk 

concentrations of potentially toxic elements such as arsenic, cadmium, copper, nickel, 

lead, and antimony could lead to the Creede sphalerite releasing higher concentrations of 

these constituents into the environment upon weathering. Complete bulk chemistry for 

the sphalerite samples is found in appendix 2.

Introduction—Leaching Studies

Leaching studies were conducted on prepared (<2 mm) splits of all five zincbearing mineral samples using five different leaching tests in order to identify and 

characterize constituents that are likely to be mobilized from these minerals in a variety 

of settings. The following is a list of the leaching tests used in this study along with a 

brief description of each method.

U.S. Geological Survey Field Leach Test of Hageman (2007a)

This leach test uses deionized (DI) water (ASTM Type II) and a short agitation 

period (5 minutes) to assess the readily water-soluble, water-reactive characteristics of a 

sample. This test is very effective in identifying the constituents that would be mobilized 

due to leaching by natural precipitation. An important feature of the field leach test (FLT)

is that it uses a 20:1 leaching ratio (leachant/solid). This ratio avoids leachate saturation 

while simultaneously providing enough leachate (leach solution) to complete all the 

desired analyses. Use of this ratio also allows FLT-leachate geochemical results to be 

directly compared to the results of the EPA 1312 (synthetic precipitation leaching 

procedure) and 1311 (toxicity characteristic leaching procedure) methods (SPLP, TCLP; 

USEPA, 2004) because all three procedures use the same leaching ratio. For this study, 

both the 5-minute and 18-hour versions of the FLT were used. The only difference in the 

two procedures is the length of time samples are agitated.

U.S. Environmental Protection Agency Method 1312 (Synthetic Precipitation 

Leaching Procedure)

Synthetic precipitation leaching procedure (SPLP; USEPA, 2004) is a regulatory 

test that is used to characterize the water-soluble fraction of a sample. However, this 

leach test uses a long agitation period (18 hours) in order to quantify the sample 

constituents that would be released from samples that have been mechanically broken 

down by end-over-end agitation for 18 hours. Two versions of this procedure were used 

in this study: one using leachate adjusted to pH 4.2 to simulate rainfall for areas east of 

the Mississippi River and the other using a leachate adjusted to pH 5.0 for simulation of 

rainfall in areas west of the Mississippi River.
35

U.S. Environmental Protection Agency Method 1311 (Toxicity Characteristic 

Leaching Procedure)

The toxicity characteristic leaching procedure (TCLP; USEPA, 2004) is the most 

commonly used regulatory leaching test. It requires end-over-end agitation for 18 hours 

and was designed to simulate the leaching conditions in a mixed-waste municipal landfill.

The primary difference between the TCLP and the other leach tests used in this study is 

that the TCLP method requires the use of buffered acetic acid as the leachant.

Unfortunately, this has led to the TCLP being misused (Al-Abed and others, 2005) in 

geochemical studies that were designed to assess or characterize the water-soluble phase 

of geogenic materials. Problems arise because the TCLP was not designed to accurately

simulate or characterize the leaching potential of materials in the natural environment.

For this reason, leachate geochemical results produced using this test are not relevant to 

those produced by the other leaching tests. Thus, TCLP results are included in this study 

for comparative purposes only.

Materials, Methods, and Sample Analysis

The samples were leached according to the specific protocols of each leach test.

After leaching, pH and specific conductance (SC) data were determined on unfiltered 

aliquots of all leachates using calibrated handheld meters. Other splits of leachate were 

filtered using a 60-cubic-centimeter (cm3) plastic syringe and 0.45-micrometer (μm) 

pore-size, nitrocellulose-capsule filter. If filtration was difficult, a 0.70-μm glass-fiber 

pre-filter was used in series with the 0.45-μm filter. Approximately 15 mL of each filtrate 

was transferred to acid-washed high-density polyethylene (HDPE) bottles and preserved 

by acidification with two drops of ultra-pure nitric acid (HNO3) for analysis using ICP–

MS (Lamothe and others, 2002) and inductively coupled plasma–atomic emission 

spectrometry (ICP–AES) (Briggs, 2002). Another aliquot of filtrate (40 mL) was 

collected in HDPE bottles and preserved by refrigeration for determination of alkalinity 

(Theodorakos, 2002) and for ion chromatography (IC) analysis (Theodorakos and others, 

2002). A third subsample of filtrate (30 mL) was collected and preserved for mercury 

analysis CVAFS (Hageman, 2007b). This aliquot of filtrate was collected in acid-washed 

borosilicate glass bottles with Teflon-lined caps and preserved with 1.0-mL mercury-free 

concentrated hydrochloric acid (HCl) and 120-μL bromine chloride (BrCl) per 30-mL 

sample. 

Leachate Geochemistry 

Leachate analytical results for pH, specific conductance, ICP–MS, ICP–AES, IC, 

alkalinity, and mercury are discussed and summarized below, and complete leachate 

analytical results are provided in appendix 3.

Leachate pH

Immediately following completion of agitation, pH was determined on unfiltered 

aliquots of the leachates using a hand-held Orion pH meter and electrode. A comparison 

of the unfiltered leachate pH values for the five samples by all leach tests is presented in 

fig. 20.
36

Figure 20. Leachate pH values for five zinc minerals using five leach tests. FLT, field leach test; 

SPLP, synthetic precipitation leaching procedure; TCLP, toxicity characteristic leaching 

procedure.

These data show that the FLT 5-minute, SPLP-pH 4.2, and SPLP-pH 5.0 leach

tests all produce similar leachate pH trends for the samples. Importantly, the FLT 5-

minute leach test produced results similar to those obtained using the 18-hour leach tests. 

Thus, the mechanical breakdown of mineral surfaces due to extended agitation resulted in 

very little change to the pH of the leachates. The TCLP leachate pH values do not 

conform to the pH trends produced by the other four leach tests. It is apparent that the 

TCLP leachate pH was controlled by the acetic-acid-based extract because the pH values 

for all the samples approach the pH of the blank TCLP extract (pH ≈4.95). This finding 

suggests that the TCLP leach test does not provide an accurate indication of the pH that 

would be expected when these zinc minerals weather in a natural environment.

Excluding the TCLP results, the lowest leachate pH (6.10) value was produced by 

the sphalerite sample from Creede (FLT 5 minutes). The highest leachate pH (9.20) value 

was produced by the hemimorphite sample (FLT 18 hours). After averaging the leachate 

pH values for the five mineral samples using the FLT (5 minute), FLT (18 hour), SPLP 

pH 4.2, and SPLP pH 5.0, the ordered leachate pH ranking (highest to lowest) among the 

five samples is: hemimorphite > smithsonite > sphalerite (Balmat) > hydrozincite > 

sphalerite (Creede). 
37

Leachate Specific Conductance

Specific conductance (SC) was measured on unfiltered aliquots of leachate using 

a Myron L portable conductivity meter. Figure 21 shows the leachate specific 

conductance results of the five zinc samples.

Figure 21. Leachate specific conductance (SC) values for five zinc mineral samples using five 

leach tests. FLT, field leach test; SPLP, synthetic precipitation leaching procedure; TCLP, 

toxicity characteristic leaching procedure; μS/cm (microsiemens per centimeter).

Leachates produced using the FLT (5 minute and 18 hour), SPLP pH 4.2, and 

SPLP pH 5.0 all provide similar SC trends for the samples. Overall, the 18-hour leaching 

tests produced slightly higher SC values for the sphalerite (Balmat, New York), 

hemimorphite, and smithsonite samples relative to the 5-minute FLT data. Four of the 

five zinc minerals produced leachates with relatively low leachate SC values of <100 

μS/cm (microsiemens per centimeter). Only the hydrozincite sample produced a much 

higher average leachate SC of 1,950 μS/cm. Similar to leachate pH, specific conductance 

in the TCLP leachates is not consistent with values derived from the other four leach 

tests. TCLP SC was controlled by the specific conductance of the pre-leach (blank) TCLP 

leach solution, which has a specific conductance of approximately 4,100 μS/cm.

Ultimately, the elevated SC of the TCLP extract solution overwhelmed soluble 

constituents mobilized from the samples and produced the elevated TCLP SC values. 

Because of this, the TCLP leach test does not provide relevant specific conductance data 

in terms of accurately depicting the abundances of constituents that may be solubilized 
38

and mobilized from these minerals as they weather and are leached in the natural 

environment.

After averaging the leachate SC values for all five samples obtained using the 

FLT (5 minute), FLT (18 hour), SPLP pH 4.2, and SPLP pH 5.0 (TCLP SC data 

excluded), the hydrozincite sample produced leachate with the highest average specific 

conductance (1,950 μS/cm). The ordered average SC ranking (highest to lowest) is as 

follows: hydrozincite > sphalerite (Creede) > hemimorphite > smithsonite > sphalerite 

(Balmat). 

Major Anions by Ion Chromatography

Leachates from all five leach tests were analyzed for chloride, fluoride, and nitrate 

(NO3) using ion chromatography (IC) (Theodorakos and others, 2002). Data from all tests 

except the TCLP indicate that all of the minerals have relatively low anion concentrations 

(appendix 3). TCLP leachates have fluorine concentrations that are abnormally high—

orders of magnitude in some cases—relative to those produced by the other tests.

Alkalinity

All of the leachates were analyzed for alkalinity using the method described in 

Theodorakos (2002). Alkalinity values were relatively low for all of the samples by all of 

the leach tests except those produced by the TCLP (appendix 3). TCLP leachates indicate 

much higher (in some cases by several orders of magnitude) alkalinity for all the samples 

relative to those produced by the other leach tests.

Leachable Zinc and Other Metallic Elements

An important aspect of this study involved assessing, quantifying, and 

characterizing the potential for these minerals to release zinc either in the surficial 

environment via weathering (leaching) or in the subsurface as a result of groundwater 

moving through mineralized rocks.

The extent to which zinc was leached from the various samples as a function of 

the five leach protocols varies significantly. In figures 22–26, the zinc leachate 

concentrations are provided in milligrams leached per kilogram of sample (mg/kg). This 

was done in order to compare the leachate results given in micrograms per liter (μg/L) to 

those of the bulk zinc concentration data given as mg/kg. In order to convert the data, 

leachate zinc concentration (μg/L) was multiplied by the leaching dilution factor 

(twenty), and the resulting value was divided by one thousand to convert units. Zinc 

concentrations for SPLP (pH 5.0) leachates for the sphalerite (Creede) and the 

hydrozincite sample were not reported due to ICP–MS instrument problems.
39

Figure 22. Zinc results for sphalerite (Balmat, New York). ICP–MS, inductively coupled plasma–

mass spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic 

precipitation leaching procedure; TCLP, toxicity characteristic leaching procedure.

Figure 23. Zinc results for sphalerite (Creede). ICP–MS, inductively coupled plasma–mass 

spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic 

precipitation leaching procedure; TCLP, toxicity characteristic leaching procedure.

1

10

100

1000

10000

100000

1000000

(log) Concentration (mg/kg)

SPHALERITE (Balmat) Leached zinc vs 

bulk zinc concentration (ICP-MS) 
40

Figure 24. Zinc results for hemimorphite. ICP–MS, inductively coupled plasma–mass 

spectrometry; mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic 

precipitation leaching procedure; TCLP, toxicity characteristic leaching procedure.

Figure 25. Zinc results for smithsonite. ICP–MS, inductively coupled plasma–mass spectrometry; 

mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic precipitation leaching 

procedure; TCLP, toxicity characteristic leaching procedure.
41

Figure 26. Zinc results for hydrozincite. ICP–MS, inductively coupled plasma–mass spectrometry; 

mg/kg, milligrams per kilogram; FLT, field leach test; SPLP, synthetic precipitation leaching 

procedure; TCLP, toxicity characteristic leaching procedure.

Comparison of the leachate zinc data highlights the fact that leachates produced 

using the TCLP leach test contain elevated zinc concentrations relative to those produced 

by the other leach tests. For three of the samples, the difference was dramatic with TCLP 

zinc concentrations nearly two orders of magnitude greater for the hemimorphite and 

smithsonite samples and nearly one order of magnitude greater for the hydrozincite 

sample. Use of the TCLP leach protocol to characterize the leachability of zinc minerals 

yields results that are misleading and overestimate the amount of zinc that will be leached 

and mobilized.

Excluding the TCLP data, results for the other four leach tests indicate that, on 

average, the hydrozincite sample released the most zinc, whereas the hemimorphite 

released the least. For all five minerals, the zinc leached was <1 percent of the bulk zinc 

concentration. The relatively minimal zinc release is likely due to the samples having 

“fresh” surfaces, which have not been subjected to extensive oxidation or formation of 

soluble secondary zinc salts that can enhance metal release. 

Comparison of leachable zinc from the two sphalerite samples suggest that 

although the Balmat sphalerite had generally higher bulk concentrations of metallic 

elements including zinc, the Creede sphalerite released considerably more zinc and other 

metallic elements to solution than the Balmat sphalerite. Mineralogical studies conducted 

by Benzel and Diehl (this volume) show that the Balmat sphalerite is composed 

predominantly of sphalerite and quartz and is therefore somewhat less reactive, whereas 

the sample from Creede is a vein-filling cement in a breccia of quartz-orthoclase host 

rock and alteration minerals that enhance its reactivity. 

The intergrown hydrozincite and willemite sample releases especially elevated 

concentrations of As, Cd, Cu, and Pb. The sample composed of hydrozincite and 

willemite is the most fibrous and porous of those studied and is therefore most 

susceptible to alteration by fluids and oxygen. An example of the leachate geochemical 

signature for selected metallic elements produced using the FLT is shown in figure 27.
42

Figure 27. Field leach test (FLT) leachate plot A: Graph plot of selected elements and their 

concentration in leachates produced using the 5-minute field leach test (FLT) versus the zinc 

mineral. SEDEX, sedimentary exhalative deposit; μg/L, micrograms per liter.

Cadmium is present in soluble forms in the sphalerite from the Creede vein 

deposit as well as in the hydrozincite and smithsonite samples. The importance of 

cadmium’s mode of occurrence is demonstrated by the fact that cadmium is hosted within

the lattice structure of the smithsonite mineral as shown in figures 7C and 7D above,

whereas cadmium in the hydrozincite sample is hosted in the lattice structure of the 

accessory calcite. Calcite readily takes up cadmium into its structure and is a more 

soluble mineral than smithsonite. 

Summary of Leaching Studies

1. The simplified FLT procedure, which uses DI water and a 5-minute agitation, was 

as effective as the long-term (18-hour agitation) tests in assessing the watersoluble phase of these samples including pH, specific conductance, and watersoluble zinc concentration. Because of its short agitation time, the FLT test may 

produce results that are more representative of the readily soluble phase of a 

sample that would be mobilized as runoff under natural conditions.

2. The TCLP leaching procedure produced lower pH trends, higher conductivity 

trends, and much higher concentrations of zinc when compared to results 

produced using the other leach tests. This finding indicates that it is not 
43

appropriate to use the TCLP procedure to assess the general leachability of 

geologic materials. Using this procedure to assess general leachability suggests 

that much more zinc would be mobilized from these minerals than is actually the 

case. Using the TCLP procedure would only be appropriate if the material were

going to be disposed of in a municipal landfill.

3. All leach tests showed major leachate geochemical differences between the 

Creede sphalerite and the Balmat sphalerite. The Balmat sphalerite had a slightly 

higher bulk-zinc concentration; whereas the sphalerite from Creede produced 

leachate with a lower pH, higher conductivity, and moderately higher leachatezinc concentration. These findings show that although samples may have the same 

mineral name, subtle mineralogic differences such as trace-element content, grain 

size, and associated minerals make them act quite differently when leached.

4. This study demonstrates the importance of choosing the proper leach test to use 

for characterization of the soluble fraction of a material. Use of an improper leach 

test (that is, TCLP) to assess the general leachability of a material will lead to 

inaccurate findings. Finally, if an investigator wants to mimic the leaching 

process that occurs in the natural environment, a DI water-based leaching test 

similar to the FLT should be used. 

Acid-Base Accounting Study

The net acid production (NAP) and the acid neutralizing capacity of the five zinc 

minerals were determined using the same methods and procedures used in the study of 

copper minerals (Driscoll and others, 2012). The NAP method used in this study is a 

modification of a method described by Lapakko and Lawrence (1993). The primary 

advantage of this method is that it reacts and accounts for both the acid-producing and 

acid-neutralizing potential of the sample using one test. It should be noted, however, that 

though this test fully quantifies the acid-producing potential of the sample, it does not 

quantify the exact neutralizing potential, but instead, only determines that a sample is 

“net” neutralizing.

The method requires digesting finely ground samples using 30-percent hydrogen 

peroxide (H2O2). Hydrogen peroxide rapidly oxidizes any sulfides present in the sample 

thereby forming sulfuric acid (H2SO4), which in turn reacts with acid-neutralizing 

minerals contained in the sample. After these reactions are complete, the resulting 

digestate is filtered and titrated (if necessary) to pH 7.0 with 0.1-N (normal) sodium 

hydroxide (NaOH). 

To carry out this procedure, 1.0 g of prepared sample is weighed into a 250-mL 

Erlenmeyer glass flask. Fifty-mL 30-percent H2O2 is slowly added to the sample. After 

all reaction has ceased, another 50-mL H2O2 is added, the flask is swirled, and the 

reaction is allowed to go to completion. A final 50-mL H2O2 is added, and the flask is

swirled, placed on a hotplate and heated at 90 °C. The sample remains on the hotplate 

until the reaction is completed. The flask is then removed from the hotplate and allowed 

to cool for 15 minutes. After cooling, 1-mL copper nitrate (CuNO3) is added and the 

contents are again swirled. The flask is returned to the hotplate and brought to boil 

(≈110 °C). After 10 minutes, the flask is removed from the hotplate. When cooled to 

room temperature, the liquid is filtered into a clean 250-mL glass beaker to remove the 

solids. As a final step, the solids retained in the filter are rinsed with 1-molar (M) calcium
44

chloride (CaCl2). Following filtration, the pH of the filtrate is measured and recorded. If 

the pH is greater than 7.0, the sample does not have to be titrated, because this pH

indicates there is net buffering capacity in the sample, and the sample is determined to be 

net neutral. If the pH is less than 7.0, the filtrate is titrated with 0.1-N sodium hydroxide 

(NaOH). A stir bar is placed in the beaker, and the solution is constantly stirred during 

titration. A pH electrode is suspended in the beaker during titration, and solution pH is 

constantly monitored. When the solution pH reaches 7.0, the quantity of NaOH 

consumed during the titration is recorded.

Upon completion of the titration, final NAP is determined by multiplying the 

number of milliliters NaOH consumed by the titrant concentration (0.1). The sum is then 

multiplied by 50. Data are reported in kilograms calcium carbonate (CaCO3) equivalent 

required to neutralize 1 ton of sample (kg/t CaCO3).

Summary of Acid-Base Accounting Study

This acid-base accounting procedure indicates that only the two sphalerite 

samples are net-acid producers that require CaCO3 in order to buffer the peroxide 

digestate (fig. 28.). Once again, the two sphalerite samples acted differently with the 

Balmat sphalerite requiring more CaCO3 (555 kg/t) than the Creede sphalerite (460 kg/t) 

to neutralize the digestate. This indicates that, although both sphalerite samples are net 

acid producers, the Balmat sphalerite would most likely produce more acid in a natural 

setting. The hemimorphite, smithsonite, and hydrozincite samples are “net neutralizing” 

and thus did not require any CaCO3. These three minerals are less likely to produce acid 

as they weather.

Figure 28. Acid-base accounting (ABA) test results for five zinc-bearing mineral samples. Kg, 

kilograms.
45

In Vitro Bioaccessibility Extractions

By Suzette Morman

Leachate results by extraction fluid: Appendix 4

Introduction

The term “bioaccessibility,” which refers to the fraction of a potential toxicant in 

soil or other earth materials (that is, volcanic ash, wildfire ash, and dust) that becomes 

soluble in the stomach and is then available for absorption (Ruby and others, 1993), is 

often confused with “bioavailability,” which is the amount of a potential toxicant 

absorbed and transported to a site of toxicological action. Generally, studies to examine 

bioavailability and resulting health effects or toxicity are conducted with animal 

surrogates or cell-line tests, both of which are time consuming and expensive. In vitro 

bioaccessibility (IVBA) tests are inexpensive, physiologically based tests designed to 

estimate the bioaccessibility of elements in soils, dusts, or other environmental materials 

by measuring the dissolution of the materials of interest in fluids compositionally similar 

to human body fluids (Morman and others, 2009). Prior sections in this volume discussed 

methods to identify and quantify major-, minor-, and trace-element abundances of a suite 

of zinc minerals and presented leachate results that showed the mobility of trace metals 

and metalloids in simple solutions. The research described below evaluates 

bioaccessibility via both ingestion and inhalation exposure pathways.

Methods

The evaluated samples were previously ground (see Driscoll, this volume). For 

the gastrointestinal extractions, the samples were sieved to <250 μm using a 3-inch 

stainless steel sieve. The <250-μm-size fraction is important as it is reportedly the grain 

size most likely to adhere to the hands of children and be ingested (Van Wijnen and 

others, 1990). For the remaining extractions, the samples were sieved to <20 μm using a 

3-inch stainless steel sieve and a Retsch A S200 auto-sieve. Although particles greater 

than 10 μm are generally trapped in the nasopharyngeal region and swallowed, these 

larger particles may pass into the bronchioles during exercise and mouth breathing. The 

<20-μm-size fraction is a reasonable proxy for respirable-size particles (generally 

<5 μm); sieving to the smaller size fraction is difficult and requires additional time and 

material to obtain sufficient study material. Trace-element abundances in the extraction 

solutions were analyzed by inductively coupled plasma–mass spectrometry (ICP–MS) 

(Lamothe and others, 2002). The ICP–MS system was calibrated with multi-element 

standard solutions prepared from commercially available stock solutions. A procedural 

blank and duplicate sample were added to each sample batch in support of quality control 

evaluations. All results were blank-corrected prior to interpretation.

Ingestion Pathway 

In ingestion-pathway evaluations, most IVBA methodologies utilize either a 

single or sequential extraction with adjustments made to solution composition or pH. For 
46

this study, we selected a simulated gastric fluid (SGF) extraction protocol (Drexler and 

Brattin, 2007), identified as a standard operating procedure and approved by the United 

States Environmental Protection Agency (USEPA, 2008a), to measure bioaccessibility 

and estimate relative bioavailability of lead. The gastric extraction was followed by an 

intestinal extraction protocol, described in Basta and others (2007) and developed by 

Ohio State University researchers (OSU-IVG (in vitro gastrointestinal) method). This 

procedure is described in detail by Driscoll and others (2012).

Inhalation Pathway

Both SLF and SPF were utilized to model the inhalation pathway. These 

simulated body fluids are used to examine a variety of materials (Sun and others, 2001; 

Herting and others, 2006) and permit modeling of in vitro solubility, an important 

physiochemical factor that determines the rate and extent that particles are retained at the 

site of deposition, translocated to other tissues, or excreted (Kanapilly and others, 1973; 

Kreyling, 1992; Ansoborlo and others, 1999) relative to an inhalation pathway. The lung 

fluid has neutral pH; the phagolysosomal fluid models the lower pH (4.5) encountered 

when particles are engulfed by pulmonary alveolar macrophages, which are specialized 

lung cells involved in particle removal from the lung.

The composition of the simulated lung fluid and procedures are described by 

Morman (in Driscoll and others, 2012). A solid to liquid ratio of 1:100 was selected 

based on previous studies in our laboratory. The procedure specifies that bottles be placed 

in a preheated (37 °C) incubator to provide both constant rotation and temperature for 24 

hours. The composition of the phagolysosomal simulant fluid used is described by 

Stefaniak and others (2006). The antifungal agent Aldkylbenzyldimethylammonium 

chloride is not used in our procedure due to concerns that this chemical could introduce 

contaminants or alter leachate results. The pH of this leach solution was titrated to 4.5 by 

the addition of 0.1-M potassium hydroxide solution. The method is identical to that of the 

simulated lung fluid and uses the same solid to liquid solution ratio (1:100).

Results

The primary and secondary zinc minerals in the evaluated sample suite have 

different morphologies, solubilities (see Hageman, this volume), densities, and host 

different trace-element concentrations (see Benzel and Diehl, this volume). Therefore, 

zinc sulfide, zinc carbonate, and zinc silicate should yield different rates of dissolution. 

Comparison of individual element abundances in leachates, as a function of extraction 

fluid, allows identification of basic trends, but the number of samples studied is too few 

to allow robust statistical analysis. Some elements, including Ag, Be, Ni, Sb, Se, and V, 

were largely soluble in only the acidic SGF. Other elements, such as Al, Ba, Ca, Cd, Co, 

Fe, Pb, and Sr, demonstrated some solubility in all fluids but showed enhanced 

bioaccessibility in the acidic SGF and SPF. Only a few elements (As, Cu, Mn, U, and Zn) 

demonstrated fairly similar and sometimes greater bioaccessibility in the near-neutral 

simulated lung fluid as well as in the acidic fluids (table 3 and appendix 4).
47

Table 3. Total concentration, leachate concentration, and bioaccessibility of selected trace metals.

[*, blank-correction resulted in a negative number; mg, milligrams; kg, kilograms; ppm, parts per million; μg/L, micrograms per liter; nr, not reported due to excess 

concentration; <RL, value less than the reported limit; SGF, simulated gastric fluid; SIF, simulated intestinal fluid; SLF, simulated lung fluid; SPF, simulated 

phagolysosomal fluid. Chemistry results were corrected for dilution and blank-corrected prior to calculations]

SGF As ug/L

As

(mg leached/

kg solid)

As

(total, ppm)

As

%

Bioaccessible

Cd ug/L

Cd

(mg leached/

kg solid)

Cd

(total, ppm)

Cd

%

Bioaccessible

Sphalerite (Balmat, NY) <RL * 6.7 * 16.1 1.6 1140 0.14

Sphalerite (Creede, CO) 63 6.3 117 5 31.8 3.2 4100 0.08

DupSphalerite (Creede,CO) 65.5 6.6 117 6 33.3 3.3 4100 0.08

Hemimorphite 1820 182 1620 11 14200 1420 1770 80

Hydrozincite/Willemite 601 60.1 70.4 85 6170 617 495 125

Smithsonite 130 13 41.3 31 12100 1210 2950 41

SIF

Sphalerite (Balmat, NY) 3.3 0.3 6.7 5 13.3 1.3 1140 0.11

Sphalerite (Creede,CO) 94 9.4 117 8 5.8 0.5 4100 0.01

DupSphalerite (Creede, CO) 90 9.0 117 8 4.0 0.3 4100 0.01

Hemimorphite 1820 182 1620 11 13400 1340 1770 76

Hydrozincite/Willemite 267 26.7 70.4 38 4640 464 495 94

Smithsonite 77.3 7.7 41.3 19 8670 867 2950 29

SLF

Sphalerite (Balmat, NY) <RL <RL 6.7 * <RL <RL 1140 *

Sphalerite (Creede,CO) 24.7 2.5 117 2.1 <RL <RL 4100 *

Hemimorphite 1220 122 1620 7.5 6.4 0.64 1770 0.04

Hydrozincite/Willemite 75.2 7.5 70.4 10.7 125 12.5 495 2.5

Smithsonite 21.8 2.2 41.3 5.3 27.1 2.7 2950 0.09

DupSmithsonite 22.2 2.2 41.3 5.4 28.2 2.8 2950 0.10

SPF

Sphalerite (Balmat,NY) <RL <RL 6.7 * 0.51 0.05 1140 0.004

Sphalerite (Creede, CO) 86.2 8.6 117 7.4 4.4 0.44 4100 0.01

Hemimorphite 329 32.9 1620 2 11700 1170 1770 66

Hydrozincite/Willemite 28 2.8 70.4 4 714 71.4 495 14

Smithsonite 24.6 2.5 41.3 6 2160 216 2950 7.3

DupSmithsonite 22.4 2.2 41.3 5.4 2140 214 2950 7.3
48

Table 3. Total concentration, leachate concentration, and bioaccessibility of selected trace metals.—Continued

[*, blank-correction resulted in a negative number; mg, milligrams; kg, kilograms; ppm, parts per million; μg/L, micrograms per liter; nr, not reported due to excess 

concentration; <RL, value less than the reported limit; SGF, simulated gastric fluid; SIF, simulated intestinal fluid; SLF, simulated lung fluid; SPF, simulated 

phagolysosomal fluid. Chemistry results were corrected for dilution and blank-corrected prior to calculations]

SGF Cu ug/L

Cu

(mg leached/

kg solid)

Cu

(total, ppm)

Cu

%

Bioaccessible

Mn ug/L

Mn

(mg leached/

kg solid)

Mn

(total, ppm)

Mn

%

Bioaccessible

Sphalerite (Balmat, NY) 5.6 0.43 47.9 1 26.4 2.6 926 0.3

Sphalerite (Creede, CO) 0.86 * 792 * 293 29.3 1270 2

DupSphalerite (Creede,CO) <RL * 792 * 276 27.6 1270 2

Hemimorphite 97.7 9.6 17.6 55 4830 483 10400 5

Hydrozincite/Willemite 26200 2620 2340 112 2320 232 1340 17

Smithsonite 1290 129 165 78 1670 167 574 29

SIF

Sphalerite (Balmat, NY) 14.5 * 47.9 * 41.4 1.3 926 0.1

Sphalerite (Creede,CO) 17.6 * 792 * 232 20.4 1270 2

DupSphalerite (Creede, CO) 15 * 792 * 237 20.9 1270 2

Hemimorphite 117 9.4 17.6 54 3890 386 10400 4

Hydrozincite/Willemite 17100 1708 2340 73 1280 125 1340 9

Smithsonite 920 90 165 54 963 94 574 16

SLF

Sphalerite (Balmat, NY) <RL <RL 47.9 * 39.9 4.0 926 0.43

Sphalerite (Creede,CO) 6.1 0.61 792 0.08 165 16.5 1270 1.3

Hemimorphite 112 11.2 17.6 64 131 13.1 10400 0.13

Hydrozincite/Willemite 5060 506 2340 22 36.5 3.7 1340 0.27

Smithsonite 222 22.2 165 13 40.8 4.08 574 0.71

DupSmithsonite 229 22.9 165 14 44.1 4.41 574 0.77

SPF

Sphalerite (Balmat,NY) <RL <RL 47.9 * 88.2 8.8 926 1.0

Sphalerite (Creede, CO) <RL <RL 792 * 448 44.8 1270 3.5

Hemimorphite 79.6 8 17.6 45 2390 239 10400 2.3

Hydrozincite/Willemite 8240 824 2340 35 572 57.2 1340 4.3

Smithsonite 464 46 165 28 431 43.1 574 7.5

DupSmithsonite 467 47 165 28 441 44.1 574 7.7
49

Table 3. Total concentration, leachate concentration, and bioaccessibility of selected trace metals.—Continued

[*, blank-correction resulted in a negative number; mg, milligrams; kg, kilograms; ppm, parts per million; μg/L, micrograms per liter; nr, not reported due to excess 

concentration; <RL, value less than the reported limit; SGF, simulated gastric fluid; SIF, simulated intestinal fluid; SLF, simulated lung fluid; SPF, simulated 

phagolysosomal fluid. Chemistry results were corrected for dilution and blank-corrected prior to calculations]

SGF Pb ug/L

Pb

(mg leached/

kg solid)

Pb

(total, ppm)

Pb

%

Bioaccessible

U ug/L

U

(mg leached/

kg solid)

U

(total, ppm)

U

%

Bioaccessible

Sphalerite (Balmat, NY) 25.4 2 60.8 4 5.5 0.55 4.9 11

Sphalerite (Creede, CO) 2030 203 14400 1 0.51 0.05 0.64 8

DupSphalerite (Creede,CO) 1530 153 14400 1 0.5 0.05 0.64 8

Hemimorphite 247 25 1320 2 7.3 0.73 30.5 2

Hydrozincite/Willemite 37500 3750 4950 76 6120 612 514 119

Smithsonite 15300 1530 1180 130 11.5 1.15 0.41 280

SIF

Sphalerite (Balmat, NY) 5.9 * 60.8 * 0.83 0.08 4.9 2

Sphalerite (Creede,CO) 450 44 14400 0.3 0.24 0.02 0.64 4

DupSphalerite (Creede, CO) 469 46 14400 0.3 0.19 0.02 0.64 3

Hemimorphite 11.6 0.1 1320 0.01 1.5 0.15 30.5 0.5

Hydrozincite/Willemite 10400 1039 4950 21 132 13 514 3

Smithsonite 2030 202 1180 17 1.15 0.12 0.41 28

SLF

Sphalerite (Balmat, NY) 1.2 * 60.8 * 11.2 1.1 4.9 23

Sphalerite (Creede,CO) 214 21.2 14400 0.15 <RL <RL 0.64 *

Hemimorphite <RL <RL 1320 * 7.0 0.7 30.5 2

Hydrozincite/Willemite 109 10.7 4950 0.22 733 73 514 14

Smithsonite 21.7 2.0 1180 0.17 4.3 0.4 0.41 105

DupSmithsonite 23.2 2.1 1180 0.18 4.5 0.4 0.41 109

SPF

Sphalerite (Balmat,NY) 13.9 1.2 60.8 2.0 12.8 1.3 4.9 26

Sphalerite (Creede, CO) 6520 652 14400 4.5 < 1 <RL 0.64 *

Hemimorphite 7.2 0.56 1320 0.04 2.2 0.22 30.5 0.73

Hydrozincite/Willemite 10100 1010 4950 20 2050 205 514 40

Smithsonite 6460 646 1180 55 3.6 0.36 0.41 87

DupSmithsonite 6490 649 1180 55 3.2 0.32 0.41 79
50

Table 3. Total concentration, leachate concentration, and bioaccessibility of selected trace metals.—Continued

[*, blank-correction resulted in a negative number; mg, milligrams; kg, kilograms; ppm, parts per million; μg/L, micrograms per liter; nr, not reported due to excess 

concentration; <RL, value less than the reported limit; SGF, simulated gastric fluid; SIF, simulated intestinal fluid; SLF, simulated lung fluid; SPF, simulated 

phagolysosomal fluid. Chemistry results were corrected for dilution and blank-corrected prior to calculations]

SGF Zn ug/L

Zn

(mg leached/

kg solid)

Zn

(total, ppm)

Zn

%

Bioaccessible

Sphalerite (Balmat, NY) 6550 651 635000 0.1

Sphalerite (Creede, CO) 13800 1376 587000 0.2

DupSphalerite (Creede,CO) 14600 1456 587000 0.2

Hemimorphite 2790000 278996 382000 73

Hydrozincite/Willemite 5160000 515996 561000 92

Smithsonite 1750000 174996 504000 35

SIF

Sphalerite (Balmat, NY) 5530 482 635000 0.08

Sphalerite (Creede,CO) 10600 989 587000 0.2

DupSphalerite (Creede, CO) 9920 921 587000 0.2

Hemimorphite 2440000 243929 382000 64

Hydrozincite/Willemite 4600000 459929 561000 82

Smithsonite 1580000 157929 504000 31

SLF

Sphalerite (Balmat, NY) 3540 350 635000 0.06

Sphalerite (Creede,CO) 3940 390 587000 0.07

Hemimorphite 4230 419 382000 0.11

Hydrozincite/Willemite 4450 441 561000 0.08

Smithsonite 6520 648 504000 0.13

DupSmithsonite 6490 645 504000 0.13

SPF

Sphalerite (Balmat,NY) 15400 1535.9 635000 0.24

Sphalerite (Creede, CO) 19200 1915.9 587000 0.33

Hemimorphite 223000 22295.9 382000 5.8

Hydrozincite/Willemite 428000 42795.9 561000 7.6

Smithsonite 390000 38995.9 504000 7.7

DupSmithsonite 392000 39195.9 504000 7.8
51

Bioaccessibility is calculated as

(mg leached by extraction fluid per kg soil)/(total concentration in soil (in mg/kg)) × 100.

For some elements, in some samples, blank-correcting leachate abundances resulted in 

negative numbers and bioaccessibility could not be quantified. Concentrations of trace 

elements in some leachate samples were less than the reporting level (RL); these data are not 

reported in table 1 or appendix 4. Bioaccessibility values in excess of 100 percent may result 

from analytical error stemming from incomplete solid digestion, sample heterogeneity, or 

solid:solution ratio.

The five samples contain total zinc concentrations that range from 382,000 mg/kg 

(hemimorphite) to 635,000 mg/kg (Balmat sphalerite). Zinc bioaccessibility was low for both 

the Balmat and Creede sphalerite samples in all leachate solutions (<1 percent). Zinc 

bioaccessibility for the other three alteration minerals was significantly higher, particularly for 

the two carbonates—hydrozincite and smithsonite. The carbonate minerals exhibit greater 

bioaccessibility for several elements (arsenic, cadmium, and lead) despite having wholesample concentrations that are similar to those of the two sphalerite samples. The 

hemimorphite sample, classified as a silicate, yielded both high total concentrations and 

bioaccessibility of several trace metal(loid)s with known human and ecosystem health risks 

(table 3).

Low zinc solubility is observed (<1 percent) for all mineral samples in the neutral pH 

SLF (figs. 29–32). Given the total concentrations, however, this still amounts to elevated 

concentrations in solution (table 3). Figures 29–32 show the bioaccessibility of zinc by 

mineral and leachate type.

Figure 29. Bioaccessibility of zinc (Zn) for Balmat, New York, and Creede, Colorado, sphalerite 

samples measured in simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung 

fluid (SLF), and simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble 

in the fluid.
52

Figure 30. Bioaccessibility of zinc (Zn) for hemimorphite sample measured in simulated gastric fluid 

(SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated phagolysosomal 

fluid (SPF) presented as the percent of the solid soluble in the fluid.

Figure 31. Bioaccessibility of zinc (Zn) for hydrozincite/willemite sample measured in simulated gastric 

fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid.
53

Figure 32. Bioaccessibility of zinc (Zn) for smithsonite sample measured in simulated gastric fluid 

(SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated phagolysosomal 

fluid (SPF) presented as the percent of the solid soluble in the fluid. Dup, duplicate.

Figures 33–36 demonstrate the variability of arsenic bioaccessibility by mineral and 

extraction fluid.

Figure 33. Bioaccessibility of arsenic (As) for Balmat (Bal) and Creede sphalerite samples measured in 

simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and 

simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid. RL, 

reporting level.
54

Figure 34. Bioaccessibility of arsenic (As) for hemimorphite sample measured in simulated gastric fluid 

(SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated phagolysosomal 

fluid (SPF) presented as the percent of the solid soluble in the fluid.

Figure 35. Bioaccessibility of arsenic (As) for hydrozincite/willemite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid.
55

Figure 36. Bioaccessibility of arsenic (As) for smithsonite sample measured in simulated gastric fluid 

(SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated phagolysosomal 

fluid (SPF) presented as the percent of the solid soluble in the fluid. Dup, duplicate.

Hemimorphite, with the highest total arsenic concentration, displays bioaccessibility 

values closer to that of the zinc sulfides than the zinc carbonates. However, hemimorphite 

exhibits greater cadmium bioaccessibility than smithsonite and its cadmium bioaccessibility is 

equal to that of the hydrozincite (figs. 37–40).
56

Figure 37. Bioaccessibility of cadmium (Cd) for Balmat (Bal) and Creede sphalerite samples measured 

in simulated gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and 

simulated phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid. RL, 

reporting level; Dup, duplicate.

Figure 38. Bioaccessibility of cadmium (Cd) for hemimorphite sample measured in simulated gastric 

fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid.
57

Figure 39. Bioaccessibility of cadmium (Cd) for hydrozincite/willemite sample measured in simulated 

gastric fluid (SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated 

phagolysosomal fluid (SPF) presented as the percent of the solid soluble in the fluid.

Figure 40. Bioaccessibility of cadmium (Cd) for smithsonite sample measured in simulated gastric fluid 

(SGF), simulated intestinal fluid (SIF), simulated lung fluid (SLF), and simulated phagolysosomal 

fluid (SPF) presented as the percent of the solid soluble in the fluid. Dup, duplicate.
58

Discussion

Five zinc-bearing mineral samples were chosen for this study. Zinc, an essential 

element, has a daily intake requirement since there is no specific storage system for zinc in the 

body (National Institutes of Health, 2011). This requirement is usually easily achieved 

through meat and milk consumption, but a zinc deficiency may be a feature of certain cultures 

and can be associated with poverty. Zinc influences many physiological functions in the body

including immunity, brain development and function, growth, and nerve conduction. Zinc is 

also a catalytic metal ion in many enzymes and proteins in the body. Toxic exposures are 

generally related to occupational exposures such as zinc oxide fumes (Gordon and Fine, 

1993), although poisoning from ingestion of zinc-contaminated food and drink, which can 

result in gastrointestinal distress, has been reported (Standstead and Au, 2007). 

Although this study evaluates monomineralic samples of zinc minerals, ore-deposit 

studies indicate that the abundances of diverse elements vary in the environment as a 

consequence of associated ore-deposit type and geologic setting. Mine-waste chemistry and 

mineralogy is complex because most ore minerals host many trace elements that are not a part 

of stoichiometric formulae. These trace-element impurities affect the physical and chemical 

behavior of the minerals as they weather in place or in waste piles and tailings ponds. No 

similar studies examining zinc bioaccessibility from primarily monomineralic samples to 

determine zinc bioaccessibility in the simulated body fluids utilized here were discovered for 

comparison. 

Many studies have examined bioaccessibility of minerals in mine waste (Ruby and 

others, 1996) or soils affected by ore extraction and processing (Brumbaugh and Morman, 

2011) or smelter-related deposition (Ettler and others, 2012). Additional studies have 

examined how particle size, soil pH, the organic carbon content, and the presence of clays and 

iron or manganese oxides affect element mobility and controls on bioaccessibility. This study 

focused on mineral solubility in fluids with a composition and pH similar to that of human 

body fluids. Results presented here indicate that mineral morphology and solution pH strongly 

influence bioaccessibility. Mineral phase is an important determinant of lead and arsenic 

bioaccessibility as discussed in Ruby and others (1999). Similarly, it was demonstrated that 

copper bioaccessibility varies as a function of copper source mineral (Suzette Morman in

Driscoll and others, 2012).

Limitations of Results

The results of this study are limited by several factors. Limited research has been 

conducted utilizing these simulated fluids to examine other exposure pathways, such as 

inhalation. Further, no uptake studies have been conducted to compare how accurately the 

SLF and SPF mimic the complex physiological processes of the lung. IVBA results express 

the amount of an element that is available to the body for uptake by target organs such as the 

kidneys and liver; actual uptake is as yet undetermined. Many physiological factors control 

the uptake of elements by the body. To date, no reference values delineating high or low 

levels of bioaccessibility and the correlation or relative bioavailability with uptake or 

bioavailability have been established for metals other than lead. Finally, the minerals studied 

in this investigation were ground, artificially creating fresh reactive surfaces. Unlike naturally 

occurring minerals present in soil matrices and lacking the additional solubility controls 

present in such matrices (that is, soil pH, presence of organic carbon, clays, and iron or 

manganese oxides whose surfaces might sorb released metals), fresh surfaces may produce 

artificially high solubility values for some trace metals.
59

Summary

Variations in trace-metal bioaccessibility are related to mineral species or extraction 

solution pH, or both, for these samples. Ruby and others (1999) have focused on 

understanding aspects of the soil matrix to identify factors that influence bioaccessibility. 

Fewer studies have evaluated the effects of variable mineralogy. However, Schaider and 

others (2007) observed that zinc bioaccessibility was greater in zinc carbonates than zinc 

sulfides in size-fractionated mine-waste samples. The results described here help characterize 

the bioaccessibility of zinc-bearing minerals and provide data of potential utility in modeling 

and understanding variations in zinc bioaccessibility.
60

Procedure and Results of Metal Toxicity for Five Zinc Mineral 

Concentrates Using MetPLATETM

By LaDonna M. Choate

Introduction

Metals leached from minerals in the environment can be toxic to aquatic organisms. 

Metal bioavailability and aquatic toxicity are a complex function of water chemistry (Sunda 

and Guillard, 1976). Some factors that can affect metal toxicity include metal concentration, 

complexation by aqueous ligands, and competition with other cations (Ca2+, Mg2+, Na+, K+, 

H+) for biological uptake. Determining the half-maximal effective concentration (EC50)—

defined as the concentration that produces a response in 50 percent of the exposed 

organisms—due to the presence of metals can be expensive and time consuming if standard 

test organisms are used. It is advantageous to use a screening protocol to identify samples that

may require more in-depth evaluation. MetPLATETM is an enzymatic bioassay kit that can be 

used to determine if leached minerals will release metal in concentrations that are toxic to 

Escherichia coli (E. coli) bacteria (Bitton and others, 1994). Toxicity for the MetPLATETM is 

defined as the inhibition of ß-galactosidase enzyme production. Unstressed E. coli bacteria 

produce ß-galactosidase that catalyzes the hydrolysis (fig. 41) of a colored chromogenic 

substrate (chlorophenol red-ß-galactopyranoside) causing the experimental solution to change 

from yellow-orange to purple-red.

Figure 41. ß-galactosidase catalyzed hydrolysis of chlorophenol red-ß-galactopyranoside (Bitton and 

Koopman, 1997; Rossel and others, 1997; and Ward and others, 2005).

Bacteria stressed by the presence of metals produce less of the enzyme causing less of 

the colored chromogenic substrate to be hydrolyzed. Enzyme production inhibition is 

determined by comparison of the color (purple-red) of the sample, measured as absorbance at 

575-nanometer (nm) wavelength, to that of a control having no inhibition. Figure 42 illustrates 

a developed MetPLATETM test. Each sample column represents an individual sample with 

varying concentrations of a toxic substance, in this case copper, but a similar response is 

found for zinc. The copper concentration decreases going down each column from rows A to 

H. Inhibition (directly related to toxicity) decreases with decreasing metal concentration. The
61

less toxic the sample, the darker the color of the experimental fluid and the higher the 

measured absorbance. Relative absorption is used to determine the EC50 for E. coli.

Figure 42. A developed 96-well MetPLATETM bioassay showing negative (toxic) and positive (nontoxic) 

controls and dilution series for several water samples.

General Method

The MetPLATETM test kit, produced by M2B Research and Innovative Technologies 

(Gainesville, Fla.), is refrigerated until the day of use when it is removed and allowed to warm 

to room temperature. Mineral-concentrate samples were leached using the U.S. Geological 

Survey field leach test (USGS-FLT), 5-minute version (Hageman, 2007a). Twenty to fifty mL 

of the leachate (USGS-FLT solution) was evaluated using the MetPLATETM toxicity test. 

Dilutions were prepared by adding 10 mL of diluent (EPA moderately hard water; USEPA, 

1994) to 10 mL of the USGS-FLT solution, obtained from P. Hageman, and vortexing for 10 

seconds. Subsequent dilutions were prepared using this procedure (beginning with the 

previous dilution) until the required set of samples was obtained. Samples to be evaluated 

were prepared by transferring a 0.9-mL aliquot of the undiluted sample and each of the 

dilutions into test tubes and adding 0.1 mL of the reconstituted bacterial reagent (E. coli). The 

controls were prepared by adding 0.1 mL of bacterial reagent to test tubes containing 0.9 mL 

of diluent for the negative control and 0.9 mL of the MetPLATETM for positive control 

(copper sulfate solution at toxic levels). All sample tubes were vortexed for 10 seconds and 

incubated at 35 °C for 90 minutes. After incubation, 0.2 mL of sample was transferred by 

pipette into the corresponding well in the 96-well microplate (fig. 42) and 0.1 mL of the 
62

reconstituted chromogenic substrate was added to each well. All samples were run in 

duplicate or triplicate.

Microplates were incubated at 35 °C, and the absorbance measured at 575 nm every 15

minutes over the course of 2 hours, using a BIO-TEK PowerWave350X spectrophotometer 

with KCjunior software. When the absorbance for the most dilute sample is approximately 

equal to the negative control, the corresponding time interval is used to determine the EC50 for 

the sample. The positive control has the maximum effect—100 percent inhibition of ßgalactosidase production—and should exhibit zero absorbance. However, the nonhydrolyzed 

chromogenic substrate has some color and produces an absorbance value. If the resulting 

absorbance values do not span the range between the positive and negative controls, then the 

sample is re-tested using more or less dilute sample to increase the absorbance spread. The 

absorbance for all concentrations has been corrected for maximum toxicity by subtracting the 

absorbance of the positive control.

The percent inhibition is calculated for each sample using the following equation:

% inhibition = (1–(sample absorbance)/(control absorbance)) × 100

In some cases, the control absorbance of the most dilute sample is greater than the 

absorbance of the negative control. These leachate matrices probably contain other 

elements/compounds that enhance their absorbance at 575 nm. Accordingly, the highest 

absorbance for the leachate is used as the control absorbance to account for this matrix effect. 

A plot of metal concentration versus percent inhibition indicates the metal concentration at 

which half of the E. coli is affected (EC50).

Summary

The pH for all of the 100-percent USGS-FLT solutions were greater than or equal to 

6.1, which is compatible with MetPLATETM applicability for use with pH 5.0 to 7.5 solutions. 

Although the pH of the 100-percent hemimorphite USGS-FLT solution was 7.7, the pH of this 

solution was not adjusted, and the results are considered to be reasonably accurate despite the 

pH being slightly in excess of the MetPLATETM specifications. Leachates derived from the 

five zinc mineral concentrates yield a wide array of toxicity, as shown in figures 43–45. The 

maximum zinc concentration for the hemimorphite is less than 50-percent inhibition for the 

USGS-FLT solution and its dilutions (fig. 43) and precludes determination of the EC50 for this 

sample. The low zinc concentration produced in the hemimorphite mineral-concentrate leach 

limits its inhibition effect.
63

Figure 43. Percent inhibition of serial dilutions of the U.S. Geological Survey field leach test solution 

compared to the zinc concentrations (in micrograms per liter) for the mineral hemimorphite. EC50,

half-maximal effective concentration.
64

Figure 44. Percent inhibition of serial dilutions of the U.S. Geological Survey field leach test solution 

compared to the zinc concentrations (in micrograms per liter) for the mineral hydrozincite. EC50,

half-maximal effective concentration.
65

Figure 45. Percent inhibition of serial dilutions of the U.S. Geological Survey field leach test solutions 

compared to the zinc concentrations (in micrograms per liter) for the minerals smithsonite, sphalerite 

(Balmat, New York), and sphalerite (Creede, Colorado). EC50, half-maximal effective concentration.

Enzyme production inhibition is greater than 50 percent for the 100-percent USGSFLT solution and the next several most-concentrated dilutions produced from the hydrozincite 

USGS-FLT leachates (fig. 44) and the smithsonite and sphalerite samples (fig. 45). Due to the 

high concentration of zinc (120,000 micrograms per liter (μg/L)) in the USGS-FLT for 

hydrozincite, a dilution factor of 1:10 was used to obtain a range to calculate the EC50.

The Balmat sphalerite was leached in duplicate (fig. 46), and each leach was analyzed 

in triplicate allowing determination of the error for the analyses. An evaluation of the original 

Balmat sphalerite leachate data (fig. 46) indicates that the two datasets are statistically 

indistinguishable; all of these data were combined to determine the EC50 value for the Balmat 

sphalerite mineral concentrate.
66

Figure 46. Percent inhibition of serial dilutions of the U.S. Geological Survey field leach test solution 

compared to the zinc concentrations (in micrograms per liter) for duplicate leaches of sphalerite 

(Balmat, New York). EC50, half-maximal effective concentration.

EC50 for each of the five mineral-concentrate USGS-FLT leachate solutions was 

determined by fitting a logarithmic curve through five or more points (the number of points 

depended on obtaining the best fit) around 50-percent inhibition and determining the zinc 

concentration at the intersection of the fitted curve with the 50-percent inhibition line (figs. 47

and 48).
67

Figure 47. Percent inhibition of serial dilutions of the U.S. Geological Survey field leach test solution 

compared to the zinc concentrations (in micrograms per liter) for the mineral hydrozincite; the five 

dilutions nearest 50-percent inhibition with the fitted logarithmic curve. EC50, half-maximal effective 

concentration.
68

Figure 48. Percent inhibition of serial dilutions of the U.S. Geological Survey field leach test solution 

compared to the zinc concentrations (in micrograms per liter) for the minerals smithsonite, sphalerite 

(Balmat, New York), and sphalerite (Creede, Colorado); the 7 (smithsonite), 6 (sphalerite (Creede)), 

or 10 (sphalerite (Balmat)) dilutions nearest 50-percent inhibition with the fitted logarithmic curve.

EC50, half-maximal effective concentration.

Tabulated inhibition results may not reflect the effect of zinc alone. The observed 

inhibition may be enhanced or diminished by other metals and constituents derived from the 

five mineral-concentrate leachates (table 4).
69

Table 4. Summary of the EC50 (half-maximal effective concentration) values and 100-percent leachate 

concentrations for zinc, cadmium, lead, calcium, magnesium, sulfate chlorine, and pH (from P. 

Hageman, this volume) for five zinc-bearing minerals and diluent.

[nc = no chronic toxicity at the leach concentration; EC50 is outside of the concentration range. μg/L, micrograms 

per liter; mg/L, milligrams per liter]

Mineral → Diluent

Hemimorphite

Zn4Si2O7(OH)2 –

H2O

Hydrozincite

Zn5(CO3)2(OH)6

Smithsonite

ZnCO3

Sphalerite

(Balmat)

ZnS

Sphalerite

(Creede)

ZnS

EC50

(Zinc μg/L) na nc 112.7 87.3 79.1 126.0

100-percent leachate ↓

 concentration ↓

Zinc μg/L 75 120000 2400 390 6500

Cadium μg/L 25 1300 43 0.12 16

Lead μg/L 0.3 430 21 1.7 140

Calcium mg/L 7.0 2.7 240 1.4 <0.1 0.53

Magnesium mg/L 6.1 <0.1 15 0.13 <0.1 0.18

Sulfate (SO4

-2

) mg/L <1 41 <1 <1 9.3

pH 7.6 7.7 6.6 7.1 7.2 6.1

The EC50 for the hemimorphite mineral concentrate is indeterminate because the 

derived leachate concentration for zinc does not produce inhibition for 50 percent or more of 

the organisms. Determined EC50 zinc values for the hydrozincite (112.7 μg/L) and the Creede 

sphalerite (126.0 μg/L) are higher (table 4) than the EC50 determined for smithsonite (87.3 

μg/L) and the Balmat sphalerite (79.1 μg/L). The 100-percent leachate derived from the 

hydrozincite and the Creede sphalerite mineral concentrates contain significant sulfate, which 

may form soluble complexes with the zinc; both leachates also contain calcium and 

magnesium that can compete and limit the uptake of zinc. The reduction of inhibition due to 

the competition of calcium and magnesium with zinc may have increased the EC50 for the 

hydrozincite mineral concentrate; since 1:10 dilutions were required to achieve the necessary 

range, the concentration of calcium (7 milligrams per liter (mg/L)) and magnesium (6.1 mg/L) 

in the diluent are important because they can compete with zinc for uptake by the organism. 

The carbonate molecule can form soluble complexes with the zinc and is a significant 

constituent of smithsonite; also, the leachate contains significant amounts of calcium and 

magnesium reducing the zinc uptake and increasing the EC50.

Determined EC50 values for the five minerals are similar, which suggests that replicate 

MetPLATETM analyses should be performed to determine if the EC50 values are statistically 

different. In order to more broadly establish the toxicity of leachates derived from the five 

zinc mineral concentrates, additional toxicity tests using an array of different aquatic species 

should be conducted to determine the EC50 for additional organisms.

Conclusion

Metallic zinc is used in the production of alloys and as a coating for iron and steel 

structures to prevent corrosion in most atmospheres (Zinc Information Center, 2012). Zinc 

oxide is used as an additive in rubber, ceramics, various chemicals, paints, and 

pharmaceuticals (International Zinc Association, 2012).
70

As a discrete element, zinc poses almost no risk to human health or the natural 

environment; the World Health Organization recommends pure zinc supplementation for 

severe malnutrition, diarrhea, and disease prevention, and to reduce premature mortality in 

poor, underdeveloped countries (WHO, 2006). The U.S. Environmental Protection Agency 

classifies zinc as a nuisance element, one that may produce an unpleasant odor or metallic 

taste in water but is otherwise neutral, or nontoxic (USEPA, 2006). Neutrality though, as this 

assessment demonstrated, is not characteristic of zinc-bearing minerals.

Findings presented in this volume demonstrate that the selected zinc-bearing minerals 

contain significant cadmium and lead. As revealed by Benzel and Diehl (this volume), 

hydrozincite contains lead as a replacement element and both lead and cadmium as surficial 

carbonate precipitates. The authors suggest that the precipitates are susceptible to dissolution 

and discharge of primary metals. Through a series of chemical analyses and leach tests, 

Hageman (this volume) confirmed the presence of lead and cadmium in the hemimorphite 

sample; his FLT test produced a pH value for hemimorphite of 9.20—evidence of the 

solubility and subsequent migration of acid-producing heavy metals. The simulated extraction 

fluid studies demonstrated that cadmium and lead, present in all of the study minerals, are not

only soluble in all experimental biological fluids, but are readily absorbed, or bioaccessible, in 

acidic bio-environments, such as the stomach. The results of the toxicity study suggest that 

elevated levels of lead and cadmium, as well as other metals, may inhibit production of a 

critical reproduction enzyme.

These elements, if liberated from the mineral structure (through either artificial 

processes such as mining or smelting, or through a natural process like water infiltration),

have the potential to compromise the health of most living organisms. Cadmium, particularly, 

is an element of concern. The EPA classifies cadmium as a probable human carcinogen, and 

both the International Agency for Research on Cancer and the National Toxicity Program 

classify cadmium as a known human carcinogen (ATSDR, 2008). Lead, also recognized as a 

toxic metal, contributes to fine-motor-function deficits, increased blood pressure, cataracts, 

and cognitive impairment (NIEHS, 2012). Both of these elements were detected in the studied 

zinc minerals.

Although zinc is the dominant metal in these minerals, the abundance and toxicity of 

soluble cadmium and lead suggest that rocks that contain appreciable quantities of zinc 

minerals pose a significant health risk and that exposure to common zinc-bearing minerals 

should be limited and (or) regulated.

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http://water.epa.gov/drink/contaminants/secondarystandards.cfm.

U.S. Environmental Protection Agency (USEPA), 2008, Standard operating procedure for an 

in vitro bioaccessibility assay for lead in soil: U.S. Environmental Protection Agency

9200.1-86, November 2008, available at 

http://www.epa.gov/superfund/bioavailability/pb_ivba_sop_final.pdf.
75

Van Wijnen, J.H., Clausing, P., and Brunekreef, B., 1990, Estimated soil ingestion by 

children: Environmental Research, v. 51, p. 147–162.

Ward, M.L., Bitton G., and Townsend T., 2005, Heavy metal binding capacity (HMBC) of 

municipal solid waste landfill leachates: Chemosphere, v. 60, p. 206–215.

World Health Organization (WHO), 2006, The impact of zinc supplementation on childhood 

mortality and severe morbidity: World Health Organization Workshop, Geneva, 

Switzerland, September 15–16, 2006, 11 p., available at 

http://www.who.int/maternal_child_adolescent/documents/zinc_mortality/en/index.html.

Young, R., 1995, The Rietveld method: New York, University Press, International Union of 

Crystallography Monographs no. 5, 289 p. (See appendix 1.)

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September 4, 2012, at http://www.zincinfocentre.org.
76

Appendix A

Panalytical X’PERT X-ray Diffractometer Instrument Set-Up

The following instrument conditions were used to collect scan data:

• voltage (kV) 45

• current (mA) 40

• scan range (o2θ) 5 to 90 o2θ

• step 0.0167 o2θ

• seconds per step 5

• tube divergence slit 2°

• tube scatter slit 4°

• detector scatter slit 0.5°

• detector reference slit 0.2°

• strip detector with an active area of 127 points in 2.12 o2θ

The Panalytical X’Pert x-ray diffractometer was calibrated using National Institute of 

Standards and Technology Silicon x-ray Diffraction standard SRM 640. The calibration 

involves measuring the full width half maximum (FWHM) of the silicon standard over full 

scan range of the instrument, 0 to 90 o2θ for the Scintag. The line broadening and goniometer 

aberrations are plotted to create a calibration curve specific to each instrument. The dataprocessing software described below uses this calibration to correct the measured sample 

scans.

In general, XRD has a detection limit of approximately 3 weight percent. Highly 

crystalline minerals have a lower detection limit (approximately 1–3 percent) and poorly 

crystalline minerals have a higher detection limit (approximately 3–5 percent). Amorphous 

materials do not produce unique reflections; however, they do contribute to the background 

intensity of the scan, which allows an estimate to be made of the total amorphous content.

Data Processing

First, the observed pattern, or data, is acquired from the XRD instrument. X-ray 

powder diffraction scans were reviewed for mineral phases present in each specimen. Mineral 

phases are identified in a scan by comparing observed reflections, both two-theta position and 

intensity, with reference standards. Mineral databases from the International Center for 

Diffraction Data (ICDD) and the National Institute of Standards and Technology Inorganic 

Crystal Structure Database (ICSD) (Belsky, 2002) were used to search for phases. Once all 

the reflections are assigned to mineral phases, the data was analyzed using Jade Whole Pattern 
77

Fit Analyses software (WPF v.9.0.0). With the Jade software, whole pattern fitting of the 

observed data and Rietveld refinement of crystal structures are performed simultaneously 

(Young, 1995).

Limitations of the data

The WPF software normalizes the data to 100 percent for all identified phases. The 

typical detection limit by X-ray diffraction is between 1 and 3 weight percent, depending on 

the crystallinity of the phase and interference from overlapping lines from other phases. Thus, 

there may be trace phases present, but not identified, and they are not included in the model. 

Furthermore, the amorphous content is calculated based on the internal standard. Any error 

introduced by grinding and blending that alters the weight ratio of the sample to internal 

standard will yield anomalous amorphous content, which in turn will be carried forward into 

the normalization and effect the mineral contents.

The WPF software calculates the unit cell for phases selected by the user. The unit 

cell (or lattice parameter) calculation produces the best results for major components because 

the XRD scan includes sufficient information (reflections and intensity). Minor and trace 

components having less intensity and often weak reflections do not show up in the scan. 

Lastly, as the number of phases increase in the sample, the line overlap (interference) 

increases, which reduces the accuracy of the unit-cell calculation. 

Appendix B

Complete bulk geochemistry results for five zinc-bearing minerals.

Appendix B. Bulk chemical composition of five zinc minerals.

[All results by inductively coupled plasma‒mass spectrometry except mercury (cold-vapor atomic fluorescence), and 

total carbon and total sulfur by LECO; na = not analyzed; < = less than; mg/L = milligrams per liter; μS/cm = micro 

Siemens per centimeter; mg/kg = milligrams per kilogram; wt.% = weight percent]

Mineral Ag Al As Ba Be Bi CO2

Carbonate 

C Ca

(mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (wt. %) (wt. %) (mg/kg)

Sphalerite/Balmat 1.3 200 6.7 2.9 < 0.03 < 0.06 na na 1360

Sphalerite/Creede 17 5300 120 29 0.23 0.12 na na 103

Hemimorphite 12 650 1600 48 31 < 0.06 na na 18000

Smithsonite 6.9 1300 41 2.4 0.27 9.0 20 5.5 10200

Hydrozincite 31 560 70 6.2 0.31 0.23 13 3.6 13400

Mineral Cd Ce Co Cr Cs Cu Fe Hg Ga

(mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)

Sphalerite/Balmat 1100 0.3 0.91 1 0.02 48 62000 17 21

Sphalerite/Creede 4100 16 23 1.9 0.12 792 26000 1.1 23

Hemimorphite 1800 0.99 3.9 1.9 0.19 18 130000 0.75 2.3

Smithsonite 3000 0.82 910 1.7 0.12 170 2000 0.86 1.3

Hydrozincite 500 1.3 3.6 5.1 0.09 2300 240 2.9 0.78
78

Appendix B. Bulk chemical composition of five zinc minerals.—Continued

[All results by inductively coupled plasma‒mass spectrometry except mercury cold-vapor atomic fluorescence, and 

total carbon and total sulfur by LECO; na = not analyzed; < = less than; mg/L = milligrams per liter; μS/cm = micro 

Siemens per centimeter; mg/kg = milligrams per kilogram; wt.% = weight percent] 

Mineral K La Li Mg Mn Mo Na Nb Ni

(mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)

Sphalerite/Balmat < 20 0.2 < 0.3 330 930 0.52 < 20 < 0.1 < 0.3

Sphalerite/Creede 1200 10 < 0.3 2300 1300 37 < 20 < 0.1 0.9

Hemimorphite < 20 1.1 < 0.3 630 10000 48 < 20 < 0.1 14

Smithsonite 19 1 < 0.3 2900 570 4.5 < 20 < 0.1 4.2

Hydrozincite 100 2 < 0.3 2000 1300 1.3 650 < 0.1 14

Mineral P Pb Rb Total S Sb Sc Se Sr Th

(mg/kg) (mg/kg) (mg/kg) (wt. %) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)

Sphalerite/Balmat 230 61 0.24 33 0.73 < 0.04 9.5 3.2 0.32

Sphalerite/Creede < 5 14000 5 31 17 0.5 9.7 2.6 0.35

Hemimorphite < 5 1300 0.29 na 8.2 < 0.04 5.3 3.9 < 0.1

Smithsonite < 5 1200 0.47 na 3.6 < 0.04 8 2.9 < 0.1

Hydrozincite < 5 5000 0.46 na 50 < 0.04 9.9 30 < 0.1

Mineral Ti Tl U V Y Zn

(mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)

Sphalerite/Balmat < 40 <0.08 4.9 < 0.2 0.36 640000

Sphalerite/Creede 110 0.66 0.64 6.4 1.9 590000

Hemimorphite < 40 0.67 31 37 4.2 380000

Smithsonite 54 <0.08 0.41 2.9 3.1 500000

Hydrozincite 42 <0.08 510 22 12 560000

Appendix C 

Appendix C. Complete leachate analytical chemistry results for five zinc-bearing minerals

(AppendixC.xlsx) 

Appendix D 

Appendix D. Leachate concentrations as milligrams leached per kilogram solid for measured 

elements by mineral and as measured in simulated gastric fluid (SGF), simulated intestinal fluid 

(SIF), simulated lung fluid (SLF), and simulated phagolysosomal fluid (SPF). (AppendixD.xlsx) 

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20135223