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Native Elements, Sulfides, and Sulfosalts

GLY 4200

Lab 2 - Fall, 2014

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Native Elements

• The native elements may be divided into metallic, non-metallic, and semi-metallic

• Native elements are composed of a single element, uncombined with other elements

• They are often ores, especially the metallic native elements Gold, Silver, Copper, Platinum

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Native Element Luster

• The luster shows distinctive changes from one group to another Metallic group – metallic to dull metallic Semi-metal group – submetallic to dull metallic Non-metallic – various non-metallic lusters

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Sulfides

• A sulfide is a compound between sulfur, or another sulfur group element, and a metallic element

• Most of these minerals are opaque, and they often have characteristic colors, as well as distinctive streak colors

• A few are transparent to translucent – examples cinnabar, realgar, and orpiment

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Sulfide Formulas

• The general formula is AmXn where A represents metallic elements, X represents a sulfide group element whose oxidation state is -2, and m and n are integers

• The X anions are usually in the negative two oxidation state Exception: Minerals with the S2 dimer, such as

pyrite, marcasite, and arsenopyrite

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Sulfide Formulas continued

• X can be Sulfur Selenium Tellurium Arsenic Antimony

Position in Periodic Table

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Other Oxidation States

• All of the sulfide group elements occur in other oxidation states, for example As5+ forms arsenates

• But in the 2- state they belong to the sulfide group

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Anion Size

• All of the sulfide group anions are large

• The large anionic size tends to favor coordination with large cations As a result sulfide minerals are very often

important economic minerals (ores) because they associate with the relatively rare larger cations

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Anion Size Versus Coordination Number

Anion IV VI VIII

O2- O.130 0.132 0.134

S2- 0.156 0.172 0.178

Se2- 0.188 ______ 0.190

All sizes are in nanometers

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Sulfide Ores• Major sulfide ore minerals include:

Mineral Element

Galena Lead

Sphalerite Zinc

Chalcopyrite, Chalcocite, Bornite, Covellite

Copper

Greenockite Cadmium

Argentite Silver

Pentlandite, Niccoline, Millerite Nickel

Cinnabar Mercury

Stibnite Antimony

Arsenopyrite Arsenic

Molybdenite Molybdenum

Cobaltite Cobalt

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Solid Solution

• The sulfides are also characterized by extensive solid solutions

• Many of the cations are of similar size and charge and they can substitute extensively

• Anionic substitution also occurs• The physical characteristics of solid

solutions will be between their end-member minerals

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Solid Solution Example

• Galena is PbS, the only ore of lead

• Galena is found with inclusions of other elements Ag, Zn, Cd, Sb, As & Bi

• Se can substitute for S and forms a complete solid solution series

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Sulfide Characteristics

• Many sulfide minerals have resonant bonding, with a partial metallic characteristic Results in metallic to dull metallic luster

• SG is often high, because of the presence of metallic elements, and sometimes heavy anions like Se or Te

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Sulfide Hardness

• Most sulfide group minerals are soft to soft-moderate hardness Exception: Sulfides with the S2 dimer, such as

pyrite, marcasite, and arsenopyrite, are moderately hard

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Sulfosalts

• This term signifies a mineral in which a semi metallic element replaces the metals Examples: enargite, tetrahedrite

Text Reference

• See chapter 15 in the text for more information on native elements, sulfides, and sulfosalts, pp. 331-367

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