Sulfide minerals are a fascinating group with diverse structures and compositions. They're formed when sulfur bonds with metal cations, creating crystals with unique properties. From simple galena to complex chalcopyrite, these minerals showcase a range of coordination and packing arrangements.

The chemistry of sulfides is equally intriguing. Their formulas vary from simple 1:1 ratios to more complex stoichiometries. Factors like temperature, pressure, and chemical environment affect their stability and can lead to transformations. Understanding sulfide structures is key to grasping their mechanical, physical, and chemical properties.

Crystal structures of sulfide minerals

Coordination and packing arrangements

• Sulfide minerals contain sulfur as the major anion bonded with various metal cations
• Crystal structures determined by coordination preferences of metal cations and size ratio between cation and sulfur anion
• Common structures include cubic close-packed (galena), hexagonal close-packed (wurtzite), and more complex arrangements (sphalerite)
• Coordination numbers typically range from 4 to 6
  • Tetrahedral coordination most common in zinc-blende structure
  • Octahedral coordination seen in minerals like pyrite
• Examples of sulfide structures:
  • Galena (PbS): cubic close-packed, NaCl-type structure
  • Sphalerite (ZnS): cubic close-packed, diamond-like structure

Bonding characteristics

• Bonding predominantly covalent with some ionic character
• Strength and nature of metal-sulfur bond influences properties:
  • Hardness
  • Melting point
  • Electrical conductivity
• Metallic or semi-metallic luster results from electronic structure and bonding
• Bonding affects band structure and electron mobility:
  • Contributes to electrical and thermal conductivity
  • Influences optical properties like color and reflectance
• Examples of bonding effects:
  • Strong covalent bonds in pyrite (FeS2) lead to high hardness
  • Weaker bonding in galena (PbS) results in perfect cubic cleavage

Chemical compositions of sulfide minerals

Stoichiometry and formulas

• General formula X₁Y₁, where X represents metal cation and Y represents sulfur
• Stoichiometry varies from simple 1:1 ratios to more complex ratios
  • Simple ratios: FeS (troilite), PbS (galena)
  • Complex ratios: FeS₂ (pyrite), CuFeS₂ (chalcopyrite)
• Solid solution series allow varying proportions of cations to substitute within crystal structure
  • Example: (Zn,Fe)S in sphalerite-wurtzite series
• Valence state crucial for understanding compositions
  • Many metals exist in multiple oxidation states when bonded with sulfur
  • Example: Iron in Fe2+ state in pyrrhotite (Fe1-xS) vs Fe3+ in greigite (Fe3S4)

Compositional variations and impurities

• Common metal cations include iron, copper, zinc, lead, nickel, and mercury
• Trace element substitutions affect properties and aid in geochemical studies
  • Example: Trace amounts of silver in galena (PbS) important for silver production
• Impurities and non-stoichiometric compositions lead to variations in:
  • Color (e.g., iron content affecting sphalerite color)
  • Electrical properties (e.g., semiconducting behavior in non-stoichiometric pyrrhotite)
  • Other physical characteristics (e.g., magnetic properties in monoclinic pyrrhotite)
• Examples of compositional effects:
  • Copper content in bornite (Cu5FeS4) causes iridescent tarnish
  • Nickel content in pentlandite ((Fe,Ni)9S8) crucial for nickel ore deposits

Stability of sulfide minerals

Environmental factors affecting stability

• Stability varies under different temperature, pressure, and chemical environment conditions
• Phase diagrams essential for understanding stability fields and transformations
• Oxygen fugacity heavily influences stability
  • Many sulfides oxidize to form sulfates or oxides under oxidizing conditions
  • Example: Pyrite (FeS2) oxidizing to form iron oxides and sulfuric acid
• Hydrothermal alteration processes affect stability and transformation in ore deposits
  • Can lead to formation of new sulfide minerals or alteration of existing ones
  • Example: Chalcopyrite (CuFeS2) altering to bornite (Cu5FeS4) in porphyry copper deposits
• Weathering in near-surface environments leads to secondary mineral formation
  • Contributes to acid mine drainage through sulfide oxidation
  • Example: Pyrite weathering to form iron hydroxides and sulfuric acid

Phase relations and transformations

• Polymorphic transformations occur at specific temperatures or pressures
  • Result in different crystal structures with same chemical composition
  • Example: Sphalerite to wurtzite transformation at high temperatures
• Non-stoichiometric compositions exhibit complex phase relations
  • Dependent on temperature and sulfur fugacity
  • Example: Pyrrhotite (Fe1-xS) showing multiple phases with varying iron content
• Pressure effects on sulfide stability
  • Some sulfides undergo phase transitions at high pressures
  • Example: High-pressure polymorph of pyrite (FeS2) formed in deep subduction zones

Structure vs properties of sulfide minerals

Mechanical and physical properties

• Crystal structure directly influences:
  • Cleavage (e.g., perfect cubic cleavage in galena due to NaCl-type structure)
  • Fracture patterns (e.g., conchoidal fracture in sphalerite)
  • Overall mechanical strength (e.g., high hardness in pyrite due to strong covalent bonds)
• Thermal properties related to bonding strength and lattice vibrations
  • Heat capacity and thermal conductivity vary among sulfides
  • Example: Pyrite has higher thermal conductivity than most other sulfides
• Magnetic properties result from specific crystal structures and cation ordering
  • Example: Ferrimagnetic behavior in monoclinic pyrrhotite due to Fe vacancies

Electronic and chemical properties

• Electrical conductivity related to electronic band structure
  • Determined by arrangement and bonding of atoms in crystal lattice
  • Example: Metallic conduction in pyrite vs semiconducting behavior in sphalerite
• Optical properties tied to crystal structure and electronic transitions
  • Color and reflectance vary among sulfides
  • Example: High reflectance and brass-yellow color of pyrite due to its electronic structure
• Reactivity in chemical environments influenced by accessibility of metal cations
  • Crystal structure affects surface reactivity and dissolution kinetics
  • Example: Faster dissolution of galena compared to more stable pyrite in acidic solutions
• Solubility and dissolution kinetics dependent on crystal structure stability
  • Strength of metal-sulfur bonds affects mineral's resistance to weathering
  • Example: Rapid weathering of marcasite (FeS2) compared to more stable pyrite polymorph