7.3 Sulfide Mineral Structure and Chemistry
4 min read•july 31, 2024
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
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- 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 (galena), hexagonal close-packed (wurtzite), and more complex arrangements (sphalerite)
- Coordination numbers typically range from 4 to 6
- most common in
- seen in minerals like
- Examples of sulfide structures:
- Galena (PbS): cubic close-packed, NaCl-type structure
- Sphalerite (ZnS): cubic close-packed,
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- 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 (), PbS (galena)
- Complex ratios: FeS₂ (pyrite), CuFeS₂ (chalcopyrite)
- 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 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
Key Terms to Review (22)
Chalcophile Elements: Chalcophile elements are a group of chemical elements that have a strong affinity for sulfur and tend to bond with it to form sulfide minerals. This term is important in understanding the distribution and behavior of these elements within sulfide mineral structures and their significance in various geological and industrial processes. Typically, these elements include metals like copper, lead, and zinc, which are commonly found in ore deposits alongside sulfide minerals.
Cubic close-packed: Cubic close-packed (CCP) is a crystal structure where atoms are densely packed in a cubic arrangement, maximizing the use of space and minimizing empty volume. This arrangement is characterized by the stacking of layers of atoms, resulting in a coordination number of 12, which means each atom is surrounded by 12 others. This structure plays a crucial role in the organization and properties of various minerals, especially sulfides and oxides.
Cubic structure: A cubic structure refers to a crystal system where the unit cell is shaped like a cube, exhibiting equal dimensions along all three axes and 90-degree angles between them. This geometric arrangement is significant because it leads to various properties in minerals, influencing their symmetry, packing efficiency, and overall stability. Many minerals exhibit this structure, which can greatly affect their chemical behavior and interactions with light.
Diamond-like structure: A diamond-like structure refers to a specific arrangement of atoms in a crystalline solid, characterized by a tetrahedral coordination of atoms similar to that found in diamond. This structure is significant because it leads to properties such as high hardness and exceptional thermal conductivity, which are crucial in the context of various sulfide minerals that exhibit similar arrangements.
Galena - PbS: Galena is a lead sulfide mineral with the chemical formula PbS, and it serves as the primary ore of lead. This mineral typically crystallizes in the cubic system and often forms in hydrothermal veins associated with other sulfide minerals. Its unique structure and composition make galena an important subject of study in understanding sulfide mineral chemistry and the geological processes that produce these minerals.
Hexagonal Structure: A hexagonal structure is a crystalline arrangement where atoms are arranged in a hexagonal lattice, characterized by six-fold symmetry. This structure plays a crucial role in defining the properties and behaviors of various minerals, particularly sulfide minerals, which can display distinct physical and chemical characteristics based on their crystallography.
High Density: High density refers to the mass per unit volume of a material, indicating that it has a significant amount of mass packed into a relatively small space. In mineralogy, high density is a critical characteristic that can influence the formation, stability, and classification of minerals, especially within sulfide mineral structures where heavy metal components often contribute to their overall density.
Hydrothermal deposition: Hydrothermal deposition is the process through which minerals precipitate from hydrothermal fluids, typically hot water solutions rich in dissolved minerals, as these fluids cool or react with surrounding rocks. This mechanism plays a crucial role in forming various mineral deposits, particularly native elements and sulfides, by allowing minerals to crystallize as the conditions of pressure and temperature change.
Isomorphism: Isomorphism refers to the phenomenon where two or more minerals share the same crystal structure but differ in chemical composition. This concept is significant as it helps in understanding how different minerals can exhibit similar physical properties, such as hardness and cleavage, even though they are composed of different elements or compounds.
Metallic luster: Metallic luster refers to the reflective quality of minerals that resembles that of metals, giving them a shiny and often opaque appearance. This characteristic is influenced by the arrangement of electrons in the mineral’s structure, allowing it to reflect light effectively. Minerals with metallic luster typically exhibit colors that are more vibrant and can often be identified by their distinct sheen, which is crucial for mineral identification and classification.
Mineral resource: A mineral resource is a naturally occurring substance that is valuable for economic extraction and can be utilized in various industries, including construction, technology, and energy production. These resources include metals, non-metals, and industrial minerals, all of which have specific uses based on their chemical and physical properties. Understanding the composition and structure of mineral resources, especially sulfide minerals, is essential for efficient extraction and utilization.
Non-sulfide minerals: Non-sulfide minerals are a broad category of minerals that do not contain sulfide ions (S^2−) in their chemical composition. These minerals typically include oxides, carbonates, halides, and phosphates, and they often have distinct structural and chemical properties that differentiate them from sulfide minerals. Understanding non-sulfide minerals is crucial as they often play a significant role in various geological processes and have different applications in industries such as agriculture, electronics, and construction.
Octahedral Coordination: Octahedral coordination refers to a specific arrangement of atoms or ions around a central atom, where six surrounding atoms are positioned at the corners of an octahedron. This geometric configuration is significant in understanding how various minerals are structured and how they interact with other elements, particularly in sulfide, sulfate, and halide minerals. The octahedral shape allows for specific bonding angles and distances that influence the stability and properties of the minerals formed.
Ore: Ore is a naturally occurring solid material from which a metal or valuable mineral can be extracted profitably. It typically contains a mixture of minerals, and its economic value comes from the concentration of these valuable components, often found in sulfide minerals. Understanding ore is essential in mineralogy, particularly when examining the structure and chemistry of sulfide minerals, as many ores are composed primarily of sulfides that host significant metal deposits.
Pyrite: Pyrite, commonly known as 'fool's gold', is a sulfide mineral with the chemical formula FeS₂. This mineral is often found in sedimentary and metamorphic rocks and is characterized by its brassy-yellow color and metallic luster, making it visually similar to gold. In the context of sulfide mineral structure and chemistry, pyrite plays an important role due to its unique crystal structure, properties, and its involvement in various geological processes.
Scanning Electron Microscopy: Scanning electron microscopy (SEM) is an advanced imaging technique that uses focused beams of electrons to produce high-resolution images of the surface of materials, revealing detailed information about their morphology and composition. SEM is crucial for studying minerals as it allows researchers to visualize fine details and analyze the elemental composition of mineral samples, providing insights into their structure and properties.
Solid Solution: A solid solution is a homogeneous mixture of two or more minerals or elements where the atoms can substitute for one another in the crystal lattice without changing the overall structure. This concept highlights how minerals can vary in composition while maintaining their essential properties, which is crucial for understanding mineral formation, structural diversity, and classification in various mineral types.
Sphalerite - ZnS: Sphalerite is a sulfide mineral composed primarily of zinc sulfide (ZnS) and is known for its importance as a major ore of zinc. This mineral typically forms in hydrothermal deposits and is characterized by its distinctive cleavage, high refractive index, and wide range of colors, which can include yellow, brown, and even colorless varieties. Sphalerite's crystal structure is an example of how sulfide minerals can exhibit varied physical properties depending on their composition and formation conditions.
Tetrahedral coordination: Tetrahedral coordination refers to the arrangement of four atoms or ions around a central atom, forming the shape of a tetrahedron. This geometric configuration is essential in understanding the structural and chemical properties of various minerals, influencing their stability and reactivity. In minerals, tetrahedral coordination is often seen in sulfide and sulfate minerals, playing a key role in their crystal structures and overall chemistry.
Troilite: Troilite is an iron sulfide mineral with the chemical formula FeS, commonly found in meteorites and certain geological settings on Earth. It typically forms under reducing conditions and is recognized for its unique structure and properties, particularly its role in the study of sulfide minerals and their formation processes. The presence of troilite can provide insights into the thermal history of meteorites and the conditions under which they formed.
X-Ray Diffraction: X-ray diffraction is a powerful analytical technique used to study the structure of crystalline materials by measuring the angles and intensities of X-rays scattered by the crystals. This method is crucial for understanding mineral structures, identifying minerals, and determining their properties, linking it closely to various aspects of mineralogy and crystallography.
Zinc-blende structure: The zinc-blende structure is a crystal lattice configuration commonly found in zinc sulfide (ZnS) and various other metal sulfide minerals. In this structure, zinc and sulfur atoms are arranged in a face-centered cubic lattice, where each zinc atom is tetrahedrally coordinated by sulfur atoms. This arrangement gives rise to unique physical and chemical properties characteristic of sulfide minerals, influencing their behavior in geological processes.