Chemical formulas are the language of minerals, revealing their composition and structure. They show us how elements combine to form these natural wonders. Understanding formulas is key to grasping mineral properties and behavior in various geological processes.

Minerals are classified based on their chemistry and structure. This system helps us organize the vast world of minerals, from simple native elements to complex silicates. Knowing how minerals are grouped aids in identifying them and understanding their relationships to one another.

Chemical formulas of minerals

Composition and structure representation

• Chemical formulas of minerals represent relative proportions of elements in crystal structures expressed as chemical symbols and numerical subscripts
• Empirical formula shows simplest whole-number ratio of atoms in mineral structure
• Structural formula provides more detailed information about atom arrangement
• Cations (positively charged ions) written first, followed by anions (negatively charged ions) reflecting electrostatic interactions
• Oxidation states of elements provide information about electronic configuration and bonding behavior of atoms
• Solid solution series represented by parentheses indicating variable composition between end-members (olivine (Mg,Fe)₂SiO₄)

Water and hydroxyl groups

• Hydrous minerals include water molecules or hydroxyl groups in formulas
• Water molecules denoted as "·nH₂O" (gypsum CaSO₄·2H₂O)
• Hydroxyl groups denoted as "(OH)" (kaolinite Al₂Si₂O₅(OH)₄)
• Presence of water or hydroxyl groups significantly affects mineral properties (cleavage, hardness)

Mineral classification

Chemical composition-based classification

• Primary classification based on anionic groups divides minerals into categories
  • Native elements (gold Au, silver Ag)
  • Sulfides (pyrite FeS₂, galena PbS)
  • Oxides (hematite Fe₂O₃, magnetite Fe₃O₄)
  • Halides (halite NaCl, fluorite CaF₂)
  • Carbonates (calcite CaCO₃, dolomite CaMg(CO₃)₂)
  • Sulfates (gypsum CaSO₄·2H₂O, barite BaSO₄)
  • Silicates (quartz SiO₂, feldspar KAlSi₃O₈)
• Silicate minerals further classified based on silica tetrahedra arrangement
  • Nesosilicates (olivine (Mg,Fe)₂SiO₄)
  • Sorosilicates (epidote Ca₂(Al,Fe)₃(SiO₄)₃(OH))
  • Cyclosilicates (beryl Be₃Al₂Si₆O₁₈)
  • Inosilicates (pyroxenes, amphiboles)
  • Phyllosilicates (micas, clays)
  • Tectosilicates (quartz, feldspars)

Structural classification and phenomena

• Crystal system determined by internal atomic arrangement crucial for classification
  • Seven possible systems cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal
• Isomorphous substitution occurs when ions of similar size and charge replace each other in crystal structure
  • Leads to solid solution series (plagioclase feldspar series)
  • Affects mineral classification and properties
• Polymorphism refers to substances with identical chemical compositions but different crystal structures
  • Results in distinct mineral species (diamond and graphite, both composed of carbon)
• Mineral groups collections of minerals with similar chemical compositions and crystal structures
  • Share same anionic group but differ in dominant cation (garnet group Mg, Fe, Mn, Ca garnets)

Mineral reactions

Types of mineral reactions

• Mineral formation reactions involve combination of aqueous ions or alteration of existing minerals
  • Occurs under specific pressure and temperature conditions
  • Example $Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3$ (calcite precipitation)
• Weathering reactions typically involve hydrolysis, oxidation, or dissolution processes
  • Often result in formation of clay minerals or soluble ions
  • Example $2KAlSi_3O_8 + 2H^+ + H_2O \rightarrow Al_2Si_2O_5(OH)_4 + 4SiO_2 + 2K^+$ (feldspar weathering to kaolinite)
• Metamorphic mineral reactions involve solid-state transformations, dehydration, or decarbonation processes
  • Reflect changes in pressure and temperature conditions
  • Example $CaCO_3 \rightarrow CaO + CO_2$ (decarbonation of calcite to lime)
• Redox reactions involve transfer of electrons between species, changing oxidation states of elements
  • Often result in formation of new mineral phases
  • Example $4FeS_2 + 15O_2 + 8H_2O \rightarrow 4Fe(OH)_3 + 8H_2SO_4$ (pyrite oxidation)

Reaction balancing and environmental factors

• Balanced chemical equations for mineral reactions adhere to law of conservation of mass
  • Number and types of atoms equal on both sides of equation
• Acid-base reactions common in carbonate and silicate weathering processes
  • Involve interaction of H⁺ ions with mineral surfaces
  • Example $CaCO_3 + H^+ \rightarrow Ca^{2+} + HCO_3^-$ (calcite dissolution)
• Precipitation and dissolution reactions governed by solubility products and saturation indices
  • Depend on ion concentrations and solution conditions
  • Example $BaSO_4 \rightleftharpoons Ba^{2+} + SO_4^{2-}$ (barite equilibrium)

Element percentages in minerals

Calculation methods

• Weight percentage of element calculated by dividing total mass of element by total mass of mineral and multiplying by 100
  • Formula \text{Weight % } = \frac{\text{Mass of element}}{\text{Total mass of mineral}} \times 100
• To calculate elemental weight percentages
  • Multiply number of atoms of each element in mineral formula by its atomic weight
  • Divide by total molecular weight of mineral
• Molecular weight of mineral determined by summing products of each element's atomic weight and its stoichiometric coefficient in mineral formula
  • Example For calcite (CaCO₃) $M_w = 40.08 + 12.01 + 3(16.00) = 100.09 \text{ g/mol}$

Compositional analysis and applications

• Minerals with variable compositions (solid solutions) often express weight percentages as ranges or averages
  • Reflects compositional variability (olivine (Mg,Fe)₂SiO₄)
• Electron microprobe analysis common technique for determining precise elemental weight percentages
  • Provides quantitative data on mineral composition at microscale
• Conversion between weight percentages and oxide percentages often necessary in mineralogy and petrology
  • Requires knowledge of oxide formulas and molecular weights
• Understanding elemental weight percentages crucial for
  • Mineral identification
  • Geochemical analysis
  • Determining economic viability of mineral deposits