Nesosilicates, sorosilicates, and cyclosilicates are key silicate subclasses with distinct structures. Nesosilicates have isolated tetrahedra, sorosilicates feature paired tetrahedra, and cyclosilicates form rings. These differences impact their properties and geological occurrences.

Understanding these silicate subclasses is crucial for grasping mineral formation and behavior. Their varying structures influence physical properties, stability, and formation environments, from high-temperature magmas to metamorphic and hydrothermal settings. This knowledge helps geologists interpret rock compositions and histories.

Silicate Subclasses

Structural Characteristics

• Nesosilicates feature isolated SiO4 tetrahedra not linked to other silicon tetrahedra resulting in independent silicate units
• Sorosilicates contain two SiO4 tetrahedra sharing one oxygen atom forming Si2O7 groups with Si-O-Si bridges
• Cyclosilicates comprise ring structures of linked SiO4 tetrahedra typically forming three-membered (Si3O9) or six-membered (Si6O18) rings
• Polymerization increases from nesosilicates (isolated) to sorosilicates (paired) to cyclosilicates (rings) affecting physical and chemical properties
• Cation-to-silicon ratio decreases from nesosilicates (highest) to cyclosilicates (lower) due to structural differences
  • Impacts mineral stability and formation conditions

Bonding and Stability

• Varying bonding strengths and arrangements influence stability and geological occurrence
• Nesosilicates exhibit stronger ionic bonding between isolated tetrahedra and cations
  • Results in higher melting points (olivine melts at ~1890°C)
• Sorosilicates have moderate stability due to Si-O-Si bridges
  • Epidote decomposes at ~850°C
• Cyclosilicates display unique bonding patterns within ring structures
  • Beryl has a melting point of ~1650°C, lower than many nesosilicates

Physical Properties

• Nesosilicates generally have higher density and hardness due to closer atomic packing
  • Olivine has a density of ~3.3-4.3 g/cm³ and hardness of 6.5-7 on Mohs scale
• Sorosilicates exhibit intermediate physical properties reflecting paired tetrahedral structure
  • Epidote has a density of ~3.3-3.6 g/cm³ and hardness of 6-7 on Mohs scale
• Cyclosilicates often display unique optical properties due to ring structures
  • Tourmaline shows strong pleochroism (changes color when viewed from different angles)
• Crystal symmetry influences properties like cleavage and fracture patterns
  • Beryl's hexagonal structure results in poor cleavage and conchoidal fracture

Common Silicate Minerals

Nesosilicates

• Olivine ((Mg,Fe)2SiO4) forms solid solution series between forsterite (Mg2SiO4) and fayalite (Fe2SiO4)
  • Common in mafic and ultramafic igneous rocks (basalt, peridotite)
• Garnet (X3Y2(SiO4)3, X and Y are various cations) includes several species
  • Pyrope (Mg3Al2(SiO4)3), almandine (Fe3Al2(SiO4)3), grossular (Ca3Al2(SiO4)3)
  • Indicator minerals in metamorphic rocks
• Zircon (ZrSiO4) used in geochronology due to uranium content and resistance to weathering
  • Found in igneous, metamorphic, and sedimentary rocks
• Topaz (Al2SiO4(F,OH)2) prized as a gemstone and occurs in granitic rocks and pegmatites
  • Known for its hardness (8 on Mohs scale) and perfect cleavage

Sorosilicates

• Epidote (Ca2(Al,Fe)3(SiO4)3(OH)) common in low to medium-grade metamorphic rocks
  • Often forms in calcium-rich environments during regional metamorphism
• Lawsonite (CaAl2Si2O7(OH)2·H2O) key mineral in high-pressure, low-temperature metamorphic rocks
  • Indicator of subduction zone environments
• Hemimorphite (Zn4Si2O7(OH)2·H2O) found in zinc ore deposits
  • Forms in oxidized zones of zinc-bearing mineral deposits

Cyclosilicates

• Beryl (Be3Al2Si6O18) occurs in various colors including emerald (green) and aquamarine (blue)
  • Commonly found in granitic pegmatites
• Tourmaline (complex borosilicate with varying composition) displays wide range of colors
  • Occurs in granitic rocks, pegmatites, and some metamorphic rocks
• Cordierite (Mg2Al4Si5O18) found in metamorphic rocks derived from argillaceous sediments
  • Indicator of high-temperature, low-pressure metamorphism
• Benitoite (BaTiSi3O9) rare blue cyclosilicate with unique optical properties
  • Found in limited deposits, notably in San Benito County, California

Structure vs Properties of Silicates

Density and Hardness

• Nesosilicates generally exhibit higher density and hardness due to isolated tetrahedra
  • Olivine density: 3.3-4.3 g/cm³, hardness: 6.5-7 (Mohs scale)
  • Garnet density: 3.5-4.3 g/cm³, hardness: 6.5-7.5 (Mohs scale)
• Sorosilicates show intermediate values reflecting paired tetrahedral structure
  • Epidote density: 3.3-3.6 g/cm³, hardness: 6-7 (Mohs scale)
• Cyclosilicates vary based on composition and ring structure
  • Beryl density: 2.6-2.9 g/cm³, hardness: 7.5-8 (Mohs scale)
  • Tourmaline density: 3.0-3.3 g/cm³, hardness: 7-7.5 (Mohs scale)

Optical Properties

• Nesosilicates often display isotropic optical behavior due to highly symmetric structures
  • Garnets are isotropic, showing no birefringence
• Sorosilicates exhibit anisotropic optical properties
  • Epidote shows strong pleochroism and high birefringence
• Cyclosilicates demonstrate unique optical characteristics based on ring structures
  • Tourmaline displays strong pleochroism and dichroism
  • Beryl shows weak birefringence due to its hexagonal structure

Crystal Habits and Cleavage

• Nesosilicates typically form equant crystals with poor cleavage
  • Olivine forms stubby prismatic crystals with conchoidal fracture
  • Garnet often crystallizes as dodecahedral or trapezohedral forms
• Sorosilicates show varied crystal habits and moderate cleavage
  • Epidote forms elongated prismatic crystals with perfect cleavage in one direction
• Cyclosilicates exhibit distinctive habits influenced by ring structures
  • Beryl forms hexagonal prismatic crystals with poor cleavage
  • Tourmaline develops elongated trigonal prisms with no distinct cleavage

Geologic Environments of Silicate Formation

Igneous Settings

• Nesosilicates like olivine form in high-temperature magmatic environments
  • Common in mafic and ultramafic rocks (basalt, peridotite)
  • Crystallizes early in Bowen's reaction series at temperatures >1200°C
• Cyclosilicates often occur in late-stage magmatic processes
  • Beryl and tourmaline form in granitic pegmatites
  • Crystallize from residual magmas enriched in incompatible elements

Metamorphic Environments

• Garnets (nesosilicates) form across a wide range of metamorphic conditions
  • Indicator minerals for pressure and temperature (almandine in medium-grade, pyrope in high-grade)
• Sorosilicates like epidote typically form in low to medium-grade metamorphism
  • Common in greenschist and amphibolite facies rocks
• Lawsonite indicates high-pressure, low-temperature metamorphism
  • Found in blueschist facies rocks associated with subduction zones
• Cordierite (cyclosilicate) forms in high-temperature, low-pressure metamorphism
  • Occurs in contact metamorphic aureoles and regional metamorphic terranes

Hydrothermal and Weathering Environments

• Some nesosilicates like topaz can form in hydrothermal systems
  • Associated with fluorine-rich fluids in granitic environments
• Hemimorphite (sorosilicate) occurs in oxidized zones of zinc deposits
  • Forms through weathering and alteration of primary zinc minerals
• Tourmaline (cyclosilicate) often indicates boron-rich hydrothermal activity
  • Found in hydrothermal veins and altered wall rocks around granitic intrusions