Garnet Group Minerals: Composition, Structure, Properties, and Uses

Garnet Group

The minerals of the garnet group crystallize in the isometric system and belong to the Neso silicate group of silicates.

General Formula

X^{+2}_3 Y^{+3}_2 Z^{+4}_3O_{12}

Where:

• X⁺² = Ca, Mg, Fe, Mn (occupying dodecahedral sites)
• Y⁺³ = Fe, Al, Cr (occupying octahedral sites)
• Z⁺⁴ = Si, Ti (occupying tetrahedral sites)

1) Pyralspite Series (Aluminium Garnets)

• Pyrope: Magnesium-aluminium garnet – [Mg⁺²₃ Al⁺³₂(SiO₄)₃]
• Almandine: Iron-aluminium garnet – [Fe⁺²₃ Al⁺³₂(SiO₄)₃]
• Spessartine: Manganese-aluminium garnet – [Mn⁺²₃ Al⁺³₂(SiO₄)₃]

2) Ugrandite Series (Calcium Garnets)

• Uvarovite: Calcium-chromium garnet – [Ca⁺²₃ Cr⁺³₂(SiO₄)₃]
• Grossular: Calcium-aluminium garnet – [Ca⁺²₃ Al⁺³(SiO₄)₃]
• Andradite: Calcium-iron garnet – [Ca⁺²₃ Fe⁺³₂(SiO₄)₃]

Chemistry of Garnet

• Continuous compositional variation occurs within each series but not between the two main series.
• Each member represents an isomorphous series.
• No solid solution exists between the “Pyralspite” and “Ugrandite” systems due to ionic radius differences between Ca⁺² and Mg⁺² or Fe⁺².
• Grossular commonly contains water due to the substitution of OH, forming a solid solution with hydrogrossular (water-rich variety).
• A rare series exists between pyrope and knorringite (Cr variety).
• Garnet structure is stable in the Earth’s lower mantle.
• The ideal isometric symmetry may reduce to orthorhombic or triclinic, especially in the Ugrandite group, due to Al⁺³ & Fe⁺³ combination in the octahedral site.

Garnet Structure

The structure consists of alternating ZO₄ tetrahedra and YO₆ octahedra forming a 3D framework. Silicon-oxygen tetrahedra exist as independent groups. Cavities within the framework can be described as distorted cubes or triangular dodecahedra of eight oxygens that contain X ions. This tightly packed structure accounts for the high density and specific gravity of garnets.

Physical Properties

• Crystal System: Cubic
• Form: Rhomb dodecahedron or trapezohedron
• Color:
  • Pyrope: Blood red to black-red
  • Almandine: Deep red
  • Andradite: Brownish green
  • Uvarovite: Emerald green
  • Spessartine: Brownish red
  • Grossular: Olive-green
• Luster: Vitreous to sub-vitreous
• Fracture: Sub-conchoidal or uneven
• Cleavage: Absent
• Streak: Whitish or white
• Hardness: 6 – 7.5
• Specific Gravity: 3.42 – 4.27

Optical Properties

• Non-pleochroic, colorless or sometimes pale pink. Uvarovite is green.
• Refractive Index: Very high

Paragenesis

• Garnet is found in metamorphic, igneous, and sedimentary rocks (resistant to abrasion & chemical attacks).
• Pyrope: Occurs in ultramafic rocks such as peridotites, kimberlites, serpentinites, and eclogites.
• Almandine: Commonest garnet, typical of regional metamorphism.
• Spessartine: Common in granitic pegmatites and skarn deposits; also found in Mn ores.
• Grossular & Andradite: Occur in thermally and regionally metamorphosed impure calcareous rocks.
• Uvarovite: Rarest garnet, mostly found in serpentinite with chromite.
• Hydrogrossular: Occurs in metamorphosed marls, altered gabbroic rocks, and areas of mineral redistribution.

Garnet: Uses & Distribution

• Uses: Abrasives, gemstones
• Distribution: Metamorphosed rocks of the Eastern Ghats, beach sands of east and west coasts.
• Varieties: Transparent varieties used as gemstones.

Varieties of Garnet

• Melanite: Titanian Andradite
• Majorite: A high-pressure garnet found in the lower mantle, similar to pyrope with minor Fe.
• Kimzeyite: Zr-bearing garnet
• Goldmanite: Vanadium-bearing garnet
• Uvarovite: Excellent gemstone variety
• Demantoid: Green garnets cut as gemstones