Mineralogy: Nesosilicates and Olivine Crystallography

6 Questions

It is a compound with a structure in which independent silicate tetrahedrons are present.

All olivines crystallize in the orthorhombic crystal system.

Garnet has the general chemical formula A3B2Si3O12, where A is a divalent cation and B is a trivalent cation.

The isolated SiO4 tetrahedra contribute to high hardness, density, less pronounced cleavage, high refractive index, and stability.

Nesosilicates form in igneous, metamorphic, sedimentary, hydrothermal, and pegmatitic environments.

Sorosilicates are characterized by tetrahedra that share one corner oxygen, leading to double tetrahedra.

Study Notes

Defined as compounds with a structure in which independent silicate tetrahedra (SiO4) are present
Each tetrahedron has a central silicon atom surrounded by four oxygen atoms at the corners of a tetrahedron
Examples of nesosilicates include Phenakite, Willemite, Forsterite, Fayalite, Tephroite, Pyrope, Almandine, Grossular, Zircon, Thorite, Hafnon, and Topaz

Olivine crystallizes in the orthorhombic crystal system
Classified as a nesosilicate with isolated (SiO4)4- tetrahedra bound to each other only by ionic bonds from interstitial cations

General chemical formula: A3B2Si3O12, where A is a divalent cation (Fe2+, Ca2+, Mg2+, Mn2+) and B is a trivalent cation (Fe3+, Al3+, Cr3+)
End-members pyrope, almandine, and spessartine form one solid solution series, while the end-members grossular, andradite, and uvarovite form another

Isolated silicate tetrahedra (SiO4) contribute to high hardness due to strong ionic-covalent bonds within each tetrahedron
High densities due to close packing of tetrahedra with metal cations
Lack of shared oxygen atoms means cleavage planes are less pronounced, often resulting in conchoidal fracture
High refractive indices due to dense packing of SiO4 tetrahedra and presence of heavy metal cations
Strong internal bonds within each SiO4 tetrahedron contribute to thermal and chemical stability

Igneous environments:
Plutonic rocks (gabbro, peridotite, basalt) form from slow cooling of magma deep within the Earth's crust
Volcanic rocks (basalts, andesites) form from rapid cooling of lava
Metamorphic environments:
High-grade metamorphism (schist, gneiss) forms under high pressure and temperature conditions
Contact metamorphism forms in the contact aureole around intrusive bodies
Sedimentary environments:
Detrital minerals (zircon, garnet) accumulate in sedimentary rocks
Chemical sediments (zircon) form in silica-rich fluids during diagenesis
Hydrothermal environments:
Hydrothermal veins form where hot, mineral-rich fluids interact with surrounding rock
Pegmatitic environments:
Pegmatites form during late stages of magma crystallization, producing coarse-grained igneous rocks

Formed when one corner oxygen is shared with another tetrahedron, giving rise to the sorosilicate group
Basic structural unit is Si2O7-

6 Questions