Orthosilicates or Island Silicates

• No tetrahedral Oxygens shared with other tetrahedra
• Based on SiO₄ ^4-
• Also called nesosilicates
• A diverse group of minerals, examples covered or touched on here or in lab:
  • olivine
  • garnet
  • aluminosilicates
    • kyanite
    • andalusite
    • sillimanite
  • topaz
  • staurolite
  • zircon
  • titanite or sphene
  • chloritoid
  • humite group
• structures determined by size(s) and charge(s) of cations in the structure
• often rather dense minerals, high S.G., hard (lots of references to the Gems course), high R.I. - mantle phases
• equant grains with generally poor cleavage
• low amounts of Al substitution for Si

OLIVINE group

Orthorhombic 2/m2/m2/m, poor cleavage - see a picture

One basic (low-temperature) structure, several compositions: see Fig. 13.8 p. 448

• Forsterite Mg₂SiO₄
• Fayalite Fe₂SiO₄
• Tephroite Mn₂SiO₄
• Kirschsteinite CaFeSiO₄
• Monticellite CaMgSiO₄

(Mg,Fe)₂SiO₄ (color changes with Fe content - see peridot).

Occurrence: common in mafic and ultramafic rocks, important mantle mineral. Fayalite-rich olivine more commonly found in more felsic and alkaline plutonic rocks.

Also found in metamorphic rocks (e.g., metasediments such as metacarbonates) - essentially pure forsterite is possible (not likely in igneous rocks).

Structure: two types of octahedral sites: M1 and M2.

M1 and M2 differ in number of shared edges; Fe, Mg rather randomly distributed between these.

All O in structure associated with Si cations. These are arranged so as the unsatisfied charge on tetrahedral O is distributed in a pattern that creates octahedral sites (M1, M2).

GARNET group

An important mineral in igneous and metamorphic rocks, also in the mantle.

Identified by their hardness, color (see some examples), shape (a dodecahedron).

chemistry: (M⁺²)₃(M⁺³)₂(SiO₄)₃

extremely flexible structure, can accommodate just about anything (including rare earth elements, etc.)

Garnet crystal structure (220K)

M⁺² sites are large, 8-coordinated sites... accommodate Ca and REE, as well as smaller metals.

M⁺³ sites are 6-coordinated.

OVERALL: structure is cubic (except for some special cases)

Likely 2+ cations

• Ca⁺²
• Mg⁺²
• Mn⁺²
• Fe⁺²

• Al
• Cr⁺³
• Fe⁺³

GROUP 1: Ca-garnets:

• Ca₃ Cr₂ Si₃ O₁₂ .... Uvarovite (rare)
• Ca₃ Al₂ Si₃ O₁₂ .... Grossular (commonly in metacarbonates)
• Ca₃ Fe₂ Si₃ O₁₂ .... Andradite (found in iron-rich skarns)

  THIS GROUP CALLED " U GR AND ITE" (ugrandite)

• Mg₃ Al₂ Si₃ O₁₂ .... Pyrope (high pressure rocks)
• Fe₃ Al₂Si₃ O₁₂ .... Almandine (normal metamorphic rocks)
• Mn₃ Al₂ Si₃ O₁₂ .... Spessartine (early low grade metamorphic)

  THIS GROUP CALLED " PYR AL SP ITE" (pyralspite)

A third group: HYDROGARNETS - involves the 'hydrogarnet substitution":

4H⁺ <=> Si⁺⁴. Occurs to variable extent. These may have lower symmetry.

NOTE that natural garnets are solid solutions of several of these end-member compositions.

The ALUMINOSILICATE polymorphs

• Andalusite
• Kyanite
• Sillimanite

Composition: Al₂SiO₅

Medium to high grade metamorphic rocks

Polymorphs: One Al in 4, 5, or 6 coordination, the other in 6 coordinated sites. 6-coordinated sites (octahedra) form chains parallel to the c axis.

KYANITE:

Triclinic: All the Al is octahedrally coordinated. (no movie) Octahedral Al-bearing chains // c are cross-linked by Si tetrahedra, which are separated by the second type of Al octahedra.

SILLIMANITE: (269K)

Orthorhombic: Same octahedral chains are crosslinked by both Si and Al tetrahedra.

THUS: structure contains chains in which tetrahedral cations alternate: Al,Si,Al,Si... (Al avoidance principal - not much likelihood of Al-Si disorder in tetrahedral sites!

ANDALUSITE: (235K)

Orthorhombic: 5-coordinated Al! 5-coordinated sites link Si tetrahedra. Same octahedral chains.

Clearly, changes in structure are in response to changing P and T. Result is changes in Al coordination.

Phase transformations require rebonding of Al. Reconstructive polymorphism requires more energy than do displacive transformations.

TOPAZ

no movie but see the Gems Course treatment for lots of details

Composition: Al₂ SiO₄ (F,OH)₂

Orthorhombic

AlO₄F₂ octahedral chains parallel to c, crosslinked by Si tetrahedra.

Fluorine-bearing vapors in late crystallization of igneous rocks.

Heat and radiation treatment change its color.

STAUROLITE

No movie.

Composition: complex, hydrous, Fe,- aluminosilicate

Monoclinic, beta=90° NOT beta is identical to 90° as required by symmetry, pseudo-orthorhombic. Recognized by its characteristic crystals and twins.

Metamorphic mineral in Al-rich rocks. Often associated with kyanite, which bears a special relationship to it. Note kyanite slabs separated by Al, Fe - hydroxide layers.

ZIRCON (264K)

ZrSiO₄

Tetragonal - more from the Gems Course

Structure: Zr in large, 8-coordinated sites, linked by Si tetrahedra (distorted) - why?

IMPURITIES: U, Th, Hf, etc. - radiation damage, age dating, etc.

like topaz, a gem material.

Common accessory mineral

TITANITE (= sphene)

Monoclinic: wedge shaped crystals - sphenoids

Si tetrahedra link Ti octahedra (which form corner linked chains) Ca sit in large 7-coordinated sites.

A common accessory mineral in igneous and metamorphic rocks. May be major Ti mineral in some cases.

CHLORITOID

Structurally and physically similar to a layer silicate.

The structure (234K) is characterized by sheets of ISOLATED tetrahedra

sheets (chains, rings) of octahedral cations (Fe2+, Al) support these tetrahedra.

Composition: (Fe⁺²)₂Al₄O₂ (SiO₄)₂ (OH)₄

Layer stagger results in a monoclinic unit cell.

HUMITE GROUP

Similar to olivine structure, but with 4 octahedra in zig and zag pattern.