Orthosilcates or Island Silicates

GARNET group


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

chemistry: (M2+)3 (M3+)2 (SiO4)3

extremely flexible structure, can accommodate just about anything (GGG, YAG, and other rare earth elements, etc.)

Garnet crystal structure (220K) M2+ sites are large, 8-coordinated sites... accommodate Ca and REE, as well as smaller metals.

M3+ sites are 6-coordinated.

OVERALL: structure is cubic (except for some special cases) List of likely 2+ and 3+ cations?

List of likely 3+ cations Putting these together, make two groups:

GROUP 1: Ca-garnets:

GROUP 2: Ca-free garnets:

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

4H+ <=> Si4+. Occurs to variable extent. These may have lower symmetry.



Composition: Al2 Si O5

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.


Triclinic: 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.


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 likelyhood of Al-Si disorder in tetrahedral sites!


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. Require energy!


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: (Fe2+)2 Al O2 (Al3 O8) Si2 (OH)4

Layer stagger results in a monoclinic unit cell.


No movie.

Composition: complex, hydrous, Fe,- aluminosilicate


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.


no movie

Composition: Al2 SiO4 (F,OH)2


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


Zr Si O4


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

Composition: CaTiO SiO4

Monoclinic: wedge shaped crystals!

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.


One basic (low-temperature) structure, several compositions

Of these, we will only focus on the very important forsterite - fayalite solid solution series. (Mg,Fe)2 SiO4 (color changes with Fe content - see peridot.

Occurrence: common in mafic and ultramafic rocks, important mantle mineral ( - discuss next lecture its behavior as a function of P and T). Fayalite-rich olivine more commonly found in more felsic and alkaline plutonic rocks.

Also found in metamorphic rocks (e.g., metasediments such as metacrabonates) - 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. Olivine, Fe, Mg rather random 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).

Note: octahedral (zig zag) chains! 3 repeat chains


Covered in previous lecture: similar to olivine structure, but with 4 octahedra in zig and zag.