1.  Isolated (Neso-) Silicates
        -  SiO₄ tetrahedra are isolated (not polymerized), Si:O = 1:4,  [SiO₄]^4-
        -  Si tetrahedra are ionically bonded to interstial (spaces in structure) cations (i.e. Mg²⁺).
        -  They dense atomic packing that leads to have similar physical properties including: high
            specific gravity (G), high hardness, equidimensional crystal habit, lack of cleavage.

• Olivine Group
• Chemistry:   (Mg,Fe)₂SiO₄
• complete solid solution between forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄).
 
• Geologic Occurrence:
• Primary constituent of ultramafic (i.e. peridotites, dunites) from the mantle; common in dark-colored (mafic) igneous rocks (basalts, gabbros) and some marbles.
 
• Structure:
• Layers of octahedral (6-fold) coordinated Mg²⁺ and/or Fe²⁺ that link independent Si tetrahedra.  Octahedra are in layers in the b-c crystallographic "plane" or (100, if you like Miller Indices).

• Garnet Group
• Chemistry:
• general formula   A₃B₂Si₃O₁₂ or   A₃B₂(SiO₄)₃
• A = 8-fold (distorted cube) site that has Ca²⁺, Mg²⁺, Fe²⁺, Mn²⁺.
• B = 6-fold (octahedral) site with Al³⁺, Fe³⁺, Cr³⁺.
• Two Main Chemical Divisions:
Al-series garnets (all deep red color)              Ca-series garnets (a variety of colors)

pyrope         Mg3Al2Si3O12	uvarovite  Ca3Cr2Si3O12  (emerald green)
almandine     Fe3Al2Si3O12	grossular   Ca3Al2Si3O12  (green, yellow)
spessartine    Mn3Al2Si3O12	andradite   Ca3Fe2Si3O12 (brown, black)

 
• Geologic Occurrence:
• Extremely common metamorphic mineral.  Al-series garnets are found in regional metamorphosed sequences of clastic sedimentary rocks (shale protoliths) and commonly form large crystals (porphyroblasts) in garnet-bearing schists.
• Garnet is an important index mineral for moderate P-T conditions of metamorphism.
• Ca-series garnets are common in both contact and regional metamorphosed dolomitic limestones and other calc-silicate rocks.
 
• Structure
• 3-D network of isolated Si tetrahedra connected (bonded) to octahedra (B-site) and distorted cubic polyhedra (A-site).
 
• Al₂SiO₅ Group
• Chemistry:  Al₂SiO₅
• 3 polymorphs (same composition, different structure): kyanite, sillimanite, andalusite.
 
• Geologic Occurrence:
• Common high-grade metamorphic mineral found in regional metamorphosed sequences of clastic sedimentary rocks (shale protoliths).  Andalusite is most common as a contact metamorphic mineral.
• Important as a P-T indicator (see Fig. 13.19 in KH).  For example, all three minerals are only stable at a unique P-T condition, namely ~4 kilobars pressure and ~500C.
 
• Structure:
• All three polymorphs have a common structure with chains of edge-sharing Al-octahedra that run parallel to the c-axis and link the isolated Si tetrahedra.
• The main structural difference is that sillimanite has some of the Al in 5-fold coordination and andalusite has some Al in 4-fold (tetrahedral) coordination, whereas kyanite has all Al in 6-fold (octahedral) coordination.
 
• Zircon  (ZrSiO₄)
• Common accessory mineral (i.e. usually in small quanitities) in S-rich, felsic igneous rocks such as granites and rhyolites and also is common in high-grade gneisses and schists.
• Contains small amounts of U and Th and is radioactive, thus zircon is extremely important for radiometric dating of igneous and metamorphic rocks, particularly Precambrian rocks.  Oldest material an Earth dated so far is a 4.2 b.y. zircon grain from a sandstone in Australia.
• Zr²⁺ is in 8-fold coordination as distorted cubelike polyhedra that link Si tetrahedra.
 

• Epidote Group (have mixed paired and isolated silicate structure)
• Chemistry:
• general formula   X₂Y₃(SiO₄)(Si₂O₇)(OH)
• X = 8-fold site that has mostly Ca²⁺, Na⁺, but sometimes rare earths (i.e. Ce³⁺)
• Y = 6-fold (octahedral) site with mostly Al³⁺, with some Fe³⁺ and Mn³⁺.
• three main isostuctural members (same structure, different chemistry)

    1.  epidote  (Ca₂(Fe³⁺,Al)Al₂O(SiO₄)(Si₂O₇)(OH)
    2.  clinozoisite  (Ca₂Al₃O(SiO₄)(Si₂O₇)(OH)
    3.  allanite  (Ca,Ce)₂(Fe²⁺,Fe³⁺)Al₂O(SiO₄)(Si₂O₇)(OH)
 
• Geologic Occurrence:
• Epidote and clinozoisite are common mineral in rocks of the upper greenschist facies metamorphism (medium to high P-T conditions) and are commonly found in metamorphosed limestones and as veins/joints in granitic rocks where they are secondary replacement minerals.
• Allanite occurs as an acessory mineral in granitic igneous rocks.
 
• Structure:
• Chains of edge sharing AlO₆ and AlO₄(OH)₂ octahedra run parallel to the b-axis and are linked by independent SiO₄ tetrahedra and Si₂O₇ groups.  Elongation along the b-axis gives all epidote group minerals a monoclinic symmetry.
• There is an additional octahedral site oustide the chains, mostly occupied by Al³⁺ and Fe³⁺.  Ca²⁺ is housed in the larger 8-fold site.
 

• Si₆O₁₈ Group
• Chemistry:
• beryl    (Be₃Al₂Si₆O₁₈);
• tourmaline  [(Ca,Na)(Li,Mg,Al)₃(Al,Fe,Mn)₆(BO₃)₃(Si₆O₁₈)(OH)₄]
• corderite [(Mg,Fe)₂Al₄Si₅O18 lnH₂O]
 
• Geologic Occurrence:
• Beryl and tourmaline are common mineral in granitic pegmatites and also in some metamorphic rocks, particularly schists.
• Corderite is stricly a metamorphic mineral common in metamorphosed Al-rich shales.
 
• Beryl Structure:
• Six SiO₄ tetrahedra are linked in hexagonal Si₆O₁₈ rings in a horizontal plane perpendicular to the c-axis (0001) or in other words parallel to the a-b plane.
• Be is in 4-fold tetrahedral coordination; Al is in 6-fold octahedral coordination.  These polyhedra link independent Si₆O₁₈ rings both horizontally and vertically.
• Large "channel" (interstices) are formed in the center of the Si₆O₁₈ rings and run parallel to the c-axis.  The channels can house a variety of large cations (Rb⁺, Cs⁺, Na⁺, K⁺), anions (Cl^-, F^-, OH^-), and molecules (H₂O, organics).