Disilicates and Ring Silicates Disilicates Not a common group of minerals not rock forming Widely distributed Most common are the epidote group Epidote Zoisite Clinozoisite
Allanite Characteristic structure Pair of single tetrahedrons Share single O Z/O ratio = 2/7 General Structure
Complex structure Leads to much substitution and complicated compositions Links of silica tetrahedrons Edge sharing Al octahedrons
Octahedrons linked by double silica tetrahedrons Other cations link tetrahedrons Structure Epidote Gp One edge sharing octahedral chain M2 sites Second edge sharing octahedral chain
M1 site with Al M3 site with other cations (usually Fe and Mn) Chains and cross linked tetrahedron
produce 7 to 11 fold coordination sites A sites Commonly contain Ca Overall 5 sites: Tetrahedron, 3 octahedral sites (M1, M2, & M3), one high coordination number site - A M2 M1 Crosslinked double tetrahedro
M3 n A = 7 to 11 fold coordination Fig. 15-2 Chains are parallel to b axis Cleavage commonly occurs between b axis
Commonly elongate parallel to b axis Chemistry Epidote Gp M3 site Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) A site M1, M2, & M3 site Tetrahedral Site Variation of cations in A and M3 sites control
names Epidote Gp Minerals A Site Epidote Ca Clinozoisite Ca Piemontite Ca Allanite Ca, Ce3+, La3+, Na M3 site Al, Fe3+ Al Mn, Fe3+, Al
Fe3+, Fe2+, Mg, Al Occurrence Zoisite Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks Amphibolite from mafic igneous rocks
Clinozoisite, epidote Regional and contact metamorphic rocks Pelites, metacarbonates, felsic to mafic meta-igneous rocks Clinozoisite in Al-rich rocks Epidote in Fe-rich rocks Resistant to weathering, so they are common detrital grains, heavy minerals
Allanite Common accessory mineral in felsic igneous rocks Granite, granodiorite, monzonite, syenite Ring Silicates
Only three common species Tourmaline Beryl Isostructural Cordierite All are 6 member rings Rings stacked on top of each other
Ring Z/O = 1/3 Beryl and Cordierite Tourmaline Fig. 15-1 Beryl Al2Be3Si6O18 Structure
Rings rotated relative to the ones above and below Each column are concentric cylinders of oxygen atoms Columns arranged hexagonally Al in distorted 6-fold coordination sites Be in 4-fold sites between Si tetrahedrons Could be considered tectosilicates Si tetrahedral rings
Be in 4 fold coordination Al in 6 fold coordination Fig. 15-6 Compositional variations Inclusion of Li, Na, K, and Cs Li substitutes for Be and Al
Other located in channels Charge balance from substitution of Fe, Mg, Li for Al or vacancies in Be sites Cr substitution makes emerald Occurrence Common in granitic pegmatites Associations are quartz, K-spar, albite,
muscovite, biotite, tourmaline Use Source of Be Gemstone: aquamarine (blue-green), emerald (green) Cordierite Identical to Beryl except
Substitute Mg2+ for Al3+ in octahedral sites Substitute Al for Be in cross linked tetrahedral sites Replace one Si4+ with one Al3+ in rings for charge balance Tourmaline Hydrated Na, Mg, Fe, Li, Al, B silicate
Structure Single 6-member ring 3 octahedral sites at base with X cations coordinating One large W cation site in axis of rings B trigonally coordinated as base Columns held together with octahedrally coordinated Y cation
All rings point in same direction No center of symmetry piezoelectric W = Na, less commonly Ca, X = Mg & Fe, less commonly Mn, Li, Al Y = Al, less
commonly Fe, Mg Trigonally coordinated B Fig. 15-6 Composition General formula WX3Y6(Si6O8)(BO3)3(O,OH,F)4
W = Na, less commonly Ca, K X = Mg and Fe, less commonly Mn, Li, Al Y = Al, less commonly Fe, Mg End members Schorl Dravite
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