2.8 Disilicates and Ring silicates

2.8 Disilicates and Ring silicates - Disilicates and Ring...

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Unformatted text preview: 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 Links of silica tetrahedrons Leads to much substitution and complicated compositions Edge sharing Al octahedrons Octahedrons linked by double silica tetrahedrons Other cations link tetrahedrons Structure Epidote Gp One edge sharing octahedral chain Second edge sharing octahedral chain M2 sites 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 tetrahedron M3 A = 7 to 11 fold coordination Fig. 152 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) M1, M2, & M3 site A site Tetrahedral Site Variation of cations in A and M3 sites control names Epidote Gp Minerals Epidote Clinozoisite Piemontite Allanite A Site Ca Ca Ca Ca, Ce3+, La3+, Na M3 site Al, Fe3+ Al Mn, Fe3+, Al Fe3+, Fe2+, Mg, Al Occurrence Zoisite Medium grade metamorphic rocks derived from Carich sedimentary rocks Amphibolite from mafic igneous rocks Clinozoisite, epidote Regional and contact metamorphic rocks Pelites, metacarbonates, felsic to mafic meta igneous rocks Clinozoisite in Alrich rocks Epidote in Ferich 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 Cordierite Isostructural All are 6 member rings Rings stacked on top of each other Ring Z/O = 1/3 Beryl and Cordierite Tourmaline Fig. 151 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 6fold coordination sites Be in 4fold sites between Si tetrahedrons Could be considered tectosilicates Si tetrahedral rings Be in 4 fold coordination Al in 6 fold coordination Fig. 156 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 Use Common in granitic pegmatites Associations are quartz, Kspar, albite, muscovite, biotite, tourmaline Source of Be Gemstone: aquamarine (bluegreen), 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 6member 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, K X = Mg & Fe, less commonly Mn, Li, Al Y = Al, less commonly Fe, Mg Trigonally coordinated B Fig. 156 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 NaFe3Al6(Si6O8)(BO3)3(O,OH,F)4 NaMg3Al6(Si6O8)(BO3)3(O,OH,F)4 Na(Li,Al)3Al6(Si6O8)(BO3)3(O,OH,F)4 Elbaite Occurrence Use Common in pegmatites, other igneous and metamorphic rocks Resistant to weathering heavy mineral fractions Gemstone Pressure gauges and other electrical components ...
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