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Disilicates and Ring Silicates Disilicates Disilicates Not a common group of minerals – not rock forming Not a common group of minerals – not rock forming.

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Presentation on theme: "Disilicates and Ring Silicates Disilicates Disilicates Not a common group of minerals – not rock forming Not a common group of minerals – not rock forming."— Presentation transcript:

1 Disilicates and Ring Silicates Disilicates Disilicates Not a common group of minerals – not rock forming Not a common group of minerals – not rock forming Widely distributed Widely distributed Most common are the epidote group Most common are the epidote group Epidote Epidote Zoisite Zoisite Clinozoisite Clinozoisite Allanite Allanite

2 Characteristic structure Characteristic structure Pair of single tetrahedrons Pair of single tetrahedrons Share single O Share single O Z/O ratio = 2/7 Z/O ratio = 2/7

3 General Structure Complex structure Complex structure Leads to much substitution and complicated compositions Leads to much substitution and complicated compositions Links of silica tetrahedrons Links of silica tetrahedrons Edge sharing Al octahedrons Edge sharing Al octahedrons Octahedrons linked by double silica tetrahedrons Octahedrons linked by double silica tetrahedrons Other cations link tetrahedrons Other cations link tetrahedrons

4 Structure – Epidote Gp One edge sharing octahedral chain One edge sharing octahedral chain M2 sites M2 sites Second edge sharing octahedral chain Second edge sharing octahedral chain M1 site with Al M1 site with Al M3 site with other cations (usually Fe and Mn) M3 site with other cations (usually Fe and Mn)

5 Chains and cross linked tetrahedron Chains and cross linked tetrahedron produce 7 to 11 fold coordination sites – A sites produce 7 to 11 fold coordination sites – A sites Commonly contain Ca Commonly contain Ca Overall – 5 sites: Overall – 5 sites: Tetrahedron, 3 octahedral sites (M1, M2, & M3), one high coordination number site - A Tetrahedron, 3 octahedral sites (M1, M2, & M3), one high coordination number site - A

6 Fig M2 M1 M3 A = 7 to 11 fold coordination Cross-linked double tetrahedron

7 Chains are parallel to b axis Chains are parallel to b axis Cleavage commonly occurs between b axis Cleavage commonly occurs between b axis Commonly elongate parallel to b axis Commonly elongate parallel to b axis

8 Chemistry – Epidote Gp Ca 2 (Al,Fe)Al 2 O(SiO 4 )(Si 2 O 7 )(OH) Ca 2 (Al,Fe)Al 2 O(SiO 4 )(Si 2 O 7 )(OH) A site M1, M2, & M3 site Tetrahedral Site M3 site Variation of cations in A and M3 sites control names

9 A SiteM3 site EpidoteCaAl, Fe 3+ ClinozoisiteCaAl PiemontiteCaMn, Fe 3+, Al AllaniteCa, Ce 3+, La 3+, NaFe 3+, Fe 2+, Mg, Al Epidote Gp Minerals

10 Occurrence Zoisite Zoisite Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks Medium grade metamorphic rocks derived from Ca-rich sedimentary rocks Amphibolite from mafic igneous rocks Amphibolite from mafic igneous rocks

11 Clinozoisite, epidote Clinozoisite, epidote Regional and contact metamorphic rocks Regional and contact metamorphic rocks Pelites, metacarbonates, felsic to mafic meta- igneous rocks Pelites, metacarbonates, felsic to mafic meta- igneous rocks Clinozoisite in Al-rich rocks Clinozoisite in Al-rich rocks Epidote in Fe-rich rocks Epidote in Fe-rich rocks Resistant to weathering, so they are common detrital grains, heavy minerals Resistant to weathering, so they are common detrital grains, heavy minerals

12 Allanite Allanite Common accessory mineral in felsic igneous rocks Common accessory mineral in felsic igneous rocks Granite, granodiorite, monzonite, syenite Granite, granodiorite, monzonite, syenite

13 Ring Silicates Only three common species Only three common species Tourmaline Tourmaline Beryl Beryl Cordierite Cordierite All are 6 member rings All are 6 member rings Rings stacked on top of each other Rings stacked on top of each other Isostructural

14 Fig Ring – Z/O = 1/3 Beryl and Cordierite Tourmaline

15 Beryl – Al 2 Be 3 Si 6 O 18 Structure Structure Rings rotated relative to the ones above and below Rings rotated relative to the ones above and below Each column are concentric cylinders of oxygen atoms Each column are concentric cylinders of oxygen atoms Columns arranged hexagonally Columns arranged hexagonally Al in distorted 6-fold coordination sites Al in distorted 6-fold coordination sites Be in 4-fold sites between Si tetrahedrons Be in 4-fold sites between Si tetrahedrons Could be considered tectosilicates Could be considered tectosilicates

16 Fig Si tetrahedral rings Al in 6 fold coordination Be in 4 fold coordination

17 Compositional variations Inclusion of Li, Na, K, and Cs Inclusion of Li, Na, K, and Cs Li substitutes for Be and Al Li substitutes for Be and Al Other located in channels Other located in channels Charge balance from substitution of Fe, Mg, Li for Al or vacancies in Be sites Charge balance from substitution of Fe, Mg, Li for Al or vacancies in Be sites Cr substitution makes emerald Cr substitution makes emerald

18 Occurrence Occurrence Common in granitic pegmatites Common in granitic pegmatites Associations are quartz, K-spar, albite, muscovite, biotite, tourmaline Associations are quartz, K-spar, albite, muscovite, biotite, tourmaline Use Use Source of Be Source of Be Gemstone: aquamarine (blue-green), emerald (green) Gemstone: aquamarine (blue-green), emerald (green)

19 Cordierite Identical to Beryl except Identical to Beryl except Substitute Mg 2+ for Al 3+ in octahedral sites Substitute Mg 2+ for Al 3+ in octahedral sites Substitute Al for Be in cross linked tetrahedral sites Substitute Al for Be in cross linked tetrahedral sites Replace one Si 4+ with one Al 3+ in rings for charge balance Replace one Si 4+ with one Al 3+ in rings for charge balance

20 Tourmaline Hydrated Na, Mg, Fe, Li, Al, B silicate Hydrated Na, Mg, Fe, Li, Al, B silicate Structure Structure Single 6-member ring Single 6-member ring 3 octahedral sites at base with X cations coordinating 3 octahedral sites at base with X cations coordinating One large W cation site in axis of rings One large W cation site in axis of rings B trigonally coordinated as base B trigonally coordinated as base Columns held together with octahedrally coordinated Y cation Columns held together with octahedrally coordinated Y cation

21 All rings point in same direction All rings point in same direction No center of symmetry No center of symmetry piezoelectric piezoelectric

22 Fig W = Na, less commonly Ca, K X = Mg & Fe, less commonly Mn, Li, Al Y = Al, less commonly Fe, Mg Trigonally coordinated B

23 Composition General formula General formula WX 3 Y 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 WX 3 Y 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 W = Na, less commonly Ca, K W = Na, less commonly Ca, K X = Mg and Fe, less commonly Mn, Li, Al X = Mg and Fe, less commonly Mn, Li, Al Y = Al, less commonly Fe, Mg Y = Al, less commonly Fe, Mg

24 End members Schorl Schorl NaFe 3 Al 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 NaFe 3 Al 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 Dravite Dravite NaMg 3 Al 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 NaMg 3 Al 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 Elbaite Elbaite Na(Li,Al) 3 Al 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4 Na(Li,Al) 3 Al 6 (Si 6 O 8 )(BO 3 ) 3 (O,OH,F) 4

25 Occurrence Occurrence Common in pegmatites, other igneous and metamorphic rocks Common in pegmatites, other igneous and metamorphic rocks Resistant to weathering – heavy mineral fractions Resistant to weathering – heavy mineral fractions Use Use Gemstone Gemstone Pressure gauges and other electrical components Pressure gauges and other electrical components


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