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Clino-Amphiboles Structure Crystallographic Sites

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Clino-Amphiboles Structure Crystallographic Sites Clino-Amphiboles (Monoclinic Amphiboles) A0-1B2C5T8O22(OH,F,Cl)2 Presented by Joyleen Desai Paul Sandlin T1: dk blue A: purple T2: lt blue O1, O2, O4: red (non-bridging) Structure M1: orange O5, O6, O7: pink (bridging) M2: dk green 2- Space group: C2/m O3: OH, Fl, Cl, O M3: yellow M4: lt green Crystallographic Sites A M4 M3 M1 M2 T1 T2 O4 Coordination 12 6-8 6 6 6 4 4 Cation K Na Mn Mn Al Al Si OH Preference Na Ca Fe2+ Fe2+ Ti Si Al F Mn Mg Mg Fe3+ Cl Fe2+ Mg Mg Fe2+ Source: Spear (1995) 1 Structural Cell Parameters Mineral Formula a b c ß ? V Z ang ang ang g/cm3 ang3 (calc) Hornblende (Na,K)0-1(Ca,Na,Fe, 9.887 18.174 5.308 105 2.96 921.28 2 Mg) 2(Mg,Fe,Al) 5 (Si,Al) 8O22 (OH) 2 Cummingtonite (Mg,Fe)7Si8O22 (OH) 2 9.534 18.231 5.323 102 3.13 905.18 2 Grunerite Fe7Si8O22(OH)2 9.57 18.22 5.33 102 3.66 908.72 2 Ca Mg Si O Tremolite 2 5 8 22 9.83 18.05 5.27 105 2.98 906.64 2 (OH)2 Actinolite Ca2(Mg,Fe)5Si 8O22 9.84 18.1 5.28 105 3.2 909.61 2 (OH) 2 Glaucophane Na2Mg 3Al 2Si8O22 9.541 17.74 5.3 104 2.99 870.83 2 (OH) 2 Riebeckite Na2(Fe,Mg) 3(Fe,Al) 2 9.77 18.03 5.34 104 3.4 914.24 2 Si8O22 (OH) 2 Source: www.webmineral.com Amphibole Occurrences Source: Spear (1995) Hornblende (Na,K)0-1(Ca,Na,Fe,Mg) 2(Mg,Fe,Al)5 (Si,Al)8O22 (OH)2 • Common in igneous rocks – Essential in intermediate composition igneous rocks • Metamorphosed mafic rocks – Amphibolite facies associated with plagioclase • Hornblende is the name given to amphiboles with an indeterminate composition 2 Cummingtonite Series Cummingtonite: (Mg,Fe)7Si8O22(OH) 2 Grunerite: Fe7Si 8O22 (OH)2 • Occur in med-grade metamorphism – Grunerite found in metamorphosed Fe- rich sediments • Grunerite is less pleochroic • M4 (yellow) takes Mg in cummingtonite structure Actinolite Series Tremolite:Ca 2Mg 5Si 8O 22(OH) 2 Actinolite: Ca2(Mg,Fe)5Si 8O 22(OH)2 • Found in med-grade metamorphosed mafic rocks – Calcic amphiboles are the most abundant in greenschist and amphibolite facies • Tremolite is one of the first minerals to form in metamorphosed carbonates – At high T, yields to diopside • Fe goes into either M3 or M1 (yellow) in actinolite structure Na-Amphiboles Glaucophane: Na2Mg3Al2Si8O22 (OH)2 Riebeckite: Na2(Fe,Mg)3(Fe,Al) 2Si8O22(OH)2 • Glaucophane found only in metamorphic rocks – Blueschist facies • Riebeckite most common in igneous rocks – Granites and pegmatites • Na goes in M4 site – In Riebeckite, Fe and Al in M2 site, Fe and Mg in M3 and M1 sites 3 Expansion and Compression • Clino-amphiboles are chemically similar to clino-pyroxenes • Thermal expansion is taken up in the M-O bonds parallel to the c-axis Oxidation-Dehydroxylation • Dehydrogenation occurs rapidly at high temperatures in amphiboles containing Fe2+ • Proceeds by the reaction 2+ 3+ 2- Fe + OH = Fe + O + ½H2 • Mossbauer spectra suggest that considerable cation disorder accompanies oxidation 4.
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