Clino-Amphiboles - University of Colorado Boulder

Clino-Amphiboles - University of Colorado Boulder

Clino-Amphiboles (Monoclinic Amphiboles) A0-1B2C5T8O22(OH,F,Cl)2 Presented by Joyleen Desai Paul Sandlin Structure Space group: C2/m T1: dk blue A: purple T2: lt blue O1, O2, O4: red (non-bridging) M1: orange O5, O6, O7: pink (bridging) M2: dk green O3: OH, Fl, Cl, O2- M3: yellow M4: lt green Crystallographic Sites A

M4 M3 M1 M2 T1 T2 O4 Coordinatio n 12 6-8 6 6 6 4 4 Cation Preference K Na Mn Mn

Al Al Si OH Na Ca Fe2+ Fe2+ Ti Si Al F Mn Mg Source: Spear (1995) Mg Fe3+ Fe2+

Mg Mg Fe2+ Cl Structural Cell Parameters Mineral Hornblende Formula (Na,K)0-1(Ca,Na,Fe, a ang b ang c ang 9.887 18.174 5.308

105 Mg)2(Mg,Fe,Al)5 g/cm3 (calc) V ang3 Z 2.96 921.28 2 (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 Tremolite Ca2Mg5Si8O22 9.83 18.05

5.27 105 2.98 906.64 2 Actinolite Ca2(Mg,Fe)5Si8O22 9.84 18.1 5.28 105 3.2 909.61 2 Glaucophane Na2Mg3Al2Si8O22

9.541 17.74 5.3 104 2.99 870.83 2 Riebeckite Na2(Fe,Mg)3(Fe,Al)2 9.77 18.03 5.34 104 3.4 914.24 2

(OH)2 (OH)2 (OH)2 Si8O22(OH)2 Source: 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 Cummingtonite Series Cummingtonite: (Mg,Fe)7Si8O22(OH)2 Grunerite: Fe7Si8O22(OH)2 Occur in med-grade metamorphism

Grunerite found in metamorphosed Ferich sediments Grunerite is less pleochroic M4 (yellow) takes Mg in cummingtonite structure Actinolite Series Tremolite:Ca2Mg5Si8O22(OH)2 Actinolite: Ca2(Mg,Fe)5Si8O22(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 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 Fe 2+ Proceeds by the reaction Fe2+ + OH = Fe3+ + O2- + H2 Mossbauer spectra suggest that considerable cation disorder accompanies oxidation

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