University:
Massachusetts Institute of Technology
Course:
12.109 | Petrology
Academic year:

2021
Views:

474

Pages:

3
Author:

Oliver Schroeder

12.109 Lecture Notes September 20, 2005 Rock Forming Minerals IV Structure and composition of: PYRIBOLES Pyriboles include pyroxenes, biotite (sheet silicate), and amphiboles (chain silicates) VM Goldschmidt – coined term “pyribole”, invented geochemistry, 1880s Sheet silicates (micas) Hexagonal rings of tetrahedra Octahedral sheet layer Trioctahedral – all sites filled Dioctahedral – 2/3 sites filled like MgO like Al2O3 Brucite, gibbsite, have no tetrahedral layers (not silicates) Mg3(OH)6 and Al2(OH)6 charge balancing Æ make a sheet silicate, add Si tetrahedrons, remove H+, satisfy charge with Si-O bond Amphiboles Literally a combination of 2 pyx and 1 mica Chain silicates (double chain) Basic amphibole unit, with tetrahedral layers sandwiching octahedra: Arrangement of units: 60-120 cleavage Amphibole polytypes: (+/- signifies facing direction of silicon octahedra) + + + + + + + + + + - monoclinic clinoamphibole C2/m orthorhombic orthoamphibole Pnma orthorhombic protoamphibole Pnmn 2 pyx + 1 mica = amphibole 2 [CaMgSiO6] + Mg3Si4O10(OH)2 = Ca2Mg5Si8O22(OH)2 Diopside Talc Tremolite 2 [Mg2Si2O6] + Mg3Si4O10(OH)2 = Mg7Si8O22(OH)2 Enstatite Talc Anthophyllite You can build ANY amphibole this way! Amphibole quadrilateral – graphic representation of different types of amphiboles, by composition Jim Thompson – mixed chain width biopyriboles Discovered in 1970s in Chester, VT Chesterite, jimthompsonite, clinojimthompsonite