ERSC 2P21 Lecture Notes - Lecture 17: Silicate Minerals, Bond Energy, Polymerization
19 Dec 2018
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Volume % for common minerals in the earth"s crust. Recall the elemental composition of the earth"s crust from a few lectures ago (reproduced below). Si o bonds are 50% covalent, 50% ionic. Bond energy of si4+ is equally distributed. Pauling"s 3rd rule: the existence of shared edges or faces between 2 anion polyhedra decreases the stability of an ionic structure. Never are 2-3 oxygen shared between adjacent tetrahedra! Y: medium size ions; valence of 2 4+; c. n. =6. Neso: island in greek: structures depend on size and charge of interstitial cations, generally densely packed, high specific gravity, no cleavage! Isolated tetrahedra - equal bond strength in all directions. Edge-sharing of octahedral distorts the m1 and m2 sites. Epidote group: clinozoisite ca2al3o(sio4)(sio7)(oh, allanite x2y3o(sio4)(sio7)(oh, epidote ca2(fe3+,al)al2o(sio4)(sio7)(oh, vesuvianite ca19(al,fe,mg)13(si2o7)4(sio4)10(o,oh,f)10. Pyroxene: single chain: short, blocky prisms, 2 directions of perfect cleavage at ~90 , higher temperature, anhydrous.
