Abstract

Point-dipole theory is used to calculate the indices of refraction and orientation of the optical indicatrix in selected triclinic minerals: kyanite, wollastonite, schizolite (Mn-rich pectolite), microcline, albite, walstromite (BaCa 2 Si 3 O 9 ), kaolinite, pyrophyllite-1A, talc-1A, and triclinic forms of clinochlore and chloritoid. Electronic polarizabilities for the constituent species (atoms and OH groups) are optimized for the best agreement between observed and calculated optical properties. As many as six electronic polarizabilities can be determined in this way for each mineral. The same cations in symmetrically distinct sites were assigned the same polarizability. Four kinds of oxygen atoms were distinguished according to coordination: bridging oxygen (T-O-T, T = Si or (super [4]) Al), nonbridging oxygen (T-O-X n , X representing a cation other than Si, (super [4]) Al, or H), oxygen not bonded to Si, (super [4]) Al, or H (i.e., O-X n ), and hydroxyl oxygen (H-O-X n ). The calculations show that the electronic polarizability of an atom (or OH group) depends on the local structural setting, i.e., polarizabilities are site-specific. Electronic polarizabilities are not readily transferable and therefore not strictly additive.

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