CRYSTAL-CHEMICAL CHARACTERIZATION OF CLINOPYROXENES BASED ON EIGHT NEW STRUCTURE REFINEMENTS'

Comparison of new crystal-structure refinements for spodumene, LiFe3tSi,O" synthetic ureyite, acmite, diopside, augite, a CZ/c omphacite, and a PZ omphacite with published refinements for jadeite, johannsenite, NaIn'+Si,O" fassaite, and another PZ omphacite indicates that the structures are strikingly similar throughout the range of chemical compositions. The "end-member" clinopyroxenes are ordered within the limits of resolution. Bonding can be satisfactorily explained in terms of an essentially ionic model without invoking additional covalent effects. Except for PZ omphacites, the charge and size of the Mz cations determine the structure type: if Mz is Ca or Na, CZ/c is expected; if Mz is Li, CZ; and if Mz is Mg or Fe", PZ,jc or orthorhombic. Precise bond distances for ordered "end-member" clinopyroxenes can be used in determining T and M1 site contents of intermediate-composition clinopyroxenes, and ferrous iron is disordered in such clinopyroxenes. "Ideal" PZ omphacite, Cao.,Na,,.,Mgo.5Alo.,Si,O,, is expected to have Mg and AI fully ordered in the M1 octahedral chains, but in the Mz sites the maximum amount of order in alternating sites is jCa, j Na, and jNa, j Ca.

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