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Abstract

The writer has synthesized pure diopside and diopsides in which Na+1 and Mg+2 have been substituted for Ca+2; in which Al +3, Fe+3, Cr+3, Mn+2, Ni+2, and Ti+4 have been substituted for Mg+2; and in which Al+3 and Fe+3 have been substituted for Si+4 . The unit-cell dimensions of these synthetic clinopyroxenes have been determined. From these data, the effects of individual (e.g., Ni+2 for Mg+2) and coupled (e.g., Na+1 Al+3 for Ca+2Mg+2) ionic substitutions on the unit-cell dimensions of pure diopside have been obtained. On the basis of the assumption that the substitution of Na+1 for Ca+2 has a negligible effect on the diopside unit-cell dimensions, the effects of individual ionic substitutions (e.g., Al+3 for Mg4+2, Ti+4 for Mg+2, Al+3 for Si4+4…) are assessed. Many of the findings are i n accord with the principles of crystal chemistry (e.g., increase in the c dimension with substitution of Al+3 or Fe+3 for Si+4), but others are not. In the latter case, discrepancies are probably due to errors in the data. The probable nature and magnitude of of such errors are discussed. The unit-cell dimensions of pure diopside are: a = 9.746Å, b = 8.924Å, c = 5.247Å, β= 74°5′; a sinβ(= d100) = 9.373Å. In order of decreasing magnitude, the effects of individual ionic substitutions on these dimensions are: a sinβ decreases with substitution of Mg+2 for Ca4+2, Cr+3 for Mg+2, and Al+3 for Mg+2 and increases with substitution of Fe+3 for Si4+4; b decreases with substitution of Cr+3 for Mg+2, Al4 +3 for Mg+2, Mn4 +2 for Mg+2, Fe+3 for Si4+4, Ni4 +2 tor Mg+2, and Al+3 for Si+4; c increases with substitution of Fe+3 for Si+4 Mg+2 for Ca+2, Ti+4 for Mg+2, Al4 +3 for Mg+2, Al+3 for Si+4, and Fe+3 for Mg+2; β decreases with substitution of Mg+2 for Ca+2 and Ti+4 for Mg+2 . Effects of other individual substitutions are slight.

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