Abstract
In order to define the limits of expansion of the M2 polyhedron in theC2/c clinopyroxenes of formulaXM2MgM1 [Si2O6] as the mean ionic radius in the M2 site increases, the join CaMgSi2O6−SrMgSi2O6 (Di−SrPx) has been investigated atP=1 atm and between 1090°C and 1350°C. The extent of the clinopyroxene solid solutions is limited to the compositional range Di100−Di70SrPx30. Within this range the unit-cell parameters of the clinopyroxenes show a linear variation with the increase of Sr content. The comparison of the variations caused in the unit-cell dimensions by the increase of the mean ionic radius in the M2 site (Di−SrPx series) with those caused by the decrease of the mean ionic radius in M2 (Di−En series) displays a different trend ofb in the two series. This different trend ofb suggests a different mechanism of the structure deformation in the two solid solution series. The narrow extent of the Di−SrPx solid solutions atT=1200°C shows that the increase of the mean ionic radius in the M2 site is restricted to the range 1.12–1.16 A.
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