Crystal structures of (Li, Na)GaSi2O6 clinopyroxenes.

Abstract

Crystal structures have been refined from single-crystal X-ray diffraction data for eight synthetic (Li, Na)GaSi2O6 clinopyroxenes, crystallized at 1770 K/6 GPa or annealed at 573 k/l atm. The space group is C2/c and Z is 4. The structures of the lithium rich pyroxenes are similar to that of spodumene(LiA1Si2O6) and the structures of the sodium rich pyroxenes are similar to that of jadeite(NaA1Si2O6). Although the crystallographical Ga site is only one in the (Li, Na)GaSi2O6 pyroxene, the correlations between the mean <Ml-O1> distance and the M1-O2 distance and between the O1-Si-O2 angles and the differences, dbr-nbr (=mean <Si-O(br)>-mean <Si-O(nbr)>), where br denotes bridging and nbr nonbridging, suggest that there are two different electronic states for the octahedral Ga ions.