Pyroxene-ideal
Thompson R M, Downs R T
American Mineralogist 88 (2003) 653-666
Model pyroxenes I: Ideal pyroxene topologies
Pyroxene #2 based on stacking sequence ABAC. Ideal analogue of  protopyroxene,
high-P protopyroxene.
Note: Atoms Mg, Si, and O are assigned to atomic positions just for convenience.
_database_code_amcsd 0002997
6.5319726 6 3.4641016 90 90 90 *P2_1cn
0 .25 0
atom    x     y   z
Mg1     0  1/12 2/3
Mg2     0   .25 1/6
SiA 11/16 11/12 5/6
SiB  5/16  1/12   0
O1A   7/8 11/12 5/6
O1B   1/8  1/12   0
O2A   5/8   3/4   0
O2B   3/8   .25 1/6
O3A   5/8 11/12 1/2
O3B   3/8 11/12 1/6