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Protopyroxenes
Protopyroxene is a high-temperature form of Mg-rich orthopyroxene. It is a polymorph of the Mg-rich orthopyroxenes. These compositions arefound at the bottom left of the pyroxene triangle below.
Mg2+ Fe2+
Ca2+
Stability
Protopyroxene is not stable at room temperature. However, the addition of Sc3+ and Li+ into the M1 and M2 sites stabilizesprotopyroxene at room temperature.
At around 1190oC, orthopyroxene >> protopyroxene
Between 800 and 600oC, protopyroxene >> pigeonitePigeonite is a Ca-poor orthopyroxene.
Protopyroxene forms as orthopyroxene is heated. Protopyroxeneis cooled to form pigeonite.
Crystallography
Point group: 2/m 2/m 2/m
Space group: Pbcn
Hermann-Mauguin symbol: P 21/b 2/c 21/n
Interpretation of space group: b-glide normal to a-axisc-glide normal to b-axisn-glide normal to c-axis
I-beam diagram for Pbcn space group. Each “I” shape consists of a tetrahedral-octahedral-tetrahedral layer. The single tetrahedral and octahedral chains extend along the c axis.
In the I-beam diagram, the tetrahedra are labeled as “O”. This refers to the arrangement of the silica tetrahedral chains in relationto the octahedral chains shown behind them. The tetraheral and octahedral chains in pyroxenes are linked by the O1 atoms.
The P and N symbols on the I-beam diagram indicate the directionsof the tetrahedral and octahedral basal triangles point in relative to each other.
High pressure protopyroxene
Between 2.03 and 2.50 Gpa, a high-pressure form of protopyroxeneforms.
The space group of the high-pressure pyroxene changes from Pbcnto P21cn
The space group group change is due to the disappearance of the b-glidenormal to the a-axis. This disappearance can be detected due to 0kl reflections where k no longer equals 2n.
The disappearance of the b-glide may be due to a change from the O-rotated silicate chain to the S-rotated chain in certain sites.
Properties of Protopyroxene
Bulk modulus: 130 Gpa (with (dK/dP)=4)
Bulk modulus of high pressure protopyroxene: 111 Gpa where (dK/dP)=4
Cell lengths for protopyroxene at atmospheric pressure: a 9.255 b 8.765 c 5.333 V 432.65 Cell lengths for protopyroxene at 6.14 Gpa:
a 9.05 b 8.71 c 5.12 V 403.97
The Si-O bonds of protopyroxene are probably smaller than those of other pyroxenes. But the M1-O and M2-O distances are greater.
