We
have here apparently a polysynthetic or parallel growth, and a simple
interpenetrative twinning according to P. The optical peculiarities of
perovskite have long been a subject of discussion, and diverse views
are at the present time held as to the crystalline system to which it
belongs. Some crystallographers ' hold that perovskite is a regular
mineral, with optical anomalies which are due to strain ; while others 2 believe it to be a rhombic mineral, occurring in compound twins belonging to the class of ' mimetic ' minerals.
The
brighter colours of single lamellae, and of certain portions of the
compound crystals in the Kimberley rock, suggest that the very weak
double refraction exhibited by the crystals, as a whole, may be due to
the overlapping of two or more simple components producing a
compensation ; a weak double refraction belonging to the untwinned
mineral. The re-entrant angles, so common in the Kimberley crystals,
seem to have been rarely observed
