Inverted Pigeonite

 Pigeonite is a high-temperature low-calcium clinopyroxene that has the appearance of augite, but has a low 2V (0-25o).  Although it is restricted to lavas and shallow hypabyssal rocks, the same mineral can form in plutonic environments and, upon slow cooling, re-equilibrate into an orthopyroxene host with exsolution lamellae of clinopyroxene.  In A, lamellae of clinopyroxene stand out relative to orthopyroxene host due to incipient alteration along the contacts of the two minerals.  (The refractive indices of the two minerals are so similar that a very fresh example would not show much difference in relief between the two phases.)  The lamellae are more obvious in B, in which one set has second order interference colours typical of clinopyroxene and the host orthopyroxene is extinct.  In C, rotated slightly from B, the first order colours of the host orthopyroxene are apparent.  The “herringbone” pattern of the clinopyroxene lamellae indicate that the precursor monoclinic pigeonite was twinned on {100}, and the lamellae exsolved parallel to {001}, as seen in image D, both features characteristic of monoclinic pyroxenes.  Following inversion of the host to orthorhombic symmetry, continued exsolution of clinopyroxene from the host orthopyroxene can occur parallel to {100}.  A, B and C from a gabbro from an unknown location and D is from the Logan Sill, near McDiarmid, Ontario.  All views are 2.2 mm across.  A ppl, B and C  x-nicols.