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. |