#clinopyroxene

(These rocks are gay!)

Viewed in XPL

Here are some nice simple twins for you! Clinopyroxene in dolerite, from the Nuria Sill in Venezuela. The smaller, black and white laths are plagioclase feldspar.

Löhley, Üdersdorf, Daun, Eifel, Rhineland-Palatinate, Germany

Brownish-red perovskite with white needles of apatite and green prismatic clinopyroxene crystals. Collection and Photo: Edgar Müller, Saarwellingen. Olympus OMD EM5 II, tube (21 cm), USMCO 10x, DCR-150 retro, linear actor (Stack Master), Stack Master control.

This is an opaque mineral in my dolerite samples, as seen in plane-polarized light. With opaque minerals, it’s very difficult to determine what they actually are in thin section. Even at 0.03mm, light doesn’t pass through them, so you can’t use their optical properties (such as birefringence) to identify what they are. A lot of times the best you can do is hold the slide up to the light and see what color the opaques reflect! In this case, I have a good sense of this rock’s composition, which helps me make an educated guess. As a mafic rock, dolerite has more magnesium and iron than its felsic counterparts, so these opaques are most likely an iron oxide (magnetite, likely). In other mafic rocks, a titanium oxide wouldn’t be a bad guess, either, except that whole rock analysis has revealed these rocks to be very titanium-poor. The surrounding minerals are plagioclase (low-relief, relatively colorless) and clinopyroxene, with maaaaybe some olivine in the bottom right of the less-magnified snapshot.