Diopside Occurrence and Compostion  Diopside is found in mafic and ultramafic igneous rocks and their metamorphosed equivalents. It is also very common in marbles and some marls, typically associated with garnet. It is an end member clinopyroxene with compostion CaMgSi2O6. Identification  High relief, clear to light green color, upper second-order interference colors, near 90o cleavage angle, and a large extinction angle facilitate identification. Important properties:  ·Appearance and habit - Short prismatic crystal or well-formed equant cross-sections are typical.  ·Color - colorless or pleochroic in light greens  ·Interference colors - Maximum inteference colors are upper second order.  ·Cleavage - Diopside has classic pyroxene cleavage. Basal sections show a cleavage angle near 90o. Longitudinal sections show only one cleavage. Similar minerals:  ·Orthopyroxene is similar but has lower birefringence and parallel extinction.  ·Augite (another pyroxene) is more deeply colored.

Diopside in a Dolmitic Marble These photos show diopside in a marble from the Adirondack Mountains, New York. Field of view is about 2 mm. The football shaped diopside grain in the center shows classic near 90o angle between cleavages - diagnostic of pyroxene. It also shows incipient twinning (XP). Diopside's interference colors range up to mid second order but in views that show two cleavages tend to be lower. This diopside is in a matrix of dolomite. Here the dolomite appears slightly pinkish because the thin section was etched and stained with alizarin red stain to help distinguish calcite from dolomite (calcite stains a darker red color). The dolomite is twinned and shows very high order interference colors; they appear as pastels, in places almost pearly white.

Wollastonite, Diopside and Garnet in a Skarn This photograph shows a large grain of wollastonite (clear, diamond shaped, center of photo), a smaller grain of diopside (higher relief, upper left part of photo) and garnet (isotropic; brownish PP; extinct XP). This rock comes from a classic wollastonite deposit near Willsboro, New York. The field of view is about 2 mm.