Diopside is a monoclinic pyroxene mineral with
composition MgCaSi2O6. It is an important rock-forming
mineral in several metamorphic and basic to ultrabasic
igneous rocks, and is also found in meteorites. It forms
complete solid-solution series with “hedenbergite”
(FeCaSi2O6) and “augite” [(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6]
and partial solid solutions with “orthopyroxene”
[(Mg,Ca)(Si,Al)2O6] and “pigeonite” [(Ca,Mg,-
Fe)(Mg,Fe)Si2O6)]. The diopside–hedenbergite series is
analogous to the amphibole series. Diopside has several
varieties, including a chromium-rich gem variety called
chrome diopside. Diopside forms variably colored, but
typically dull green crystals in the monoclinic prismatic
class. It has two distinct prismatic cleavages at 87 and
93 typical of the pyroxene series. It has a Mohs hardness
of six, a Vickers hardness of 7.7 GPa at a load of
0.98 N, and a density of 3.25 to 3.55 g/cc. It is transparent
to translucent with indices of refraction of
η1 = 1.663–1.699, η2 = 1.671–1.705, and η3 = 1.693–1.728.
Habit: Prismatic, Blocky, Granular.
Color: white, yellowish green, black, or grayish blue.
Diaphaneity: transparent to translucent.
Luster: vitreous (i.e., glassy).
Streak: white green.
Cleavage: (110) good.
Twinning: {001}, {100}.
Fracture: brittle, conchoidal.
Occurrence: basic and ultrabasic igneous and metamorphic rocks.
