Quartz (Labradorite)

Labradorite ((Ca, Na)(Al, Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573. Twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which form nearly right angle prisms. It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.