- A plagioclase feldspar, the final member of the Albite-Anorthite solid solution series. Anorthite is very rare on Earth, but abundant in moon rocks. This is because Anorthite weathers rapidly to clay minerals under atmospheric conditions. Good crystals have been found in lava-digested limestone blocks erupted from Mount Vesuvius. ,Chemical composition: Calcium aluminum silicate, ,Physical properties: Mohs Hardness: 6.0 Specific gravity: 2.76 Color: Shades of gray, often with a blue, purple, or green cast Luster: glassy Fracture: conchoidal Cleavage: two good at about 94º to each other Habit: Triclinic - pinacoidal. Crystals are well developed, forming massive clumps, and are both twinned and striated.