Are shale, mica and limestone crystals?
Amorphous refers to a rock or mineral which is composed of microscopically fine mineral crystals. Some black, glassy lavas which have been rapidly chilled in water or the air are amorphous, because the minerals which make up the lava were unable to grow into normal sized crystals. The same lava which didn’t encounter water may be composed of interlocking crystals of a number of minerals which are visible to the naked eye – this rock is described as “holocrystalline”, meaning that we can observe the crystals in the rock with the naked eye or with a hand lens. Shale is a rock composed of microscopically fine crystals of mica, quartz and clay. These are the ground down, weathered and eroded and altered fragments of pre-existing rocks, which were themselves masses of crystalline minerals. Mica is a mineral which naturally exists as crystals. Mica crystals are common in granites, schists, phyllites, and as spectacular, very coarse crystals in pegmatites. Limestone is composed of calcium car
