Rocks, minerals, crystals, and sediments form the physical substance of the solid Earth. Learn how these materials originate, acquire their properties, change under different conditions, and provide evidence about geological processes.
If a granite countertop, a pale beach sand grain, or a piece of volcanic rock contains a light-colored mineral, there is a good chance feldspar is involved. Feldspar is not a single mineral but a family of closely related aluminosilicates, and together they are estimated to make up about 60 percent...
Quartz is chemically simple: silicon dioxide, or SiO₂. In its purest form, a well-formed crystal can be nearly colorless. Yet quartz also appears purple, smoky gray, yellow, pink, green, milky white, and almost black. That range does not mean each color is a different mineral. In most cases, the...
The enormous translucent beams beneath Naica, Mexico, are often called crystals in the broadest sense, but their mineral identity is specific: they are gypsum, not quartz. Some extend more than 11 meters through cavities in limestone, showing what calcium sulfate can become when crystal growth...
Halite begins as separated sodium and chloride ions moving through brine. It becomes rock salt only when water loss concentrates those ions beyond the solution’s ability to hold them. At that threshold, atoms join an orderly cubic lattice, new crystals nucleate, and existing faces advance through...
A drop of weak acid can make an apparently solid calcite crystal fizz. The bubbles are not trapped air escaping from pores; they are carbon dioxide created as acid dismantles calcium carbonate. This visible response is so dependable that geologists use it as a rapid identification test, yet the same...
Olivine is built for conditions unlike those at Earth’s surface. Its crystals form abundantly in the mantle and in magnesium- and iron-rich magmas, where heat is intense and free liquid water is scarce. When tectonic activity brings olivine-bearing rock into contact with circulating water, the...