Mica is a group of minerals. Mica is a sheet silicate and the sheets are easy to separate from each other because there are weak chemical bonds holding them together. This gives them perfect cleavage — the most characteristic feature of mica minerals.
The term “mica” is, however, not easy to define. If we try to do it structurally then we should include illite and glauconite but the former is usually considererd to be a clay mineral and the latter is mostly not considered to be mica because it tends to occur as rounded pellets, not as thin sheets.
Mica flakes are elastic, this separates them from chlorite (another sheet silicate) for example. There are almost 30 mica minerals but only four are important (abundant in geological record).
These four are muscovite, paragonite, biotite, and lepidolite. Phlogopite and sericite are sometimes included but phlogopite is just a variety of biotite and sericite is a very fine-grained muscovite.
Large flakes of biotite on the left and muscovite on the right. Biotite often looks black but here it is seen that thin sheets of biotite are actually brown.
Mica minerals are really important, they make up 5% of the Earth’s crust. They occur in many different rock types.
Muscovite is a common mineral in felsic igneous rocks (mostly plutonic). It could also be an alteration product of other silicates, mostly feldspars (in this case it is usually called sericite).
Paragonite is similar to muscovite and is most likely more common than usually believed because it is often misidentified as muscovite. It is found in low-grade metamorphic rocks.
Biotite is a very common mineral just like muscovite. It occurs in igneous rocks as well but forms before muscovite (crystallization temperature range is higher), contains iron and magnesium, and is therefore much darker-colored than muscovite.
Lepidolite contains lithium. It is pink in color and occurs mostly in pegmatites with other minerals (like beryl, spodumene, topaz, and tourmaline) that need unusual chemical elements in their crystal structure. It may be sometimes hard to distinguish from muscovite.
I have also written about zinnwaldite which definitely belongs to the mica group but is not a valid mineral species anymore.
Mica minerals are moderately resistant in the weathering environment and are therefore frequently present in sand and sandstone. The most micaceous sand seems to be river sand, followed by beach sand. Eolian (wind-blown) sand usually contains no or very little mica.
Beach sand containing lots of transparent muscovite and dark biotite flakes. Beach of Cap Coz, Commune Fouesnant, Finistere, France. Width of view 20 mm.