January 29, 2023
Geology Word of the Week: M is for Mica
Posted by Evelyn Mervine
A term used to describe a group of minerals that form in flat layers (or sheets) and have a vitreous or pearly luster (they are shiny!). Micas are phyllosilicate minerals, also known as “sheet silicate” minerals. Micas are common rock-forming minerals, although some varieties are harder to find than others. Micas come in many different colors. Common mica minerals include muscovite (clear), biotite (black), and phlogopite (dark brown). The different colors of mica can be explained by their large range of chemical compositions. Micas always contain silica and often contain elements such as potassium, sodium, or calcium (in one structural position) and aluminum, magnesium, or iron (in another structural position). More rarely, other elements can fill these structural positions, which can lead to some amazing color variations. For example, lepidolite contains lithium and can be purple or pink in color. As another example, fuchsite contains chromium and is bright green in color. Micas are soft minerals, generally about 2.5-3 on the Mohs hardness scale.
Mica is one of my favorite minerals. Who doesn’t love a shiny mineral that comes in a variety of fun colors?
The photograph above shows a large piece of muscovite that I have in my personal rock collection. I collected it from a pegmatite, a rock that commonly contains large crystals.
I recently saw some gorgeous specimens of mica when I visited the Museum of Mines and Metal in Belo Horizonte, Brazil. Below are some pictures of the mica specimens on display at the museum. Enjoy!
I’ve always been interested in soil genesis, and I guess I could call mica the mother of all clays.