August 21, 2011
A pink, lilac, or gray-white, lithium-rich, Mica Group mineral with the formula K(Li, Al)2-3(AlSi3O10)(O,OH,F)2.
Geologists (and laypeople) often talk about the “mineral” mica, but mica is not really a mineral. Rather, mica is the name of a group of minerals. The micas are phyllosilicate minerals, which means that they are comprised of flat sheets. In Micas, these flat sheets are piled together in stacks. The word “phyllo” comes from Greek and means “leaf.” To remember the word phyllosilicate, I always think of phyllo dough, which is a dough made up of thin, flat sheets piled up and used to make pastries such as baklava or spinach pie. Phyllo dough is generally made up of flour, water, and a little sugar. Phyllosilicates, on the other hand, are made up of thin sheets of silicon and oxygen in a 2:5 ratio. Micas also have some aluminum and potassium thrown into the structure. Micas are basically stacked sheets of aluminum, silicon, and oxygen that are held together by charged potassium (K+). If you want to read more about the structure of phyllosilicates and Micas in particular, I recommend these excellent notes I found on a Smith College Geoscience website.
Lepidolite is similar in composition and structure to the silver plates of muscovite mica and the brown-black plates of biotite mica that are common rock-forming minerals in rocks such as granites. However, lepidolite contains a significant amount of the element lithium (in the same place where aluminum sits in muscovite and other micas). In fact, lepidolite is sometimes mined for lithium although generally only because it is associated with other lithium-rich minerals such as spodumene (formula: LiAl(SiO3)2).
Lepidolite is a fairly rare mineral, generally found in something called a pegmatite, which is a very coarse-grained igneous deposit in which large crystals (sometimes amazingly large crystals!) were able to grow because of special conditions. In order to grow large crystals and form pegmatites, igneous bodies must cool very slowly and also have high rates of diffusion (high rates of transport of elements, basically), generally aided through the presence of water or vapor or both. Pegmatites often have high concentrations of lithium because lithium (as well as boron and other large elements) lowers the solidification temperatures (basically by being big edit: when thinking about relative sizes of elements, remember to consider ionic raidus as elements are generally in charged forms in crystal structures) of magmas, giving the crystals more time to grow. Lepidolites often form intermixed with muscovite and other mica minerals as well as with other lithium-bearing minerals such as spodumene, amblygonite, beryl, and tourmaline. Lepidolite most often occurs in pegmatites associated with granite bodies.
Lepidolite is a gorgeous mineral, especially when it is bright pink and lavender. My fellow AGU blogger Jessica Ball recently observed and collected some lepidolite during her visit to the Harding Pegmatite Mine in New Mexico. Jessica was kind enough to send me some pictures of gorgeous purple lepidolite from the Harding Pegmatite:
Klein, Cornelius. 2002. The 22nd Edition of the Manual of Mineral Science. John Wiley & Sons.
Deer, W.A., Howie, R.A., and Zussman, J. 1992. An Introduction to the Rock-Forming Minerals, 2nd Edition. New York: Pearson Education Limited.