anorthosite - teaching hand specimen of a dark anorthosite from the Laramie Range, Wyoming
Anorthosite is a rock with a composition of greater than 90% plagioclase feldspar. Plagioclase ranges in color from white and light gray to almost black. The anorthosite in the Laramie Anorthosite Complex of Wyoming’s Laramie Range is composed of plagioclase that is almost black. This is the Poe Mountain anorthosite, emplaced during the Proterozoic around 1.43 billion years ago.
Anorthosites appear to form in two stages. First, a basaltic magma intruded near the base of the crust differentiates. Olivine and pyroxine crystallize and sink to the bottom of the melt, while later-crystallizing plagioclase floats to the top of the magma chamber. In the second stage, denser residual liquid is removed from the plagioclase-crystal mush by filter pressing, where the pressure of overlying rocks squeezes the crystals together, expelling the liquid.
An anorthosite composed of a light gray to white plagioclase forms a body in the San Gabriel Mountains in California. That anorthosite resembles the anorthosite brought back from the moon by the apollo astronauts, though it varies in chemical composition. We usually have that anorthosite in stock.
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