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Pinalite
Pinalite is a rare lead tungstate–chloride mineral with formula: Pb3WO5Cl2.
Pinalite crystallizes in the orthorhombic crystal system.〔Clark, A.M. (1993) Hey's Mineral Index (Third edition). 548 p. The Natural History Museum, London, UK.〕 The orthorhombic system is described as having three crystallographic axes of unequal lengths, normally referred to as c (the longest axes), b (the second longest axes), and a (the smallest axes). These axes all have corresponding angles of 90 degrees.〔Klein, Cornelis and Dutrow, Barbara. (2007) The Manual of Mineral Science (Twenty-third edition) 187-188 p. United States of America.〕 Pinalite belongs to the biaxial optical class, and is negative.〔 The main difference between biaxial and axial crystals is that biaxial crystals have two optic axes.〔
Pinalite's structure is significant in that it is one of a kind. It is therefore important in seeing how the structures in new lead oxyhalides (containing square pyramids incorporated into sheets) are arranged. Its structure contains an extra oxygen, which therefore has a very small amount of free space and consequently protrudes into neighboring layers. The oxygen then pushes aside the chlorine atoms, leaving almost no room for the lead to occupy. This is of interest to geologists for it is such a rare occurrence of structure seen only in pinalite and its barium analog. Therefore, it could be of use to structuralists to see how this mineral was formed. Experiments have been done, using lead oxyhalides, to see the reason and sequence of events in which this mineral is executed.〔Charkin, Dmitri and Lightfoot, Phillip. "Synthesis of novel lead–molybdenum and lead–tungsten oxyhalides with the pinalite structure, Pb3MoO5Cl2 and Pb3WO5Br2." American Mineralogist; November 2006; v. 91; no. 11-12; p. 1918-1921. Mineralogical Society of America. 2006.〕
==Discovery and occurrence==
Pinalite was first discovered in 1989 in the St. Anthony deposit, Tiger, Mammoth District, Pinal Co., Arizona, USA, and was named for the location county.〔 Pinalite has otherwise only been seen in the Grand Reef mine, near Klondyke in the Santa Teresa Mountains in Graham County, Arizona, so is therefore found around orogenic activity.〔
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