Pinalite is a rare

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

tungstate In chemistry, a tungstate is a Chemical compound, compound that contains an oxyanion of tungsten or is a mixed oxide containing tungsten. The simplest tungstate ion is , "orthotungstate". Many other tungstates belong to a large group of polyatomi ...

–

chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...

mineral with formula: Pb₃WO₅Cl₂. Pinalite crystallizes in the

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

crystal system In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices (an infinite array of discrete points). Space groups (symmetry groups ...

. The orthorhombic system is described as having three crystallographic axes of unequal lengths, normally referred to as c (the longest axis), b (the second longest axis), and a (the smallest axis). 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

oxyhalide In chemistry, oxohalides or oxyhalides are a group of chemical compounds with the chemical formula , where X is a halogen, and A is an element different than O and X. Known oxohalides have fluorine (F), chlorine (Cl), bromine (Br), and/or iodine ( ...

s (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 Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

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 The tiger (''Panthera tigris'') is a large Felidae, cat and a member of the genus ''Panthera'' native to Asia. It has a powerful, muscular body with a large head and paws, a long tail and orange fur with black, mostly vertical stripes. It is ...

, Mammoth District, Pinal Co.,

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

, US, 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 The Santa Teresa Mountains are a mountain range located within the Coronado National Forest and partly within the San Carlos Apache Indian Reservation, in western Graham County, Arizona Arizona is a U.S. state, state in the Southwestern ...

Graham County, Arizona Graham County is a county in the southeastern part of the U.S. state of Arizona. As of the 2020 census, the population was 38,533, making it the third-least populous county in Arizona. The county seat is Safford. Graham County composes the ...

, so is therefore found around orogenic activity.

References

{{Reflist Lead minerals Tungstate minerals Chloride minerals Orthorhombic minerals Minerals in space group 63