Pinalite is a rare

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

tungstate In chemistry, a tungstate is a 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 polyatomic ions that are ter ...

–

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...

mineral with formula: Pb₃WO₅Cl₂. Pinalite crystallizes in the

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

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. Space groups are classified into crystal systems according to their po ...

. 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, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula , where X = fluorine (F), chlor ...

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 with the 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 the largest living cat species and a member of the genus '' Panthera''. It is most recognisable for its dark vertical stripes on orange fur with a white underside. An apex predator, it primarily preys on u ...

, Mammoth District, Pinal Co.,

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

, 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 ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣona ...

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 Halide minerals Orthorhombic minerals Minerals in space group 63