Macdonaldite is a rare barium

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

with a chemical formula of BaCa₄Si₁₆O₃₆(OH)₂·10H₂O.Alfors, J.T., Stinson, M.C., Matthews, R.A., and Pabst, A. (1965) Seven new barium minerals from eastern Fresno County, California. American Mineralogist, 50, 314-340. Macdonaldite was first described in 1965 and named for Gordon A. Macdonald (1911–1978) an American volcanologist at the

University of Hawaii A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, th ...

. Macdonaldite crystallises in the orthorhombic system. Macdonaldite is anisotropic with low relief.Bloss, F.D. (1999) Optical Crystallography. Mineralogical Society of America, 5, 161-163. Macdonaldite appears as

veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...

and fracture coatings in a sanbornite and

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

bearing

metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

. Macdonaldite was first described in 1965 for an occurrence near the Big Creek-Rush Creek area in

Fresno County, California Fresno County (), officially the County of Fresno, is a county located in the central portion of the U.S. state of California. As of the 2020 Census, the population was 1,008,654. The county seat is Fresno, the fifth-most populous city in Ca ...

. It has also been reported from Mariposa and Tulare counties in California. It has also been reported from a quarry in

San Venanzo San Venanzo is a ''comune'' (municipality) in the Province of Terni in the Italian region Umbria, located about southwest of Perugia and about northwest of Terni. San Venanzo borders the following municipalities: Ficulle, Fratta Todina, Marsci ...

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, Italy.

References

Barium minerals Calcium minerals Phyllosilicates Orthorhombic minerals Minerals in space group 63 {{silicate-mineral-stub