White crystals of the very rare magnesium borate mineral associated to gypsum and halite from the type locality: Eagle Borax Spring, Death Valley, California, United States of America. — Photo: David Hospital · CC BY-SA 4.0
Mcallisterite
Mcallisterite is an IMA-recognized mineral species in the borate class, with the chemical formula Mg₂[B₆O₇(OH)₆]₂·9H₂O.
It crystallizes in the hexagonal crystal system (space group R-3c).
Key properties: Mohs hardness 2.5; specific gravity 1.867; optically uniaxial.
IMA symbol Mcl, IMA status: approved, named in 1963.
A secondary mineral derived from colemanite and priceite veins in altered basalt (Furnace Creek district, California, USA); a primary mineral in a metamorphosed borate playa deposit (Tincalayu deposit, Argentina); at a saline lake (China).
It is found associated with Sassolite, ginorite, gypsum (Furnace Creek district, California, USA); starkeyite, gypsum (Sterling Hill, New Jersey, USA); rivadavite (Tincalayu deposit.
The mineral was named to honor James Franklin McAllister (1911–2000), geologist, U.S. Geological Survey, Menlo Park, California, USA, who collected the originally described material.