Macdonaldite

Macdonaldite is a rare barium silicate mineral with a chemical formula of BaCa₄Si₁₆O₃₆(OH)₂·10H₂O.^[7] Macdonaldite was first described in 1965 and named for Gordon A. Macdonald (1911–1978) an American volcanologist at the University of Hawaii.^[7]

Macdonaldite
White sprays of macdonaldite and blue botryoidal crust of mcguinnessite
General
Category	Phyllosilicate
Formula	BaCa4Si16O36(OH)2·10H2O
IMA symbol	Mcd[1]
Strunz classification	9.EB.05
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Cmcm
Unit cell	a = 14.06 Å, b = 23.52 Å, c = 13.08 Å; Z = 4
Identification
Color	Colorless, white
Crystal habit	Acicular also fibrous, in radiating aggregates; granular
Cleavage	{010} perfect, {001} good, {100} indistinct
Mohs scale hardness	3.5–4.0
Luster	Vitreous – silky
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.27
Optical properties	Biaxial (+/−), surface relief – low,
Refractive index	nα = 1.518 nβ = 1.524 nγ = 1.530
Birefringence	0.012
2V angle	Measured: 90°
Dispersion	Weak
References	[2][3][4][5][6]

Macdonaldite crystallises in the orthorhombic system. Macdonaldite is anisotropic with low relief.^[8]

Macdonaldite appears as veins and fracture coatings in a sanbornite and quartz bearing metamorphic rock. Macdonaldite was first described in 1965 for an occurrence near the Big Creek-Rush Creek area in Fresno County, California. It has also been reported from Mariposa and Tulare counties in California;^[4][7] and from a quarry in San Venanzo, Umbria, Italy.^[4]