| Macdonaldite | |
|---|---|
 White sprays of macdonaldite and blue botryoidal crust of mcguinnessite | |
| General | |
| Category | Phyllosilicate |
| Formula (repeating unit) | 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 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 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] It has also been reported from a quarry in San Venanzo, Umbria, Italy.[4]