| Macdonaldite | |
| Category: | Phyllosilicate |
| Formula: | BaCa4Si16O36(OH)2·10H2O |
| Imasymbol: | Mcd[1] |
| Strunz: | 9.EB.05 |
| System: | Orthorhombic |
| Class: | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Symmetry: | Cmcm |
| Unit Cell: | a = 14.06 Å, b = 23.52 Å, c = 13.08 Å; Z = 4 |
| Color: | Colorless, white |
| Habit: | Acicular also fibrous, in radiating aggregates; granular |
| Cleavage: | perfect, good, indistinct |
| Mohs: | 3.5–4.0 |
| Luster: | Vitreous – silky |
| Refractive: | nα = 1.518 nβ = 1.524 nγ = 1.530 |
| Opticalprop: | Biaxial (+/−), surface relief – low, |
| Dispersion: | Weak |
| Birefringence: | 0.012 |
| 2V: | Measured: 90° |
| Streak: | White |
| Gravity: | 2.27 |
| Diaphaneity: | Transparent to translucent |
| 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]