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White sprays of macdonaldite and blue botryoidal crust of mcguinnessite
Category Phyllosilicate
(repeating unit)
Strunz classification 09.EB.05
Crystal system Orthorhombic
Unit cell a = 14.06 Å, b = 23.52 Å,
c = 13.08 Å; Z = 4
Color Colorless, white
Crystal habit Acicular also fibrous, in radiating aggregates; granular
Crystal symmetry Orthorhombic dipyramidal
H-M symbol: (2/m 2/m 2/m)
Space group: C mcm
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 [1][2][3][4]

Macdonaldite is a rare barium silicate mineral with a chemical formula of BaCa4Si16O36(OH)2·10H2O.[5] Macdonaldite was first described in 1965 and named for Gordon A. MacDonald (1911–1978) an American volcanologist at the University of Hawaii.[5]

Macdonaldite crystallises in the orthorhombic system. Macdonaldite is anisotropic with low relief.[6]

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.[2][5] It has also been reported from a quarry in San Venanzo, Umbria, Italy.[2]


  1. ^ "Macdonaldite at". 
  2. ^ a b c "Macdonaldite at". 
  3. ^ "Macdonaldite". 
  4. ^ Handbook of Mineralogy
  5. ^ a b c 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.
  6. ^ Bloss, F.D. (1999) Optical Crystallography. Mineralogical Society of America, 5, 161-163.