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White sprays of macdonaldite and blue botryoidal crust of mcguinnessite
(repeating unit)
IMA symbolMcd[1]
Strunz classification9.EB.05
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupCmcm
Unit cella = 14.06 Å, b = 23.52 Å,
c = 13.08 Å; Z = 4
ColorColorless, white
Crystal habitAcicular also fibrous, in radiating aggregates; granular
Cleavage{010} perfect, {001} good, {100} indistinct
Mohs scale hardness3.5-4.0
LusterVitreous - silky
DiaphaneityTransparent to translucent
Specific gravity2.27
Optical propertiesBiaxial (+/-), surface relief - low,
Refractive indexnα = 1.518 nβ = 1.524 nγ = 1.530
2V angleMeasured: 90°

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]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Mineralienatlas
  3. ^ "Macdonaldite at".
  4. ^ a b c "Macdonaldite at".
  5. ^ "Macdonaldite".
  6. ^ Handbook of Mineralogy
  7. ^ 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.
  8. ^ Bloss, F.D. (1999) Optical Crystallography. Mineralogical Society of America, 5, 161-163.