Macdonaldite

{{Infobox mineral

| name = Macdonaldite

| category = Phyllosilicate

| image = Macdonaldite-356329.jpg

| imagesize = 260px

| caption = White sprays of macdonaldite and blue botryoidal crust of mcguinnessite

| formula = BaCa₄Si₁₆O₃₆(OH)₂·10H₂O

| IMAsymbol = Mcd{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| 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

| twinning =

| cleavage = {010} perfect, {001} good, {100} indistinct

| fracture =

| tenacity =

| mohs = 3.5–4.0

| luster = Vitreous – silky

| refractive = n_α = 1.518 n_β = 1.524 n_γ = 1.530

| opticalprop = Biaxial (+/−), surface relief – low,

| dispersion = Weak

| birefringence = 0.012

| pleochroism =

| 2V = Measured: 90°

| streak = White

| gravity = 2.27

| density =

| melt =

| fusibility =

| diagnostic =

| solubility =

| diaphaneity = Transparent to translucent

| other =

| references = [https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Macdonaldite Mineralienatlas]{{Cite web |url=http://webmineral.com/data/Macdonaldite.shtml |title=Macdonaldite at webmineral.com}}{{Cite web |url=http://www.mindat.org/min-2506.html |title=Macdonaldite at mindat.org}}{{Cite web |url=http://euromin.w3sites.net//mineraux/MACDONALDITE.html |title=Macdonaldite}}[http://rruff.geo.arizona.edu/doclib/hom/macdonaldite.pdf Handbook of Mineralogy]

}}

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

Macdonaldite crystallises in the orthorhombic system. Macdonaldite is anisotropic with low relief.Bloss, F.D. (1999) Optical Crystallography. Mineralogical Society of America, 5, 161–163.

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; and from a quarry in San Venanzo, Umbria, Italy.