Buddingtonite

Buddingtonite is an ammonium feldspar with formula: NH₄AlSi₃O₈ (note: some sources add 0.5H₂O to the formula). It forms by hydrothermal alteration of primary feldspar minerals. It is an indicator of possible gold and silver deposits, as they can become concentrated by hydrothermal processes. It crystallizes in the monoclinic crystal system and is colorless to white with a vitreous luster. Its structure is analogous to that of high sanidine (KAlSi₃O₈). Buddingtonite has a hardness of 5.5 and a specific gravity of 2.32.

Buddingtonite
General
Category	Tectosilicate minerals, feldspar group
Formula	NH4AlSi3O8
IMA symbol	Bud
Strunz classification	9.FA.30
Crystal system	Monoclinic
Crystal class	Prismatic (2) or sphenoidal (2/m)
Space group	C2/m (no. 12)
Unit cell	a = 8.57 Å, b = 13.03 Å, c = 7.18 Å; β = 112.73°; Z = 4
Identification
Color	Colorless
Crystal habit	Compact masses replacing plagioclase as pseudomorphs
Cleavage	Good on {001}, distinct on {010}
Tenacity	Brittle
Mohs scale hardness	5.5
Luster	Vitreous
Streak	Light grey to yellow (impure specimens)
Diaphaneity	Transparent to translucent
Specific gravity	2.32
Optical properties	Biaxial (+)
Refractive index	nα = 1.530 nβ = 1.531 nγ = 1.534
Birefringence	δ = 0.004
2V angle	Calculated: 60°
References

Buddingtonite was discovered in 1964 at the Sulfur Bank mine near Clear Lake in Lake County, California. Clear Lake is at the north end of The Geysers geothermal area. It also occurs in the Tonopah, Nevada area and in hydrothermal areas in New Zealand and Japan. It has also been reported from the sedimentary Phosphoria Formation in Idaho, South Dakota, Wyoming, and Montana. It occurs in the oil shale deposit, near Proserpine, Queensland, Australia.

It was named for Arthur Francis Buddington (1890–1980), a petrologist at Princeton University.

Other reading

• Voncken JHL, van Roermund HLM, van der Eerden AMJ, Jansen JBH, Erd RC (1993). "Holotype buddingtonite, an ammonium feldspar without zeolitic H₂O". American Mineralogist. 78: 204–209. Retrieved 13 June 2022.
• Voncken JHL, Konings RJM, Jansen JBH, Woensdregt CF (1988). "Hydrothermally grown buddingtonite, an anhydrous ammonium feldspar (NH₄AlSi₃O₈)". Physics and Chemistry of Minerals. 15 (4): 323–328. Bibcode:1988PCM....15..323V. doi:10.1007/BF00311036. S2CID 93352487.