Buddingtonite is an ammonium feldspar with formula: NH4AlSi3O8 (note: some sources add 0.5H2O to the formula). It forms in hydrothermal areas by 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 was discovered in 1964 at the Sulfur Bank mine near Clear Lake in Lake County, California (Erd et al., 1964). Clear Lake is at the north end of The Geysers geothermal area. It also occurs in the Tonopah, Nevada(Felzer et al., 1994) area and in hydrothermal areas in New Zealand(Yang et al., 2001) and Japan. It has also been reported from the sedimentary Phosphoria Formation in Idaho(Gulbrandsen, 1974), South Dakota(Solomon & Rossman, 1988), Wyoming, and Montana. It occurs in the oil shale deposit, near Proserpine, Queensland, Australia (Loughan, et al., 1983).
- Erd RC, White DE, Fahey JJ, Lee DE (1964). "Buddingtonite, an ammonium feldspar with zeolitic water". American Mineralogist 49 (7-8): 831–50.
- Felzer B, Hauff P, Goetz AFH (1994). "Quantitative reflectance spectroscopy of buddingtonite from the Cuprite mining district, Nevada". Journal of Geophysical Research 99 (B2): 2887–95.
- Gulbrandsen RA (1974). "Buddingtonite, ammonium feldspar, in the Phosphoria Formation, southeastern Idaho". USGS Journal of Research 2 (6): 693–7.
- Loughan FC, Roberts FI, Linder AW (1983). "Buddingtonite (NH4-feldspar) in the Condor Oilshale Deposit, Queensland, Australia". Mineralogical Magazine 47: 327–34.
- Solomon GC, Rossman GR (1988). "NH4<super>+</super> in pegmatitic feldspars from the southern Black Hills, South Dakota". American Mineralogist 73: 818–21.
- Yang K, Browne PRL, Huntington JF, Walshe JL (2001). "Characterizing the hydrothermal alteration of the Broadlands-Ohaaki geothermal system, New Zealand, using short-wave infrared spectroscopy". Journal of Volcanology and Geothermal Research 106: 53–65. DOI:10.1016/S0377-0273(00)00264-X.
- 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