Buddingtonite Identification Guide

What Buddingtonite Looks Like

Buddingtonite is a rare ammonium-bearing feldspar (NH₄AlSi₃O₈), essentially the ammonium analogue of orthoclase/sanidine where ammonium (NH₄⁺) substitutes for potassium. It is typically colorless, white, pale gray, or faintly cream, with a vitreous to slightly pearly luster, and is translucent to opaque. It rarely forms showy crystals; instead it usually occurs as a fine-grained alteration product replacing feldspar in hydrothermally altered or geothermal rocks.

Crystal habit / form

Monoclinic, but visible crystals are uncommon. Buddingtonite is usually massive, microcrystalline, or as replacement masses in altered volcanic and sedimentary rocks. Because it is so fine-grained, hand-specimen identification is difficult—confirmation typically requires infrared spectroscopy (for the ammonium signature) or XRD.

Step-by-Step Field ID Checklist

1. Note the setting. Pale feldspar-like material in hydrothermally altered, geothermal, or mineralized volcanic rock is the first clue.
2. Check feldspar properties. Hardness ~5.5–6, two cleavage directions near 90°.
3. Look for associated alteration minerals (silica, clays, pyrite, gold-bearing systems).
4. Assess color—pale, dull, nondescript white/gray.
5. Test hardness—scratches glass marginally; close to feldspar.
6. Confirm with lab methods if precise ID is needed (IR/XRD), as it mimics ordinary feldspar.

Key Diagnostic Tests

• Mohs hardness: ~5.5–6 (feldspar range). Scratches glass with effort.
• Streak: White.
• Cleavage: Two directions intersecting near 90° (feldspar cleavage).
• Density: ~2.3–2.4 g/cm³, slightly lower than common K-feldspar.
• Acid: Inert.
• Definitive test: Infrared spectroscopy detects N–H absorption from ammonium—the only reliable hand-sample-independent confirmation.

Common Look-Alikes and How to Tell Them Apart

• Orthoclase/sanidine (K-feldspar): Visually nearly identical; buddingtonite differs only by ammonium replacing potassium, confirmable by IR spectroscopy or chemical analysis.
• Albite/plagioclase: May show twinning striations; buddingtonite lacks plagioclase albite twinning lamellae.
• Kaolinite/clay alteration: Softer (Mohs ~2), earthy, no feldspar cleavage.
• Opal/silica replacement: No cleavage, conchoidal fracture, often higher translucency.
• Zeolites: Often lower hardness and different (fibrous/blocky) habits in the same altered settings.

Where It Is Found

Buddingtonite forms by hydrothermal alteration where ammonium-bearing (often organic-derived) fluids react with feldspar, notably in geothermal fields, epithermal gold deposits, and hot-spring systems. The type locality is the Sulphur Bank mercury mine, California. It is also reported from other western U.S. geothermal and ore districts and similar settings worldwide, and is of interest as a pathfinder/alteration indicator in gold exploration.