Bixbyite Identification Guide

What Bixbyite Looks Like

Bixbyite is a manganese-iron oxide, (Mn,Fe)2O3, prized by collectors for sharp, jet-black crystals. It is opaque with a metallic to submetallic luster and almost never transparent.

• Color: black, sometimes with a brownish or bluish-black tint
• Habit: equant cubic crystals, frequently with modified edges and corners — its blocky cubes are its signature
• Associations: commonly perched on or near topaz, red beryl (bixbite), garnet, and pseudobrookite in rhyolite cavities
• Size: crystals are usually small (millimeters to ~1 cm), highly lustrous and well-formed

Step-by-Step Field-ID Checklist

1. Look for black cubes. Sharp isometric cubic crystals in a rhyolite vug are the strongest clue.
2. Check luster. Metallic to submetallic and shiny, not earthy or dull.
3. Streak test. Drag on unglazed porcelain — bixbyite gives a black to brownish-black streak.
4. Hardness test. Hard for an oxide, ~6–6.5; it scratches glass.
5. Magnet test. Typically non-magnetic to very weakly magnetic — this separates it from magnetite.
6. Check the host. Topaz-bearing rhyolite with red beryl is the classic setting.

Key Diagnostic Tests

• Mohs hardness: 6–6.5; scratches glass and feldspar.
• Streak: black to brownish-black.
• Cleavage/fracture: poor octahedral cleavage; uneven to conchoidal fracture; brittle.
• Specific gravity: ~4.9–5.0 — distinctly heavy.
• Magnetism: generally non-magnetic (unlike magnetite). A weak response can occur if intergrown with other oxides.
• No acid reaction.

Common Look-Alikes and How to Tell Them Apart

• Magnetite: also black and metallic, but strongly magnetic and forms octahedra rather than cubes. The magnet test is decisive.
• Hematite: gives a red-brown streak (bixbyite's is black/brown-black) and often occurs as platy or botryoidal forms.
• Franklinite: similar black oxide but typically rounded octahedra, slightly magnetic, and tied to the Franklin, NJ zinc deposits.
• Hausmannite: brownish-black, with a chestnut-brown streak and pseudo-octahedral crystals.
• Pseudobrookite: often associated in the same rhyolite, but forms bladed/prismatic crystals, not cubes.
• Galena: black-gray but much softer (2.5), with perfect cubic cleavage and a far higher density.

The combination of sharp black cubes, hardness above 6, high density, and weak/no magnetism is essentially unique among common black minerals.

Where Bixbyite Is Found

The classic locality is the Thomas Range and Wah Wah Mountains of Utah, USA, where bixbyite crystals line cavities in topaz rhyolite alongside red beryl and topaz. Other notable occurrences include Italy (where the closely related species was first described), and various rhyolitic and metamorphic manganese deposits worldwide.

Quick Confirmation

A small, brilliant black cube on rhyolite, hardness ~6.5, black-brown streak, heavy in the hand, and not pulled by a magnet, is almost certainly bixbyite — especially with topaz or red beryl nearby.