Aurichalcite
Aurichalcite is a hydrated zinc copper carbonate, (Zn,Cu)5(CO3)2(OH)6, that forms as a secondary mineral in the weathered, oxidized portions of zinc and copper ore bodies. It typically appears as delicate, pale blue-green sprays of needle-like crystals with a distinctly pearly luster. The name derives from Greek words for gold and copper, an old and somewhat misleading reference to brass-like alloys rather than the mineral's actual color.
Aurichalcite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | (Zn,Cu)5(CO3)2(OH)6 |
| Mohs hardness | 1–2 |
| Crystal system | Monoclinic |
| Luster | Silky, pearly |
| Streak | Pale blue-green |
| Cleavage | Perfect, one direction (basal) |
| Fracture | Sectile |
| Specific gravity | 3.6–3.9 |
| Transparency | Translucent |
| Common colors | Pale green, Blue-green, Sky blue |
| Rarity | common |
Geology & background
Aurichalcite forms in the oxidized zone above zinc-copper sulfide deposits, where descending groundwater dissolves sphalerite and chalcopyrite and redeposits the metals as carbonates near the water table. It commonly crystallizes in cavities and fractures alongside smithsonite, hemimorphite, malachite, and azurite, forming delicate encrustations rather than large masses. Well-known localities include the Rowley Mine and the Bisbee district of Arizona, Mexico's Ojuela Mine, and Tsumeb in Namibia, all classic sources of fine oxidized-zone secondary minerals. Because its acicular crystals are so fragile and thin, aurichalcite specimens are almost always left uncleaned and displayed as delicate tufts or crusts rather than loose crystals. Its pastel blue-green color can resemble smithsonite or hemimorphite, but aurichalcite is noticeably softer and its crystals are finer and more feather-like.
How to identify Aurichalcite
Aurichalcite forms fine, hair-like or feathery crystal sprays in pale blue-green to greenish-blue shades, with a silky to pearly luster. It is very soft, around 1 to 2 on the Mohs scale, easily marked with a fingernail, and leaves a pale blue-green streak. There is no useful cleavage to observe given the crystal size, but the acicular, tufted habit is diagnostic. It can be confused with smithsonite or malachite, though both are noticeably harder.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Aurichalcite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Aurichalcite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Despite its name referencing gold and copper, aurichalcite contains neither metal; it is actually a carbonate of zinc and copper.
- Its crystals are so delicate that specimens are typically never cleaned with ultrasonic equipment for fear of destroying the sprays.
- Aurichalcite forms in the same oxidized zones that historically guided prospectors toward buried zinc and copper sulfide ore below.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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