Brochantite
Brochantite is Cu4(SO4)(OH)6, hardness 3.5 to 4, density 3.97. It shares malachite's hardness and streak, and only the acid test truly separates the two.
Brochantite is basic copper sulphate, Cu4(SO4)(OH)6, monoclinic, hardness 3.5 to 4 with a measured density of 3.97 and crystals to 5 cm. About 40 crystal forms are known. It is emerald to blackish green with a pale green streak — the same hardness, streak and near-identical density as malachite.
In short
- Hardness, streak and density all fail on this one. Brochantite is 3.5 to 4, density 3.97, streak pale green. Malachite is 3.5 to 4, density 4.05, streak pale green. Three standard tests, three identical answers.
- The test that works is chemical and it is trivial. Malachite is a carbonate and effervesces in cold dilute hydrochloric acid. Brochantite is a sulphate and does not. One drop on a fragment from the back settles it.
- Brochantite and langite are the same formula plus two waters. Brochantite is Cu4(SO4)(OH)6; langite is Cu4(SO4)(OH)6·2H2O. Langite is softer, at 2.5 to 3, and blue rather than green, but the pair are a reminder that a copper sulphate label needs care.
- It forms under low acidity, principally in arid regions. That is why the great brochantite localities are Chilean and Arizonan rather than Cornish — though Cornwall and Roughton Gill in Cumbria both produce it.
- Crystals reach 5 cm and about 40 forms are known, typically thick prismatic to acicular, commonly twinned on {100} into a pseudo-orthorhombic habit.
| Species | Formula | Hardness | Density | Streak | The test that works |
|---|---|---|---|---|---|
| Brochantite | Cu4(SO4)(OH)6 | 3.5–4 | 3.97 | Pale green | No effervescence in dilute acid |
| Malachite | Cu2(CO3)(OH)2 | 3.5–4 | 4.05 | Pale green | Effervesces in cold dilute acid |
| Pseudomalachite | Cu5(PO4)2(OH)4 | 4.5–5 | 4.15–4.35 | Pale green to pale bluish green | Distinctly harder; no effervescence |
| Atacamite | Cu2Cl(OH)3 | 3–3.5 | 3.745–3.776 | Apple-green | A chloride; softer, and the streak is apple-green |
| Antlerite | Cu3(SO4)(OH)4 | 3.5 | 3.88 | Pale green | A sulphate too — needs analysis, not a bench test |
| Langite | Cu4(SO4)(OH)6·2H2O | 2.5–3 | 3.28–3.34 | Pale greenish blue | Blue rather than green, and much softer |
Why the usual tests do not work on green copper minerals
The copper secondaries are the hardest group in a British or Cornish collection to work through by eye, and brochantite against malachite is the clearest illustration of why.
Both are 3.5 to 4 on the Mohs scale. Both give a pale green streak. Their measured densities are 3.97 and 4.05, a difference of 2%. Both are emerald to dark green, both occur as crusts, druses and acicular aggregates in the oxidised zone of copper deposits, and both are commonly intergrown with each other. Habit helps a little — malachite’s banded botryoidal masses are its own — but on a crust of green needles it does not.
This is a useful thing to know in general, because it demonstrates that the standard identification sequence has limits. Hardness, streak and heft narrow the field; they do not close it. The full sequence and its failure modes are on testing mineral hardness and identifying a mineral specimen.
The acid test, done without wrecking the specimen
Malachite is a copper carbonate; brochantite is a copper sulphate. Carbonates effervesce in cold dilute hydrochloric acid and sulphates do not. That is the whole determination.
Doing it responsibly matters more than doing it cleverly. Take a fragment that has already detached, or a grain from a hidden part of the base — never test a display face. Put it on a watch glass, not on the specimen. One drop of dilute acid under a loupe is enough; a carbonate fizzes visibly within a second or two. Rinse the fragment, or discard it.
The things that make this go wrong are worth naming. Malachite and brochantite very often occur on the same specimen, so a positive fizz tells you there is carbonate present, not that the whole crust is malachite. Calcite matrix will fizz vigorously and mislead completely. And acid on a mixed specimen can etch neighbouring species — azurite, calcite and cerussite all react. If the specimen is good, the honest answer is to record the determination as undetermined and leave it alone. A correct “we do not know” is worth more than a confident guess.
Brochantite, langite and the water in between
There is a neat pair here. Brochantite is Cu4(SO4)(OH)6. Langite is Cu4(SO4)(OH)6·2H2O. The same basic copper sulphate, with two water molecules added.
The two behave quite differently. Brochantite is hardness 3.5 to 4, density 3.97, emerald to blackish green, and forms in arid oxidised zones. Langite is 2.5 to 3, density 3.28 to 3.34, deep blue to blue-green, and the Handbook records that it may be of post-mine formation — our langite page is built on that line. Both occur in Cornwall and in the Caldbeck Fells, and both turn up on the same mine dumps.
Colour is the practical separator: brochantite green, langite blue. Hardness backs it up, since langite at 2.5 to 3 is marked by a fingernail at the low end and brochantite at 3.5 to 4 is not. A blue-green crust on an old Cornish dump specimen is worth determining rather than assuming, because a third possibility — the copper chloride atacamite, or one of the rarer Cornish arsenates — is always in play. Our Caldbeck Fells notes cover that suite.
Where brochantite forms, and why the good crystals are dry-country minerals
The Handbook’s occurrence line is unusually specific: brochantite is common in the oxidised zone of copper deposits, yet rarely an ore, formed under low acidity, principally in arid regions. That single clause explains the locality list.
The great crystallised brochantites are from dry places. Chile — Chuquicamata and Collahuasi in Antofagasta, Potrerillos in Atacama, Challacollo in Tarapacá. Arizona — large crystals from Bisbee, and the Mammoth-St Anthony mine at Tiger. Also Tintic in Utah, Socorro County in New Mexico, the Cerro Gordo mine in California, and the Yerington district in Nevada. In Europe: the Mednorudyanskoye deposit near Nizhni Tagil and Gumeshevsk in the Urals, Băița in Romania, Rosas and Sa Duchessa in Sardinia, and the Clara mine in the Black Forest.
Britain has it in quantity but not in size: the Handbook names numerous occurrences in Cornwall, and Roughton Gill in Cumbria. Tsumeb and Broken Hill produce it too. Associations to expect are malachite, azurite, tenorite, cuprite, linarite, caledonite, cerussite, atacamite, chrysocolla, cyanotrichite and iron oxides — a list that is itself a good check on a doubtful locality.
The species honours André Jean François Marie Brochant de Villiers, 1772 to 1840, of the School of Mines in Paris. Crystallised material with a firm locality is what we look for; tell us what you are after on the wanted list, or browse the rest of the species reference for the copper suite.