Rhodochrosite
Rhodochrosite is manganese carbonate, hardness 3.5 to 4, density 3.70, with perfect rhombohedral cleavage and crystals to 12 cm. Habit, localities, rhodonite.
Rhodochrosite is manganese carbonate, hexagonal, hardness 3.5 to 4, with a measured density of 3.70 and perfect rhombohedral cleavage. Rhombohedral and scalenohedral crystals reach 12 cm. It forms two series, with calcite and with siderite, which is why colour runs from cherry-red through rose to cinnamon-brown.
In short
- The cleavage is the whole story. Perfect on {1011}, in three directions, on a mineral of hardness 3.5 to 4. Rhodochrosite is the classic species where a specimen is lost to a knock that a harder mineral would have shrugged off.
- Colour is compositional, not decorative. It forms two solid-solution series, with calcite and with siderite. Manganese gives the rose-red; calcium pales it; iron drives it towards cinnamon-brown. A brown rhodochrosite is not a poor one, it is an iron-bearing one.
- It is a carbonate, so it reacts to acid — which is the single test that separates it from rhodonite, the pink manganese silicate that shares its colour and much of its market.
- Banded ornamental material and crystallised specimens are different products. The stalactitic banded rhodochrosite from Capillitas in Argentina is cut and polished; the transparent rhombohedra from Colorado and South Africa are not. They share a name and almost nothing else.
- The crystal localities that set the standard are the N'Chwaning and Hotazel mines near Kuruman in South Africa, Alma in Park County, Colorado, Cavnic and Herja in Romania, and the Huallapón mine at Pasto Bueno in Peru.
| Species | Class | Hardness | Density | The test that settles it |
|---|---|---|---|---|
| Rhodochrosite | Carbonate MnCO3 | 3.5–4 | 3.70 | Effervesces in warm dilute acid; perfect rhombohedral cleavage in three directions |
| Rhodonite | Silicate MnSiO3 | 5.5–6.5 | 3.57–3.76 | Will not react with acid, and scratches glass. Density is almost identical, so density is useless here |
| Inesite | Silicate | 6 | 3.03 | No acid reaction; lighter; typically acicular sprays rather than rhombs |
| Pink calcite | Carbonate CaCO3 | 3 | 2.71 | Also effervesces, but softer and much lighter. Density does separate this pair |
| Pink dolomite | Carbonate | 3.5–4 | 2.86 | Effervesces only in warm acid; far lighter than rhodochrosite |
| Pink smithsonite | Carbonate ZnCO3 | 4–4.5 | 4.42–4.45 | Effervesces; heavier; botryoidal habit with a silky lustre rather than rhombs |
Why rhodochrosite breaks, and what that means when you buy one
Rhodochrosite has perfect cleavage on {1011} and a hardness of 3.5 to 4. Those two facts together are the reason a disproportionate share of the rhodochrosite on the market has a repaired corner, a cleaved face presented as a natural termination, or a contact that is described as “where it was attached in the pocket”.
Perfect cleavage means the crystal has three planes along which it will part cleanly under a shock, in preference to fracturing. A cleaved face is flat, bright and reflective, and to an inexperienced eye it looks like a crystal face. The difference is that a genuine crystal face is one of a symmetrical set and carries growth features — striations, steps, slight curvature, contact marks — while a cleavage face is featureless and sits at the wrong angle to the rest of the form.
The practical checks are in our guide to telling whether a specimen has been repaired, and they apply to this species harder than to almost any other. Under raking light, ask three questions: does every bright flat face have a symmetrical partner elsewhere on the crystal, is there any face with no growth texture at all, and does the specimen have exactly one orientation in which it looks perfect. On rhodochrosite, all three are worth asking before the money moves.
Rhodochrosite or rhodonite — the test that actually works
These two are confused constantly, and it is not a beginner's mistake: they occur together, they are both manganese minerals, they are both pink, and their densities are effectively identical. The Handbook of Mineralogy gives rhodochrosite a measured density of 3.70; rhodonite runs 3.57 to 3.76. Weighing them tells you nothing.
What separates them is chemistry and hardness. Rhodochrosite is a carbonate and will effervesce in warm dilute hydrochloric acid. Rhodonite is a silicate and will not react at all. Rhodochrosite is hardness 3.5 to 4 and will not scratch glass; rhodonite is 5.5 to 6.5 and will. Either test is decisive, and the hardness test is non-destructive if you do it on an inconspicuous edge.
The habit differs too, once you know what you are looking at. Rhodochrosite forms rhombohedra and scalenohedra, and in aggregate it is bladed, columnar, stalactitic, botryoidal or compact granular. Rhodonite is typically massive with black manganese oxide veining, and its crystals, when it has them, are tabular rather than rhombohedral. Black veining through pink is rhodonite far more often than it is rhodochrosite.
One further caution: the acid test is destructive at the point of contact and will etch a display face permanently. Use a pinhead of dilute acid on the underside, or use hardness instead. Our page on identifying a mineral specimen sets out the order in which to run tests so that the destructive ones come last.
The two rhodochrosite markets, and why they should not be compared
Rhodochrosite reaches collectors as two quite different things, and treating them as one species with a range is the commonest error in how it is discussed.
Banded ornamental material comes chiefly from a large deposit at the Capillitas mine in Catamarca Province, Argentina. It is stalactitic, formed in cavities, and when sliced across the growth it shows the concentric rose-and-white banding that has made it a lapidary staple. It is cut, polished, made into beads and sold by weight. It is genuine rhodochrosite and it is not a crystallised mineral specimen.
Crystallised specimens come from a short list of hydrothermal and metamorphic localities and are collected as crystals. The Handbook singles out fine large crystals from Alma in Park County, Colorado, exceptional crystals from the Hotazel and N'Chwaning mines near Kuruman in South Africa, large twinned crystals from Mont Saint-Hilaire in Quebec, large crystals from the Huallapón mine at Pasto Bueno in Peru, and the Romanian material from Cavnic and Herja in the Baia Mare district.
These two things do not compete and should not be compared on any axis, including condition, rarity or desirability. If you are looking for one, saying which on your wanted list saves everybody a step, because a dealer who hears “rhodochrosite” with no qualifier will reasonably ask which of two entirely separate things you mean.
Occurrence, associates and reading a rhodochrosite label
Rhodochrosite is a primary mineral in low- to moderate-temperature hydrothermal veins, occurs in metamorphic deposits, is common in carbonatites, is authigenic and secondary in sediments, and is uncommon in granite pegmatites. That spread explains the variety.
The associate list tells you which environment a specimen came from without the label having to. In the hydrothermal setting: calcite, siderite, dolomite, fluorite, barite, quartz, pyrite, tetrahedrite, sphalerite and hübnerite. In the metamorphic setting: rhodonite, garnet, alabandite and hausmannite. A rhodochrosite on quartz with a little pyrite is a vein mineral; a rhodochrosite with hausmannite is a manganese orebody mineral. That is a fact about the specimen that no dealer has to supply, because it is sitting on the matrix.
Two label cautions. Colorado rhodochrosite is closely associated with Alma in Park County, and mine-level attribution there is part of what a serious label carries — “Colorado” alone is thin. And South African material from the Kalahari manganese field is normally attributed to N'Chwaning or Hotazel specifically; a label reading only “Kuruman” or “South Africa” has lost the information that mattered. Our page on what the abbreviations on a mineral label mean covers how to weigh a label that has been rewritten.