Is smithsonite a carbonate?
Is smithsonite a carbonate? Yes: Zn(CO3), calcite group, 52.14% zinc and no aluminium. How it differs from zincite, gahnite and hemimorphite, worked by script.
Yes. Smithsonite is zinc carbonate, Zn(CO3) on the IMA list, a calcite-group mineral that is 52.14% zinc by mass, hardness 4 to 4.5, density 4.43. It is not an oxide and not an aluminium oxide: it holds no aluminium. Zincite, ZnO, is the zinc oxide; gahnite, ZnAl2O4, the zinc aluminium oxide; hemimorphite the zinc silicate.
In short
- The verdict. Smithsonite is a carbonate. The September 2026 IMA-CNMNC Master List writes Zn(CO3), status G, 1832, United Kingdom; the Handbook of Mineralogy sheet writes ZnCO3, prints “Mineral Group: Calcite group”, and says it forms two series, with rhodochrosite and with siderite. A scan of the IMA formula column finds 13 rows written as one element with (CO3); smithsonite is the zinc one.
- Is smithsonite an aluminium oxide? No. Its formula has no aluminium, and worked on standard atomic weights it is 52.14% zinc, 9.58% carbon and 38.28% oxygen. The IMA list does have a zinc aluminium oxide, gahnite, ZnAl2O4, which its sheet places in the spinel group; the aluminium oxide itself is corundum, Al2O3.
- The zinc ledger. Sort seven zinc minerals by what sits beside the zinc. Carbon: smithsonite and hydrozincite are carbonates. Silicon: hemimorphite and willemite are silicates. Sulfur and no oxygen: sphalerite, a sulfide. Oxygen alone: zincite, ZnO. Aluminium and oxygen: gahnite. Each class rests on a formula from the IMA list.
- Documented failure: zinc share does not decide. Zincite is 80.34% zinc, sphalerite 67.10%, hydrozincite 59.55%, willemite 58.68%, hemimorphite 54.29%, smithsonite 52.14% and gahnite 35.66%. Two carbonates sit on either side of a silicate, and the two oxides are at opposite ends. Nor does weight: gahnite, 4.38 to 4.60, reads 15.43 to 15.65 g in water against smithsonite’s 15.49 g for 20.00 g.
- An ore of zinc. The sheet calls smithsonite a secondary mineral of the oxidized zone of zinc-bearing deposits, “where it may constitute an ore”, and names it “an ore at Leadville”, Colorado. Worked, 1.918 kg of smithsonite carries 1 kg of zinc, against 1.490 kg of sphalerite. Zinc dust is a hazard question; see toxic minerals in a collection, which is not advice.
| Mineral | IMA formula | Beside Zn | Class | Zn by mass | Al by mass | Group (sheet) | Hardness | Density (meas.) | 20.00 g in water |
|---|---|---|---|---|---|---|---|---|---|
| Smithsonite | Zn(CO3) | C, O | Carbonate | 52.14% | 0 | Calcite group | 4–4.5 | 4.43 | 15.49 g |
| Hydrozincite | Zn5(CO3)2(OH)6 | C, O, H | Carbonate | 59.55% | 0 | Not printed | 2–2.5 | 4.00 | 15.00 g |
| Hemimorphite | Zn4(Si2O7)(OH)2·H2O | Si, O, H | Silicate | 54.29% | 0 | Not printed | 4.5–5 | 3.475 | 14.24 g |
| Willemite | Zn2(SiO4) | Si, O | Silicate | 58.68% | 0 | Not printed | 5.5 | 3.89–4.19 | 14.86–15.23 g |
| Sphalerite | ZnS | S only | Sulfide | 67.10% | 0 | Sphalerite group | 3.5–4 | 3.9–4.1 | 14.87–15.12 g |
| Zincite | ZnO | O only | Oxide | 80.34% | 0 | Not printed | 4 | 5.66 | 16.47 g |
| Gahnite | ZnAl2O4 | Al, O | Oxide (zinc aluminium) | 35.66% | 29.43% | Spinel group | 7.5–8 | 4.38–4.60 | 15.43–15.65 g |
Is smithsonite a carbonate? The sheet and the IMA row
Yes. The Handbook of Mineralogy smithsonite sheet writes ZnCO3 and gives the group as the calcite group; the IMA list writes Zn(CO3), with the carbonate group in brackets, as it writes Ca(CO3) for calcite. Hexagonal, space group R3̄c; cleavage on {10-11}, nearly perfect; hardness 4 to 4.5; density 4.43 measured and calculated; streak white; lustre vitreous, may be pearly. It “Forms two series, with rhodochrosite and with siderite.”
Composition, worked. With Zn 65.38, C 12.011 and O 15.999, the formula mass is 125.39: zinc 52.14%, carbon 9.58%, oxygen 38.28%. As oxides, ZnO 64.90 and CO2 35.10, the sheet’s ideal column exactly. Its one analysis, from the Aachen district, Germany, reads ZnO 64.55: 51.86% zinc. Writing an analysis as oxides is bookkeeping, not a class; that point is made for wulfenite on is wulfenite an oxide.
Among the carbonates. A scan of the IMA formula column finds 13 rows written as one element with (CO3): aragonite, calcite and vaterite (calcium), cerussite, gaspéite, magnesite, otavite, rhodochrosite, siderite, smithsonite, spherocobaltite, strontianite and witherite. Smithsonite is the zinc row; a whole-text search finds Zn(CO3) as a complete formula nowhere else. The calcium carbonate that is also pale blue is the subject of is smithsonite the same as aragonite.
Is smithsonite an aluminium oxide?
No: it contains no aluminium. Zn(CO3) has three elements, zinc, carbon and oxygen, and aluminium is 0% of its mass. The IMA list writes all three of its oxygens inside the bracketed CO3 group, as it does for every carbonate row named above.
Where the question may come from. The IMA list does have a zinc aluminium oxide. A scan for formulas made only of zinc, aluminium and oxygen finds one row, gahnite, ZnAl2O4 (G, 1807, Sweden), which its Handbook sheet places in the spinel group; the group is on spinel. The plain aluminium oxide is corundum, Al2O3. The plain zinc oxide is zincite, ZnO (G, 1845, USA), the only row made of zinc and oxygen alone; its sheet, fetched 12 November 2026, names smithsonite and hemimorphite among its associates and Franklin and Sterling Hill among its localities.
Why it matters in the hand. The gahnite sheet gives hardness 7.5 to 8 and the smithsonite sheet 4 to 4.5. Weight will not separate them: see the next section.
Carbonate, oxide or silicate? The zinc ledger
Method. For seven zinc minerals with Handbook sheets cached here, the IMA formula was read and the class taken from what sits beside the zinc: carbon in a CO3 group makes a carbonate, silicon a silicate, sulfur without oxygen a sulfide, and oxygen alone (or with aluminium) an oxide. Zinc and aluminium were worked by mass on standard atomic weights, and the in-water weight of 20.00 g from each sheet’s measured density, on the method at how to measure specific gravity at home.
Documented failure: zinc share does not sort the classes. From most zinc to least: zincite 80.34% (oxide), sphalerite 67.10% (sulfide), hydrozincite 59.55% (carbonate), willemite 58.68% (silicate), hemimorphite 54.29% (silicate), smithsonite 52.14% (carbonate) and gahnite 35.66% (oxide). The two carbonates fall either side of a silicate, and the two oxides are at the two ends. A zinc figure from an assay says nothing about the class.
Documented failure: weight does not either. A 20.00 g piece reads 15.49 g in water as smithsonite and 15.43 to 15.65 g as gahnite. Hemimorphite, at 14.24 g, is the one the weighing does separate, which is the test on smithsonite vs hemimorphite; its sheet places it in the oxidized zone of zinc-bearing deposits too. Hydrozincite, 15.00 g, is the other carbonate, and its UV response is on is smithsonite UV reactive.
Smithsonite is an ore of which metal? Zinc
Zinc, on the sheet’s words. The Occurrence field reads: “A secondary mineral formed in the oxidized zone of zinc-bearing deposits and replacing adjacent carbonate rocks, where it may constitute an ore.” The Distribution field calls it “an ore at Leadville, Lake Co., Colorado”, “abundant at Laurium, Greece” (Laurium (Lavrion)), and lists “major ore deposits at Tarnowitz and Beuthen, Poland”. As on what metal is cerussite an ore of, “may” makes it an ore where there is enough of it, not in every specimen.
Weight per kilogram of zinc, worked. 1 kg of zinc is carried by 1.245 kg of zincite, 1.490 kg of sphalerite, 1.842 kg of hemimorphite or 1.918 kg of smithsonite. The sheet names fine examples from the Kelly mine, New Mexico, fine specimens with large crystals from Tsumeb, Namibia, and large crystals from Kabwe, Zambia.
Is smithsonite a mineral? Yes: its own IMA row, status G, which marks a species described before the IMA was founded and generally considered valid. It is named for James Lewis (Macie) Smithson, whose bequest founded the Smithsonian Institution, per its sheet. Zinc hazard questions go to toxic minerals in a collection; this page is not health or safety advice. More guides are in the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.