Cronstedtite
Cronstedtite is a serpentine-group iron silicate, hardness 3.5, density 3.34 to 3.35: black tapering prisms, dark olive-green streak. Wheal Jane is a locality.
Cronstedtite is an iron silicate of the kaolinite-serpentine group, Fe2+2Fe3+(SiFe3+)O5(OH)4, hardness 3.5, density 3.34 to 3.35, in coal-black prisms that taper to one end, with perfect {001} cleavage and a dark olive-green streak. It is 55.92% iron by mass. The Handbook names three Cornish localities, and its powder-pattern standard is from Wheal Jane.
In short
- It is a serpentine-structure silicate made almost entirely of iron. The Handbook's ideal formula puts ferric iron in the tetrahedral site as well as the octahedral one. Of the five kaolinite-serpentine sheets in this site's 176-sheet reference set, it is the one whose formula does that.
- Streak is the fast test. Dark olive-green. Of 125 sheets in the set that record a streak, three include olive-green: cronstedtite, olivenite and uraninite, and neither of the other two is a black cleavable sheet silicate. Hematite streaks red-brown, goethite brownish yellow, schorl and biotite white.
- The shape is diagnostic when present: hexagonal prisms tapering towards one end, vertically striated, and interpenetrating trigonal pyramids that make six-rayed composite crystals.
- Cornwall is on the sheet. Wheal Jane at Kea, Wheal Maudlin at Lanlivery and the Penlee quarry at Newlyn. The sheet's X-ray reference pattern, ICDD 17-470, is from Wheal Jane, and Wheal Jane appears on no other sheet in the set.
- Densest of its group in the set: 3.34 to 3.35, against greenalite 2.85 to 3.15, berthierine 3.06 calculated, amesite 2.78 and antigorite 2.65.
| Species | Hardness | Density | Streak | Tenacity or behaviour | Habit on the sheet |
|---|---|---|---|---|---|
| Cronstedtite | 3.5 | 3.34 to 3.35 | Dark olive-green | Thin laminae elastic | Tapering hexagonal prisms; six-rayed twins |
| Greenalite | Not determined | 2.85 to 3.15 | Not recorded | Moderately magnetic | Grains and oolites to 1 mm; rarely minute crystals |
| Berthierine | Soft | 3.06 calculated | Not recorded | Not recorded | Microcrystalline oolitic aggregates |
| Amesite | 2.5 to 3 | 2.78 | White, pale green tint | Brittle | Tapering pseudohexagonal prisms to 2 mm |
| Antigorite | 2.5 to 3.5 | 2.65 | White | Not recorded | Minute plates; bladed or fibrous |
| Biotite | 2.5 to 3 | 2.7 to 3.3 | White | Brittle to flexible, elastic | Pseudohexagonal plates |
| Schorl | 7 | 3.18 to 3.22 | White | Brittle | Prisms; cleavage very poor |
| Goethite | 5 to 5.5 | 4.28 | Brownish yellow, yellow-orange, ochre-yellow | Brittle | Prisms, sprays, botryoidal masses |
What is cronstedtite, and why is it in the serpentine group?
The Handbook of Mineralogy places cronstedtite in the kaolinite-serpentine group: the same group as antigorite, whose own sheet records it as polymorphous with lizardite and the chrysotiles. What sets cronstedtite apart is its chemistry. Where antigorite's formula has silicon alone in the tetrahedral site, cronstedtite's Handbook formula, Fe2+2Fe3+(SiFe3+)O5(OH)4, writes ferric iron into that site alongside silicon.
Counted across the frozen 176-sheet reference set by regular expression on the Mineral Group line, five sheets are kaolinite-serpentine: amesite (SiAl), antigorite (Si2), berthierine ((Si,Al)2), cronstedtite (SiFe3+) and greenalite (Si2). Cronstedtite is the one of the five whose formula carries iron in the tetrahedral site. On the ideal formula it is 55.92% iron by mass (method: standard atomic weights, Fe 55.845, Si 28.085, O 15.999, H 1.008; formula mass 399.49), and the calculated 15.04% SiO2 matches the Handbook's own ideal column.
The IMA list writes it more loosely, (Fe2+,Fe3+)3(Si,Fe3+)2O5(OH)4, grandfathered, year 1821, country Czech Republic. The sheet also lists nine space groups for its stacking polytypes, against three named polytypes for amesite and two for berthierine, which is why a cronstedtite crystal can be triclinic, monoclinic or hexagonal and still be the same species.
How do I tell cronstedtite from other black minerals?
Streak first. The Handbook gives cronstedtite a dark olive-green streak. Across the 176 sheets, 125 record a streak and three include olive-green: cronstedtite, olivenite and uraninite (method: case-insensitive regular expression on the Streak field). Olivenite is itself olive-green, hardness 3 and density 4.46; uraninite is hardness 5 to 6 and density 10.63 to 10.95. Neither overlaps a black, elastic, cleavable cronstedtite in the hand. The method is on how to use a streak plate.
Then tenacity and cleavage. Cronstedtite has perfect {001} cleavage and its thin laminae are elastic, springing back like a mica. Schorl, the black tourmaline, has very poor cleavage and hardness 7. Goethite is hardness 5 to 5.5 with a brownish-yellow streak. Hematite streaks cherry-red. Biotite is elastic too but streaks white. The distinction between cleavage and fracture is on cleavage or fracture, how to tell.
Then colour in thin section. The sheet records thin laminae as emerald-green by transmitted light, brown to yellow-brown on edges. A black crystal that shows green at a thin edge against a strong light, with a green streak, is a strong cronstedtite candidate.
Magnetism is not a test for it. The Handbook's greenalite sheet records greenalite as moderately magnetic; the cronstedtite sheet records nothing on magnetism, and we do not claim either way.
The Cornish cronstedtite localities
The Handbook's distribution field names three in England, all in Cornwall: Wheal Jane at Kea, Wheal Maudlin at Lanlivery, and the Penlee quarry at Newlyn. The sheet's X-ray powder pattern, the reference used to identify the species by diffraction, is also from Wheal Jane, catalogued as ICDD 17-470.
Searched across all 176 sheets in this site's reference set, Wheal Jane appears on one: cronstedtite's. It is not a name the Handbook attaches to anything else in the set.
Elsewhere, the sheet lists Příbram, Kutná Hora, Chvaletice and Chýňava in the Czech Republic; exceptional groups from Herja (Kisbánya), Romania, the source of the sheet's analysis; Hagental in the Harz; the Salsigne gold mine in Aude, France; Long Hill, Connecticut, and the Cornucopia mine, Nevada; Santa Eulalia, Mexico; Congonhas do Campo in Minas Gerais, Brazil; and Llallagua, Bolivia.
Its occurrence is a low-temperature hydrothermal product in ore veins, with siderite, pyrite, sphalerite, clinochlore and quartz, so it belongs to the vein assemblages rather than the oxidised zone. The rest of this section is indexed at the species index. There is no catalogue here and nothing here is offered for sale; the wanted list is the only commercial route.