Boracite
Boracite, Mg3B7O13Cl: a magnesium borate chloride from salt deposits, hardness 7–7.5, density 2.91–3.10, at Stassfurt and Boulby, with trembathite and ericaite.
Boracite is a magnesium borate chloride, Mg3B7O13Cl on the September 2026 IMA list: orthorhombic but pseudocubic, colourless to green, hardness 7 to 7.5 and density 2.91 to 3.10, in crystals to 2.5 cm from bedded salt and potash deposits. Its Handbook sheet names Lüneburg and the Stassfurt district in Germany and the Boulby potash mine, Yorkshire.
In short
- The mineral. The September 2026 IMA-CNMNC Master List lists boracite as Mg3B7O13Cl, status G, 1789, Germany. The Handbook of Mineralogy sheet gives the same formula: orthorhombic, pseudocubic, crystals to 2.5 cm, hardness 7 to 7.5, density 2.91 to 3.10, colourless, white, grey or yellow, green if ferroan, streak white, vitreous to adamantine. Worked, it is 18.60% magnesium, 19.30% boron and 9.04% chlorine by mass.
- Where it is found. “An uncommon component of bedded sedimentary salt and potash deposits of marine origin.” The Distribution field names Lüneburg, Lower Saxony; the Stassfurt-Westeregeln-Bernburg district, Saxony-Anhalt; Bischofferode and Sondershausen, Thuringia; Lunéville, France; the Boulby potash mine, northwest of Whitby, Yorkshire; and large crystals from Alto Chapare, Bolivia, among others.
- The two-switch rule. A script over the IMA list finds five rows built on B7O13Cl: boracite and trembathite (Mg), ericaite and congolite (Fe2+), chambersite (Mn). Two readings place a specimen among the first four: which cation leads, iron or magnesium, and the optics, biaxial for the orthorhombic pair (boracite, ericaite) and uniaxial for the trigonal pair (trembathite, congolite).
- Boulby, worked. Three of the names are on the Boulby sheet entries: boracite, trembathite and ericaite. Recast, Lüneburg boracite has Fe/(Fe+Mg) 0.019; Boulby trembathite 0.48, Boulby ericaite 0.754. Documented failure: the Boulby trembathite sits 0.02 below the halfway line, and a 20.00 g piece reads 13.13 to 13.55 g in water as boracite and 12.96 to 14.01 g as trembathite, so neither the balance nor the hardness (7 to 7.5 against 6 to 8) decides.
- Gem or mineral? A mineral, with its own IMA row. The Handbook sheet does not call it a gem and this page makes no gem claim. It is very slowly soluble in water and strongly piezoelectric and pyroelectric, as the sheet prints; it gives no care instructions, and neither does this page.
| Species | IMA formula; status, year, country | System; optics (sheet) | Fe/(Fe+Mg), worked | Hardness | Density (meas.) | 20.00 g in water | Boulby on the sheet? |
|---|---|---|---|---|---|---|---|
| Boracite | Mg3B7O13Cl; G, 1789, Germany | Orthorhombic, pseudocubic; biaxial (+) | 0.019 (Lüneburg, average of two analyses) | 7–7.5 | 2.91–3.10 | 13.13–13.55 g | Yes |
| Trembathite | Mg3B7O13Cl; A, 1991-018, Canada | Hexagonal (R3c); uniaxial (−) | 0.256 (Salt Springs); 0.48 (Boulby) | 6–8 | 2.84–3.34 | 12.96–14.01 g | Yes, large crystals |
| Ericaite | Fe2+3B7O13Cl; G, 1950, Germany | Orthorhombic, pseudocubic; biaxial (−) | 0.798 (Thuringia); 0.754 (Boulby, as stated) | [7–7.5] by analogy to boracite | 3.17–3.22 | 13.69–13.79 g | Yes, large crystals |
| Congolite | Fe2+3B7O13Cl; A, 1971-030, no country printed | Hexagonal (R3c); uniaxial (−) | 0.517 (Penobsquis, Canada) | 7 | 3.58 | 14.41 g | No |
| Chambersite | Mn3B7O13Cl; A, 1967 s.p., USA | Orthorhombic, pseudocubic; biaxial (+) | Mn 2.90 of 3 (Barber’s Hill, Texas) | 7 | 3.49 | 14.27 g | No |
What is boracite? Its formula and IMA status
Boracite is Mg3B7O13Cl on the September 2026 IMA list, status G (grandfathered), 1789, Germany, first reference Bergmannisches Journal 1 (1789), 393. The Handbook of Mineralogy boracite sheet prints the same formula. On standard atomic weights the formula mass is 392.02: 18.60% magnesium, 19.30% boron and 9.04% chlorine. Written as the sheet’s ideal column, that is B2O3 62.15, MgO 25.70 and MgCl2 12.14; the sheet prints 25.71 for MgO, a rounding difference.
What the sheet records. Orthorhombic and pseudocubic, point group mm2, space group Pca21. Euhedral crystals to 2.5 cm, described with pseudotetrahedral morphology and a dozen modifying forms; also spherulitic, plumose to fibrous and fine granular aggregates; penetration twins on {111}. Fracture conchoidal to uneven, hardness 7 to 7.5, density 2.91 to 3.10 measured and 2.97 calculated. Colourless, white, grey or yellow, and bluish green, green to dark green if ferroan; streak white; lustre vitreous to adamantine. It is “Very slowly soluble in H2O; strongly piezoelectric and pyroelectric.”
A cell check. The printed cell, a = 8.577, b = 8.553, c = 12.09 Å with Z = 4, holds 886.9 ų, and four formula units give a density of 2.936, against the printed calculated 2.97 and inside the measured 2.91 to 3.10. The sheet’s Lüneburg analysis recast on seven boron gives Mg 3.19, Fe 0.06 and Cl 1.04: close to the ideal formula, with a little ferrous iron.
Where is boracite found? Stassfurt, Lüneburg and Boulby
In salt. The Occurrence field reads: “An uncommon component of bedded sedimentary salt and potash deposits of marine origin, the boron probably derived from nearby volcanic activity.” Its associates are anhydrite, gypsum, halite, sylvite, carnallite, kainite and hilgardite. The chlorine in the formula is the same element that makes halite and sylvite halides; that classification is on is sylvite a halide.
The Distribution field, as printed. In Germany, from Lüneburg, 40 km south-southeast of Hamburg, Lower Saxony; in Saxony-Anhalt, in the Stassfurt-Westeregeln-Bernburg district, at the Douglashall, Berlepsch, Solvayhall, Wilhelmshall and other mines; in Thuringia, from Bischofferode, in the Glückauf mine, Sondershausen, and elsewhere. In France, at Lunéville, Meurthe-et-Moselle. In the Boulby potash mine, northwest of Whitby, Yorkshire, England. In the Chelkar salt dome, Kazakhstan. Large crystals from Alto Chapare, Cochabamba, Bolivia. In the USA, the Choctaw salt dome, Louisiana, and the Louann Salt Formation, Alabama. From the Penobsquis and Salt Springs evaporite deposits, near Sussex, New Brunswick, Canada.
No type locality claimed. The sheet marks none and gives type material at the Mining Academy, Freiberg; the IMA list gives only the country, Germany. The sheet’s X-ray powder pattern is labelled England. What the term means is on what is a type locality. Other boron minerals on this site, colemanite, ulexite, howlite and danburite, have their own pages.
Boracite, trembathite or ericaite? The two-switch rule
Five rows, one framework. A script over the IMA list finds five formulas containing B7O13Cl: boracite Mg3B7O13Cl (G, 1789), trembathite Mg3B7O13Cl (A, 1991-018), ericaite Fe2+3B7O13Cl (G, 1950), congolite Fe2+3B7O13Cl (A, 1971-030) and chambersite Mn3B7O13Cl (A, 1967 s.p.). The sheets pair them: boracite is “Dimorphous with trembathite; forms a series with ericaite”, and ericaite is “Dimorphous with congolite”.
The rule. Switch one is the leading cation: magnesium for boracite and trembathite, ferrous iron for ericaite and congolite, manganese for chambersite. Switch two is the structure, which shows in the optics: the trembathite and congolite sheets give hexagonal R3c and uniaxial (−); boracite and ericaite are orthorhombic Pca21 and biaxial. Magnesium-led and biaxial is boracite; magnesium-led and uniaxial, trembathite; iron-led and biaxial, ericaite; iron-led and uniaxial, congolite.
The analyses, recast. Fe/(Fe+Mg), by script from the sheets: Lüneburg boracite 0.019; trembathite 0.256 at Salt Springs and 0.48 at Boulby; ericaite 0.798 in Thuringia and 0.754 at Boulby (the sheet’s stated formula); congolite 0.517 at Penobsquis. Chambersite, from its sheet, is Mn 2.90 of 3 at Barber’s Hill, Texas, and “darkening with exposure to sunlight”, as printed for that species only.
Why a Boulby label needs optics or an analysis
Three names, one mine. The Boulby potash mine is in the Distribution field of the boracite sheet, and the trembathite and ericaite sheets both give “Large crystals from the Boulby potash mine”. The Boulby trembathite analysis, (Mg1.55Fe1.43Mn0.02), is iron-rich enough to be 0.02 from the halfway line, so a Boulby crystal can sit close to any of three names.
Documented failure: the hand tests. Hardness does not separate them: 7 to 7.5 for boracite, 6 to 8 for trembathite, 7 for congolite, and only “[7–7.5] (by analogy to boracite)” for ericaite. Density does not either: a 20.00 g piece reads 13.13 to 13.55 g in water as boracite and 12.96 to 14.01 g as trembathite, the wider range enclosing the narrower; the method is on how to measure specific gravity at home. Colour overlaps too, since boracite is green when ferroan. The pseudocubic habit is shared by boracite, ericaite and chambersite, and trembathite forms pseudocubic rhombohedra.
What does work. Uniaxial against biaxial optics on a fragment, or a chemical analysis for the iron-to-magnesium ratio. Neither is a home test. Questions about handling salt-deposit minerals go to toxic minerals in a collection; this page is not health or safety advice. More species are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.