Dumortierite

What is dumortierite? An aluminium borosilicate written two ways

Dumortierite is an aluminium borosilicate: seven aluminium, one boron and three silicon to eighteen oxygen. The Handbook of Mineralogy dumortierite sheet writes Al7(BO3)(SiO4)3O3 and gives orthorhombic, point group 2/m 2/m 2/m, space group Pmcn, as fibrous or columnar crystals with generally poor face development to 6 cm, as coarsely crystalline to intimate parallel aggregates of needles, and massive. Twinning is common on {110} and may produce trillings. Cleavage is distinct on {100} and poor on {110}, with parting on {001}. Hardness is 7 to 8.5 and measured density 3.21 to 3.41.

The boron ledger. The IMA list writes the formula AlAl6BSi3O18, status Rd, redefined by special procedure in 2013, with France as the country of first description and Canadian Mineralogist 50 (2012), 855 as its printed reference. Counted atom by atom, both versions are Al7 B Si3 O18, 571.92 g/mol. Worked on standard atomic weights, that is 31.52% SiO2, 6.09% B2O3 and 62.40% Al2O3, or 1.89% boron by mass; the sheet’s ideal column prints 31.52, 6.09 and 62.39. Its Dehesa, California analysis, by electron microprobe with B2O3 and H2O inferred from structure determination, has 29.99% SiO2, 61.49% Al2O3 and 1.56% TiO2.

Colour and optics. The sheet lists bright smalt-blue to greenish blue, violet, red-violet, purple and brown, vitreous to dull, biaxial (−), with strong pleochroism: X deep blue or violet, Y yellow to red-violet or nearly colourless, Z colourless or very pale blue. The name honours Eugène Dumortier, a French paleontologist of Lyons.

Is holtite dumortierite? Five IMA rows on one formula block

No: holtite has its own IMA row, and so do three other minerals whose formulas share dumortierite’s Al6BSi3O block. A script over the September 2026 list finds 5 rows whose formula contains Al6BSi3O. They differ at the first site: dumortierite, AlAl6BSi3O18, Rd, 2013 s.p., France; magnesiodumortierite, MgAl6BSi3O17(OH), Rd, 1992-050, Italy; holtite, (Ta0.6☐0.4)Al6BSi3O18, Rd, 1969-029, Australia; nioboholtite, (Nb0.6☐0.4)Al6BSi3O18, A, 2012-068, Poland; and titanoholtite, (Ti0.75☐0.25)Al6BSi3O18, A, 2012-069, Poland.

Where the references disagree. The International Gem Society dumortierite article lists holtite under common names and varieties of dumortierite; the IMA list treats it as a species. It also prints dumortierite as uniaxial (−) with a 2V of 13° to 56°, which describes a biaxial mineral; the Handbook sheet gives biaxial (−), 2V 13° to 55°. This page follows the sheet and the IMA list. Handbook sheets for holtite and magnesiodumortierite returned 404 pages, so no numbers are quoted for them, and this page names no group for the five rows because the nomenclature paper was not read.

As a rule. A dumortierite label on blue fibrous material is a reasonable field name. A holtite label means a tantalum-dominant species on the IMA list, and only an analysis supports it; a Ti-rich analysis such as Dehesa’s 1.56% TiO2 is still dumortierite on its own sheet.

Why was tanzanite first taken for dumortierite? The optic-sign rule

Because both are orthorhombic and strongly pleochroic in blue and violet; the optic sign separates them. Hurlbut 1969, Gem zoisite from Tanzania (American Mineralogist 54, 702) says the 1967 Tanzanian crystals, with a striking red, blue, yellow-green pleochroism and orthorhombic symmetry, were at first thought to be cordierite or dumortierite, until X-ray photographs established that they were zoisite. On the sheets zoisite is biaxial (+) and dumortierite biaxial (−). That is the optic-sign rule: on a conoscope, a positive figure rules out dumortierite.

Documented failure: the balance. Worked as 20 − 20/D, a 20.00 g piece reads 13.65 to 14.05 g in water as zoisite and 13.77 to 14.13 g as dumortierite; the windows share 13.77 to 14.05 g. GIA’s tanzanite at 3.35 reads 14.03 g, inside both. The method is on how to measure specific gravity at home.

What else differs. Zoisite is hardness 6 to 7 with a perfect {010} cleavage; dumortierite is 7 to 8.5 with a distinct {100} cleavage and grows as fibres and needles rather than as Hurlbut’s well-formed prisms. The tanzanite question itself is on is zoisite the same as tanzanite and the species on zoisite; cordierite is the third orthorhombic suspect.

Where dumortierite is found, and what dumortierite quartz is

The Distribution field lists Beaunan and Chaponost near Lyon, Rhône, France; Wolfshau near Schmiedeberg, Silesia, Poland; Kaňk near Kutná Hora, Czech Republic; in the USA, the Champion mine near Oreana, Humboldt Co., Nevada, “once mined commercially”, Clip near Yuma and Quartzsite, Arizona, Petaca, New Mexico, and Dehesa and Ogilby, California; Ashby Township, Ontario, Canada; the Erongo Mountains, Namibia; on Madagascar, Soavina, Saharina, Ambatolahinanahary, Ambositra and Riamfotsy; and Mogra, Maharashtra, India. It adds that many other minor localities are known.

Where it grows. The sheet places it in aluminium-rich regionally metamorphosed rocks, disseminated and in veinlets cutting schists, and in pegmatitic veins, with quartz, cordierite, kyanite, andalusite, sillimanite, muscovite and rutile. The International Gem Society quartz article describes dumortierite quartz as crystalline quartz coloured by dumortierite, and the gem society’s dumortierite article says Arizona dumortierite is actually a quartz-impregnated variety. How to tell that stone from quartz and from dumortierite itself is on is dumortierite quartz.

Under a lamp. A ligature-aware search of the sheet for the letters uoresc finds nothing; the gem society records blue short-wave fluorescence for French material and blue-white to violet for Californian. No cause is offered here; the background is on fluorescent minerals explained. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.

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