Montebrasite
Montebrasite, LiAl(PO4)(OH): triclinic lithium aluminium phosphate, hardness 5.5 to 6, density 2.98 to 3.04, the hydroxyl member of the amblygonite series.
Montebrasite is lithium aluminium hydroxyphosphate, LiAl(PO4)(OH) on the IMA list: triclinic, white to pale brown or pink, hardness 5.5 to 6, density 2.98 to 3.04, perfect {100} cleavage. It is the hydroxyl member of the amblygonite series, and the commoner. A measured 2V near 108° marks the boundary between the two.
In short
- The species. The September 2026 IMA-CNMNC Master List writes montebrasite LiAl(PO4)(OH), grandfathered, 1871, France, and amblygonite LiAl(PO4)F. The Handbook of Mineralogy sheet writes LiAl(PO4)(OH,F), amblygonite group, forms a series with amblygonite. The amblygonite sheet calls amblygonite much less common than montebrasite. Hardness 5.5 to 6; density 2.98 to 3.04; cleavage {100} perfect, {110} good.
- The 108° line. Greiner and Bloss, American Mineralogist 72, 617 (1987) measured 11 crystals of the series and give [F] = −66.3 + 1.08 × 2Vz, good to about 2 mol% when fluorine is below 60 mol% and 2Vz below 116°, as for the vast majority of natural specimens. Worked, the 50 mol% name boundary falls at 2Vz 107.7°. The sheet’s 2V of 66° to 101° reads as 5.0 to 42.8 mol% fluorine: all montebrasite.
- The sheet checks itself. The Tanco pegmatite analysis on the sheet, F 1.40% and H2O+ 5.25%, recasts to 11.2 mol% fluorine (the printed formula, (OH)0.85F0.11, gives 11.5%). The equation predicts a 2Vz of 71.8° for that composition, inside the sheet’s printed range.
- Documented failure: the optic sign. Greiner and Bloss write that members of the series negative in sign are called amblygonites and positive ones montebrasites, a naming by optic sign, and that their evidence places the sign change closer to amb30, 30 mol% fluorine. The equation gives 30.9 mol% at 2Vz 90°. Between about 31% and 50% fluorine a crystal is optically negative and still hydroxyl-dominant: the sheet’s “may be biaxial (−)”.
- Documented failure: the balance. A 20.00 g piece reads 13.29 to 13.42 g in water. Brazilianite, at density 2.98, reads 13.29 g, a tie, and amblygonite starts at 13.42 g. Černá, Černý and Ferguson, American Mineralogist 58, 291 (1973) found specific gravity very sensitive to alteration and inclusions.
| 2Vz | Where the figure comes from | [F] by the equation, mol% | Side of the 50% line | Optic sign |
|---|---|---|---|---|
| 66° | Montebrasite sheet, lowest 2V | 5.0 | Montebrasite | Positive |
| 71.8° | Predicted for the sheet’s Tanco analysis (11.2 mol% F, by script) | 11.2 | Montebrasite | Positive |
| 90° | Sign change (Greiner and Bloss: “closer to amb30”) | 30.9 | Montebrasite | Changes |
| 101° | Montebrasite sheet, highest 2V | 42.8 | Montebrasite | Negative |
| 107° | Amblygonite sheet, lowest 2V | 49.3 | Montebrasite, by 0.7 mol% | Negative |
| 107.7° | The 50 mol% line, worked | 50.0 | The boundary | Negative |
| 116° | Validity limit of the equation | 59.0 | Amblygonite | Negative |
| 129.5° | Amblygonite sheet, highest 2V | Outside the equation (73.6 if extrapolated) | Amblygonite | Negative |
What is montebrasite? The hydroxyl member
Montebrasite is LiAl(PO4)(OH) on the September 2026 IMA list. Status G, grandfathered, 1871, France; first reference the Comptes Rendus of the Académie des Sciences 73 (1871), 306. The Handbook of Mineralogy montebrasite sheet writes LiAl(PO4)(OH,F), places it in the amblygonite group and says it forms a series with amblygonite, LiAl(PO4)F. Worked on standard atomic weights, the IMA formula is 4.76% lithium, 10.24% Li2O and 6.17% water.
What the sheet records. Triclinic, point group 1̄. Crystals typically equant, may be short to long prismatic, to 10 cm; as coarse cleavable or blocky to rounded nodular masses. Twinning common, producing tabular composite crystals, with microscopic polysynthetic twinning. Cleavage perfect on {100}, good on {110}, distinct on a third form; fracture uneven to subconchoidal; brittle; hardness 5.5 to 6. Density 2.98 to 3.04 measured, [3.03] calculated; the IMA formula on the sheet’s cell (Z = 4) gives 3.024. Pale brown, pink, pale yellow, grey, white or colourless; colourless in transmitted light, which is what a thin section shows; vitreous, pearly on cleavages.
Montebrasite or amblygonite from the optics: the 108° line
A 2Vz below about 108° reads as montebrasite. Greiner and Bloss, American Mineralogist 72, 617 (1987) determined 2V and the principal refractive indices of 11 crystals of the series, from 4.0 to 91.8 mol% fluorine, on a spindle stage. If fluorine is below 60 mol% and so 2Vz below 116°, as for the vast majority of natural specimens, they write that the fluorine content can usually be estimated to within 2 mol% from [F] = −66.3 + 1.08 × 2Vz. Above 60 mol% the optics respond less sensitively and nonlinearly.
Worked. Setting [F] to 50 mol%, the boundary under Hatert and Burke (2008), Canadian Mineralogist 46, 717–728, gives 2Vz = 107.7°. The montebrasite sheet prints 2V 66° to 101°; read as 2Vz, that is 5.0 to 42.8 mol% fluorine, all on the montebrasite side. Its Tanco analysis, at 11.2 mol% fluorine by script, should show about 71.8°, inside the printed range. The amblygonite sheet’s range starts at 107°, which the equation puts at 49.3 mol%: by 0.7 mol%, still montebrasite, and well inside the equation’s 2 mol% uncertainty. Near 108°, then, the optics do not name a crystal: with the stated 2 mol% uncertainty, the equation separates the names below about 105.8° and above about 109.5°. The table sets out each figure.
What it needs. A measured 2V: Greiner and Bloss used a spindle stage, and it is not a hand-lens test. They note that the estimate holds despite significant sodium in place of lithium, while lattice-parameter methods may then err.
Is montebrasite positive or negative? The optic-sign trap
Either, and that is why the sign is not the name. The montebrasite sheet prints biaxial (+), may be biaxial (−); the amblygonite sheet prints biaxial (−). Greiner and Bloss (1987) write that members of the series that are negative in sign are called amblygonites, and positive ones montebrasites: a naming by optic sign, not by the fluorine-to-hydroxyl ratio the IMA formulas express. They add that Phillips and Griffen (1981) put the sign change at about amb35, and that their own evidence places it closer to amb30.
The gap, worked. On the equation, 2Vz = 90°, where the sign changes, corresponds to 30.9 mol% fluorine. A crystal between about 31% and 50% fluorine is therefore optically negative but hydroxyl-dominant: montebrasite by the IMA formula and the 50% rule, amblygonite by the optic-sign naming. The sheet’s 101° end, at 42.8 mol%, is such a crystal. A label written from the optic sign alone may name the wrong member.
Montebrasite, amblygonite or brazilianite by weight?
Weight does not settle it. Worked as 20 − 20/D, a 20.00 g piece reads 13.29 to 13.42 g in water as montebrasite (2.98 to 3.04) and 13.42 to 13.57 g as amblygonite (3.04 to 3.11): the two ranges touch at 13.42 g. Brazilianite, at 2.98, reads 13.29 g, the same as montebrasite’s lightest; that page sets out how its single good cleavage and conchoidal fracture differ from montebrasite’s perfect, good and distinct cleavages. The method is on how to measure specific gravity at home.
Why the balance is weak here. Černá, Černý and Ferguson, American Mineralogist 58, 291 (1973) studied 15 analysed members of the series to find an indirect way to their fluorine content, and found that specific gravities complied with earlier relations but proved very sensitive to alteration and inclusions. They found two X-ray methods the most practical, provided the samples contain less than about 1.5 wt% Na2O and are optically homogeneous.
Is montebrasite an ore of lithium, and a gemstone?
As its sheet and the USGS put it. The sheet says montebrasite is a late primary and secondary mineral in zoned granite pegmatites and may be an ore of lithium. USGS Professional Paper 1802-K, Lithium (2017) lists it, spelled montrebasite, in its Table K1 of commercially and (or) scientifically important lithium-bearing minerals, at 4.74 weight percent lithium, against 4.76 worked here for LiAl(PO4)(OH). This page gives no production or market information.
Gemmy crystals. The sheet records gemmy crystals from Linópolis, Galiléia, Brazil, in Minas Gerais. That is the only gem wording in the sources fetched for this page.
Where is montebrasite found? The sheet’s localities
Associates. Hydroxylapatite, spodumene, lepidolite, petalite, pollucite, tourmaline, triplite, lithiophilite, lacroixite, crandallite, muscovite and albite. The lithium silicates are on spodumene and petalite; lithiophilite is on triphylite vs lithiophilite; triphylite is its iron partner’s species page.
Distribution, as printed. A few localities for well-studied material: Montebras, Creuse, France; the Varuträsk pegmatite, 15 km northwest of Skellefteå, Västerbotten, Sweden; the Tanco pegmatite, Bernic Lake, Manitoba, Canada. In the USA, the Nevel quarry and Bell Pit, Newry, Oxford Co., and elsewhere in Maine; the Midnight Owl pegmatite, White Picacho district, Arizona; in South Dakota, the Peerless and Hugo mines near Keystone and, around Custer, the Tip Top and Tin Mountain mines, with large masses from the Beecher lode; the Nesbit mine, Gunnison Co., Colorado; the Stewart mine, Pala, California. In Brazil, gemmy crystals from Linópolis, Galiléia, and Araçuaí, Itinga, Governador Valadares and other places in Minas Gerais. The Buranga pegmatite, near Gatumba, Rwanda.
Name. For Montebras, from which it was first described; the sheet marks no type locality as such, and the term is on what is a type locality. More species are at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.