Amblygonite
Amblygonite, LiAl(PO4)F: triclinic lithium aluminium phosphate, hardness 5.5 to 6, density 3.04 to 3.11, the fluorine end of a series with montebrasite.
Amblygonite is lithium aluminium fluorophosphate, LiAl(PO4)F on the IMA list: triclinic, milk-white to pale-tinted, hardness 5.5 to 6, density 3.04 to 3.11, with a perfect {100} cleavage. It is the fluorine end of a series with montebrasite, LiAl(PO4)(OH). Its Handbook sheet calls it much less common than montebrasite, in granite pegmatites and tin greisens.
In short
- The species. The September 2026 IMA-CNMNC Master List writes amblygonite LiAl(PO4)F (G, 1818, Germany) and montebrasite LiAl(PO4)(OH). The Handbook of Mineralogy sheet writes LiAl(PO4)(F,OH), amblygonite group, and says it forms a series with montebrasite. Triclinic; crude equant crystals to 1.5 m or massive; hardness 5.5 to 6; density 3.04 to 3.11; cleavage {100} perfect, {110} good.
- The 6.47% line. Worked on standard atomic weights, LiAl(PO4)F is 12.85% fluorine. The halfway composition, F:OH = 1:1, is 6.47%: exactly the fluorine figure in both sheets’ ideal column, which the script reproduces to the hundredth. Hatert and Burke (2008), Canadian Mineralogist 46, 717–728 give two names to a two-member series, each running to the 50 mol.% mark, so a lithium–aluminium analysis above 6.47% fluorine is on the amblygonite side.
- Read on the sheets. The amblygonite sheet’s Chursdorf, Germany, analysis has 13.06% fluorine and no water: 1.01 fluorine per phosphorus by script (the sheet prints F1.02), 101.7% of the end-member figure. The montebrasite sheet’s Tanco, Canada, analysis has 1.40%, on the other side. Both sheets’ ideal columns sit exactly on the line, so neither is a name.
- The label rule. The sheet says amblygonite is much less common than montebrasite. Greiner and Bloss, American Mineralogist 72, 617 (1987) report natural members from amb0 to amb92, with specimens more fluorine-rich than amb65 rare. A piece labelled amblygonite without an analysis has not been placed on either side of the line; the optical route is on montebrasite.
- Documented failure: the lithium figure. Worked, LiAl(PO4)F is 4.69% lithium. USGS Professional Paper 1802-K, Lithium (2017) prints 3.44 for amblygonite in its Table K1, against a formula written (Li,Na)Al(PO4)(F,OH) and taken from another compilation. The two figures describe different formulas, so neither checks the other.
| Composition | Source | F, wt% | F per P | OH per P | F share of F + OH, mol% | Side of the line |
|---|---|---|---|---|---|---|
| LiAl(PO4)F | IMA amblygonite | 12.85 | 1 | 0 | 100 | Amblygonite (end member) |
| Chursdorf, Germany | Amblygonite sheet, analysis (1) | 13.06 | 1.01 (printed F1.02) | 0.00 | 100.0 | Amblygonite |
| F:OH = 1:1 | Ideal column (2) of both sheets | 6.47 | 0.5 | 0.5 | 50 | On the line: neither name |
| Tanco pegmatite, Canada | Montebrasite sheet, analysis (1) | 1.40 | 0.11 | 0.84 | 11.2 (printed formula 11.5) | Montebrasite |
| LiAl(PO4)(OH) | IMA montebrasite | 0 | 0 | 1 | 0 | Montebrasite (end member) |
What is amblygonite? The fluorine end of a series
Amblygonite is LiAl(PO4)F on the September 2026 IMA list. The row is status G, grandfathered, 1818, with Germany as the country. Its partner, montebrasite, is LiAl(PO4)(OH), grandfathered from 1871, France; both rows give American Mineralogist 88 (2003), 195 as the second reference. The Handbook of Mineralogy amblygonite sheet writes the formula LiAl(PO4)(F,OH), puts it in the amblygonite group, and says it forms a series with montebrasite.
What the sheet records. Triclinic, point group 1̄. Crude crystals, typically equant with complex form development, to 1.5 m; massive. Twinning on several laws, with microscopic polysynthetic twinning common. Cleavage {100} perfect, {110} good, {0-11} distinct, {001} imperfect; fracture uneven to subconchoidal; brittle; hardness 5.5 to 6. Density 3.04 to 3.11 measured, [3.13] calculated. Milk-white, pale yellow, beige, salmon-pink, pale green, pale blue or grey, may be colourless; vitreous to greasy, pearly on good cleavages. Biaxial (−), α 1.577 to 1.591, β 1.592 to 1.605, γ 1.596 to 1.613, 2V 107° to 129.5°.
A binary series. Greiner and Bloss (1987) write that in the common members of the series less than 10% of the lithium sites contain sodium or calcium and less than 0.1% of the aluminium sites contain iron or magnesium, so they are essentially binary solid solutions between LiAl(PO4)F and LiAl(PO4)(OH). That is why the fluorine to hydroxyl ratio is the whole naming question.
Amblygonite or montebrasite? The 6.47% line
By analysis, fluorine above 6.47% by mass puts a lithium–aluminium piece on the amblygonite side. Hatert and Burke (2008), Canadian Mineralogist 46, 717–728 state that a series with two end members gets two names, each covering the range from its end member to the 50 mol.% mark. A script on standard atomic weights gives LiAl(PO4)F 12.85% fluorine and the 50:50 composition, LiAl(PO4)F0.5(OH)0.5, 6.47% fluorine and 3.07% water. Those are the numbers both Handbook sheets print in their ideal column (2), described as F:OH = 1:1, along with P2O5 48.31, Al2O3 34.70 and Li2O 10.17. The sheets’ ideal composition is therefore the boundary itself.
The two analyses. Amblygonite from Chursdorf, Saxony, by electron microprobe: P2O5 48.44, Al2O3 33.90, Li2O 10.12, F 13.06, H2O 0.00. Recast, that is 0.99 Li, 0.97 Al and 1.01 F per phosphorus and no hydroxyl; the sheet prints Li1.00Al0.98(PO4)1.01F1.02. Montebrasite from the Tanco pegmatite, Manitoba, on its own sheet: F 1.40, H2O+ 5.25, which recasts to 0.11 F and 0.84 OH per phosphorus, 11.2 mol% fluorine; the sheet’s formula, (OH)0.85F0.11, gives 11.5%. The table sets both against the line.
Where the line fails. It is a weight percentage for a lithium–aluminium phosphate. Sodium in place of lithium changes the formula mass and moves it slightly; a mixed or altered sample gives a mixed figure. The line reads an analysis; it cannot be applied to an unanalysed specimen.
Can amblygonite be told from montebrasite by eye?
Not reliably, on the two sheets’ own numbers. Both print hardness 5.5 to 6, a perfect {100} and good {110} cleavage, uneven to subconchoidal fracture and brittle tenacity. The colour lists share pale yellow, grey, white or milk-white and colourless. Measured densities meet at 3.04: worked as 20 − 20/D, a 20.00 g piece reads 13.42 to 13.57 g in water as amblygonite and 13.29 to 13.42 g as montebrasite, on the method at how to measure specific gravity at home. Černá, Černý and Ferguson, American Mineralogist 58, 291 (1973) found specific gravity very sensitive to alteration and inclusions.
What does separate them. An analysis, read against the 6.47% line, or the optics: the amblygonite sheet prints biaxial (−) and 2V 107° to 129.5°, the montebrasite sheet biaxial (+), may be biaxial (−), and 66° to 101°. How far a 2V measurement places a crystal, and why the optic sign alone does not, is on montebrasite. Brazilianite sits at the bottom of the same weight window, at 13.29 g, which is set out on its own page.
Is amblygonite an ore of lithium?
The USGS lists it among the important lithium minerals; this page says no more than that. USGS Professional Paper 1802-K, Lithium (2017) gives a Table K1 of commercially and (or) scientifically important lithium-bearing minerals, and amblygonite is its first row: (Li,Na)Al(PO4)(F,OH), 3.44 weight percent lithium, pegmatite intrusions in orogenic belts, solid solution with montebrasite. The table takes its formulas from Barthelmy (2014), not the IMA list.
Documented failure: 3.44 against 4.69. Worked from the IMA formula, LiAl(PO4)F is 4.69% lithium and 10.10% Li2O; the Chursdorf analysis has 10.12% Li2O. The USGS figure is lower because its formula is different, with sodium sharing the lithium site, and this page does not try to reconstruct it. The same table spells the other member montrebasite and gives it 4.74%, against 4.76% worked for LiAl(PO4)(OH). The sheet itself says only that amblygonite is an uncommon accessory mineral in zoned granite pegmatites and in high-temperature tin veins and greisens. The other lithium minerals are spodumene, petalite and lepidolite.
Where is amblygonite found? The sheet’s localities
Setting. An uncommon accessory mineral in zoned granite pegmatites; in high-temperature tin veins and greisens. Associates in pegmatites: lacroixite, apatite, lithiophilite, spodumene, lepidolite, petalite, pollucite and tourmaline; in greisens, cassiterite, topaz and mica. Lithiophilite’s iron partner is on triphylite vs lithiophilite.
Distribution, as printed. Much less common than montebrasite. The few localities for analysed material include Chursdorf and Arnsdorf, near Penig, Saxony, Germany; Utö Island, near Stockholm, Sweden; Hebron, Oxford Co., Maine, USA; large crystals in San Domingo Wash, northeast of Wickenburg, Maricopa Co., Arizona; the Tanco pegmatite, Bernic Lake, Manitoba, Canada; Coolgardie, Western Australia; near Karibib, Namibia; and the Muiâne pegmatite, Alto Ligonha district, Mozambique.
Type material and name. The sheet gives type material at the Mining Academy, Freiberg, Germany, 20336, and marks no type locality; the IMA list gives Germany. The name is from the Greek for blunt and angle, for its approximately 90° cleavage angle. The term type locality is explained 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.