Petalite
Petalite, LiAlSi4O10, is a lithium aluminium silicate of pegmatites: hardness 6.5, density 2.412 to 2.422. Why three sources print three lithium figures.
Petalite is lithium aluminium silicate, LiAlSi4O10 on the IMA list and the Handbook sheet: monoclinic, hardness 6.5, density 2.412 to 2.422, one perfect cleavage. Worked from the formula it is 2.27% lithium by mass; the USGS prints 2.09 and the sheet’s Rubicon, Namibia analysis carries 2.07. The USGS says lithium was first recognised in petalite, in 1817.
In short
- One species, one formula. The September 2026 IMA-CNMNC Master List lists petalite as LiAlSi4O10, grandfathered, 1800, Sweden, and the Handbook of Mineralogy sheet prints the same formula. A script finds four IMA rows built only of lithium, aluminium, silicon and oxygen: eucryptite, petalite, spodumene and virgilite.
- The lithium ledger. On standard atomic weights LiAlSi4O10 is 2.27% lithium. USGS Professional Paper 1802-K prints 2.09 in its Table K1, while its eucryptite and spodumene figures, 5.51 and 3.73, match the same arithmetic exactly. The sheet’s Rubicon analysis, 4.45% Li2O, carries 2.07% lithium, 91.2% of the ideal. All three are printed here; we reconcile none.
- Lithium history and use, per the USGS. Professional Paper 1802-K says lithium was recognised as an element in 1817 when Johan Arfvedson analysed petalite, and that lithium minerals, mainly spodumene, petalite and lepidolite, are mined from pegmatites and used mostly as feedstock for glasses and ceramics. The sheet says petalite locally constitutes an ore of lithium.
- Lighter than its neighbours. A 20.00 g piece reads 11.71 to 11.74 g in water: lighter than spodumene, 13.40 to 13.81 g, quartz, 12.45 g, and the lightest feldspar sheet, microcline, at 12.13 g. Whether that settles the feldspar question is worked on is petalite a feldspar.
- Castorite. The International Gem Society says crystal petalites are sometimes called castorites, after Castor, paired with pollucite, named after his twin Pollux, both from Elba; castorite was later renamed petalite. The IMA list has no castorite row, by script; pollucite has one, Cs(Si2Al)O6·nH2O.
| Mineral | IMA formula | Li worked, wt% | Li, USGS Table K1 | Li in the sheet’s analysis | Density; 20.00 g piece in water | Cleavage (sheet) |
|---|---|---|---|---|---|---|
| Petalite | LiAlSi4O10 | 2.27 | 2.09 | 2.07 (Rubicon, Namibia) | 2.412 to 2.422; 11.71 to 11.74 g | Perfect {001}, poor {201}, at 38.5° |
| Spodumene | LiAlSi2O6 | 3.73 | 3.73 | 3.62 (Pala, California) | 3.03 to 3.23; 13.40 to 13.81 g | Good on {110}, about 87° |
| Eucryptite | LiAl(SiO4) | 5.51 | 5.51 | 4.52 (Bikita, Zimbabwe) | 2.657 to 2.666; 12.47 to 12.50 g | {0001} and {10-10} |
| Virgilite | LiAlSi2O6 | 3.73 | Not listed | - | 2.46 calculated; 11.87 g | Not printed |
| Lepidolite (not an IMA species) | No row | - | 3.58 | - | See the lepidolite page | - |
| Quartz | SiO2 | 0 | - | - | 2.65; 12.45 g | Rarely observable |
What is petalite? A lithium aluminium silicate
Petalite is LiAlSi4O10, a lithium aluminium silicate of granite pegmatites. The September 2026 IMA-CNMNC Master List gives it status G, grandfathered, 1800, Sweden. On standard atomic weights the formula mass is 306.25, with 78.48% SiO2, 16.65% Al2O3 and 4.88% Li2O. The Handbook of Mineralogy sheet’s Rubicon pegmatite, Namibia analysis prints 77.80% SiO2, 16.41% Al2O3 and 4.45% Li2O; recalculated on 10 oxygens it gives Li 0.924, Al 0.998 and Si 4.016, matching the printed Li0.92Al0.99Si3.99O10 to within 0.03.
What the sheet records. Monoclinic, point group 2/m; crystals tabular on {010}, or elongated parallel to [100], to 2 m, some faces striated; commonly foliated cleavable, massive. Twinning common on {001}, lamellar. Cleavage perfect on {001}, poor on {201}, the two at 38.5°; fracture imperfectly conchoidal; brittle. Hardness 6.5; density 2.412 to 2.422. Colorless, white, gray, rarely reddish or greenish; streak white; vitreous, pearly on {001}. Biaxial (+), α = 1.504 to 1.507, β = 1.510 to 1.513, γ = 1.516 to 1.523, 2V 82° to 84°. The sheet prints thermoluminescent and no fluorescence.
On paper only. LiAlSi4O10 has the composition of LiAlSi2O6 plus two SiO2, spodumene plus quartz, and of LiAl(SiO4) plus three SiO2. That is arithmetic on formulas, not a statement about how any of them forms; two IMA rows, spodumene and virgilite, share the formula LiAlSi2O6, so a formula alone names nothing.
How much lithium is in petalite? The lithium ledger
The lithium ledger: three sources, three figures, 2.27, 2.09 and 2.07% lithium. Worked on standard atomic weights (Li 6.94, Al 26.982, Si 28.085, O 15.999), lithium is 6.94 of 306.25, or 2.27%: 22.7 g in a kilogram of pure petalite. USGS Professional Paper 1802-K (Bradley and others, 2017), Table K1, gives petalite 2.09 weight per cent, noting that its compositions are from Barthelmy (2014).
A check on the table. The same arithmetic gives eucryptite, LiAl(SiO4), 5.51% and spodumene, LiAlSi2O6, 3.73%, and Table K1 prints exactly 5.51 and 3.73. Petalite alone differs, by 0.18. The sheet’s Rubicon analysis, 4.45% Li2O, works out at 2.07% lithium, 91.2% of the ideal Li2O and close to the USGS figure. This page does not say why the table differs; it prints all three.
Against spodumene. Pure petalite carries 60.8% as much lithium per kilogram as pure spodumene, worked: 22.7 g against 37.3 g. The spodumene page sets out its own arithmetic and concentrate figures, which are not repeated here: spodumene.
Is petalite a lithium ore?
Locally, on its sheet, and among the main lithium minerals mined from pegmatites, per the USGS. The petalite sheet’s Occurrence field reads: in granite pegmatites, locally constituting an ore of lithium. USGS Professional Paper 1802-K says lithium minerals, mainly spodumene, petalite and lepidolite, are mined from pegmatites and are used mostly as feedstock for glasses and ceramics, and that in terms of lithium resources the most important minerals are spodumene and petalite, both lithium aluminium silicates, and the pink mica lepidolite.
Where lithium was found. The same report says lithium was recognised as an element in 1817 when the Swedish chemist Johan Arfvedson analysed the mineral petalite, and that petalite, lepidolite and spodumene are found in granitic pegmatites. The lithium mica is on lepidolite.
What this page leaves out. Lithium production, markets and prices: none of that is mineralogy, and the report’s figures are not reproduced here.
Petalite, spodumene and the pegmatite look-alikes
Petalite grows with spodumene, albite, microcline and quartz, and the balance separates it from all four. The sheet’s associates are spodumene, pollucite, lepidolite, tourmaline, topaz, albite, microcline and quartz. A 20.00 g piece of petalite reads 11.71 to 11.74 g in water; spodumene reads 13.40 to 13.81 g, quartz 12.45 g, albite 12.31 to 12.45 g and microcline 12.13 to 12.22 g. The method is on how to measure specific gravity at home.
Cleavage. Petalite has one perfect cleavage, on {001}, and a poor one on {201} at 38.5° to it. Spodumene’s two good cleavages meet at about 87°; the pyroxene question is on is spodumene a pyroxene, which gives petalite the same 11.71 to 11.74 g. Its name, from the Greek for leaf, refers to the perfect basal cleavage.
Where the numbers disagree. The International Gem Society gives hardness 6 to 6.5 and specific gravity 2.3 to 2.5, wider than the sheet’s 6.5 and 2.412 to 2.422; at 2.5 a 20.00 g piece reads 12.00 g. Both sets are printed. The feldspar and quartz comparison is on is petalite a feldspar.
Where petalite is found, and its old name
The Distribution field: the Island of Utö, and the Varuträsk pegmatite, 15 km northwest of Skellefteå, Västerbotten, Sweden; around San Piero in Campo, Elba, Italy; Hirvikallio, Tammela, and Pajula, Finland; the Rubicon pegmatite, south of Karibib, Namibia; Bikita, Zimbabwe; Peru, Oxford County, Maine; Bolton, Massachusetts; Queen and Hiriart Mountains, Pala, and the Clark vein at Rincon, San Diego County, California; the Tanco pegmatite, Bernic Lake, Manitoba; the Maxixe, Lavra Velha and Lavra do Genipapo pegmatites near Araçuaí, and the Taquaral mine, Itinga, in Minas Gerais, Brazil; the Londonderry quarry, Coolgardie, Western Australia; and Nagatareyama, Fukuoka Prefecture, Japan. Additional minor localities are known.
Castorite and type material. The International Gem Society says crystal petalites were once called castorites, and that pollucite, discovered on Elba in 1846, was named after Pollux, Castor’s twin, because both occurred there. The sheet gives the type material as Mining Academy, Freiberg, Germany, 26512; the IMA list gives Sweden. The gem society’s localities outside the sheet are not added here.
Related pages. The lithium pyroxene is spodumene; 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.