Is petalite a feldspar?
No: petalite, LiAlSi4O10, is a lithium silicate, not a feldspar or quartz. Why the balance tells it from all ten feldspar sheets when a refractometer cannot.
No. Petalite, LiAlSi4O10, is a lithium aluminium silicate with its own IMA row; its Handbook sheet names no feldspar group, and it is not quartz either. The balance separates it: a 20.00 g piece reads 11.71 to 11.74 g in water, below the lightest of ten feldspar sheets, 12.13 g, and quartz’s 12.45 g.
In short
- The verdict. The September 2026 IMA-CNMNC Master List lists petalite as LiAlSi4O10. Its rows for albite, anorthite, orthoclase, microcline and sanidine all have four tetrahedral cations to eight oxygens, such as Na(AlSi3O8); petalite has five to ten. All 10 Handbook of Mineralogy feldspar sheets print a feldspar group line, by script; the petalite sheet prints no group line. The confusion is understandable: its sheet lists albite, microcline and quartz among its associates.
- The 12-gram line. Worked as 20 − 20/D, a 20.00 g petalite reads 11.71 to 11.74 g in water. Across the 10 feldspar sheets the lightest reading is microcline’s 12.13 g, nearly 0.4 g heavier; quartz reads 12.45 g and spodumene 13.40 to 13.81 g. Even the International Gem Society’s wider 2.3 to 2.5 tops out at 12.00 g. A clean piece reading under 12 g is not one of them.
- Choose the test by the specimen. A clean, single-species piece: the balance. A cleavable mass: one perfect cleavage, with a poor second at 38.5°, against the feldspars’ perfect {001} and second cleavage on {010}. A cut stone: the refractometer separates petalite from quartz, by 0.021, but not from orthoclase, microcline or sanidine. Hardness, 6.5 against 6 to 6.5, chooses nothing.
- Where the 12-gram line fails. Two feldspathoids read under 12 g as well: leucite, 11.84 to 12.00 g, and cancrinite, 11.74 to 12.03 g, on their sheets; neither is on petalite’s association list. Matrix pieces and intergrowths weigh as mixtures. The line separates petalite from its pegmatite company, not from every light silicate.
- Not a quartz, either. The quartz sheet gives SiO2, hardness 7, uniaxial (+), ω = 1.544, ε = 1.553. Petalite is biaxial (+), with its highest index, γ, at 1.523. Petalite carries 2.27% lithium, worked; the lithium figures are on petalite.
| Mineral (sheet) | Group line on the sheet | Density | 20.00 g piece in water | Refractive indices | Cleavage | Under 12 g? |
|---|---|---|---|---|---|---|
| Petalite | None printed | 2.412 to 2.422 | 11.71 to 11.74 g | 1.504 to 1.523, biaxial (+) | Perfect {001}, poor {201} | Yes |
| Petalite, IGS figures | - | 2.3 to 2.5 | 11.30 to 12.00 g | 1.503 to 1.523 | Perfect, 1 direction | Yes, to 12.00 g |
| Microcline | Feldspar (alkali) group | 2.54 to 2.57 | 12.13 to 12.22 g | 1.514 to 1.539 | Perfect {001} and {010}, about 90° | No |
| Orthoclase | Feldspar (alkali) group | 2.55 to 2.63 | 12.16 to 12.40 g | 1.518 to 1.525 | Perfect {001} and {010} | No |
| Sanidine | Feldspar (alkali) group | 2.56 to 2.62 | 12.19 to 12.37 g | 1.518 to 1.530 | Perfect {001}, distinct {010} | No |
| Anorthoclase | Feldspar (alkali) group | 2.57 to 2.60 | 12.22 to 12.31 g | 1.524 to 1.534 | Perfect {001}, less perfect {010} | No |
| Albite | Feldspar group, plagioclase series | 2.60 to 2.65 | 12.31 to 12.45 g | 1.526 to 1.541 | Perfect {001}, very good {010} | No |
| Oligoclase to anorthite (5 sheets) | Feldspar group, plagioclase series | 2.63 to 2.76 | 12.40 to 12.75 g | See is labradorite a feldspar | Perfect {001} | No |
| Quartz | None printed | 2.65 | 12.45 g | 1.544 to 1.553, uniaxial (+) | Rarely observable | No |
| Spodumene | Pyroxene group | 3.03 to 3.23 | 13.40 to 13.81 g | 1.648 to 1.679 | Good {110}, about 87° | No |
| Leucite (feldspathoid) | None printed | 2.45 to 2.50 | 11.84 to 12.00 g | 1.508 to 1.509 | Very poor {110} | Yes: a documented failure |
| Cancrinite (feldspathoid) | Cancrinite group | 2.42 to 2.51 | 11.74 to 12.03 g | 1.495 to 1.528 | Perfect {10-10} | Mostly: a documented failure |
Is petalite a feldspar? What the sheets and the IMA list say
Petalite is not a feldspar: it is a lithium aluminium silicate with its own IMA row, and its Handbook sheet places it in no feldspar group. The September 2026 IMA-CNMNC Master List lists petalite as LiAlSi4O10, G, 1800, Sweden. Its rows for the feldspars with sheets here have the shape M(T4O8): albite Na(AlSi3O8), orthoclase, microcline and sanidine K(AlSi3O8), anorthite Ca(Al2Si2O8). Petalite has five tetrahedral cations, one aluminium and four silicon, to ten oxygens, and lithium where a feldspar has sodium, potassium or calcium.
The sheets. A script over the 10 Handbook of Mineralogy feldspar sheets in our cache finds a feldspar group line on every one: albite and the other five plagioclase sheets, anorthoclase, microcline, orthoclase and sanidine. The petalite sheet prints no group line. It does cite Deer, Howie and Zussman’s volume on framework silicates, as all ten feldspar sheets do, by script; a shared reference book is not a shared group.
Why the question gets asked. Petalite is white to grey, glassy, cleavable and found in the same granite pegmatites: its sheet lists spodumene, pollucite, lepidolite, tourmaline, topaz, albite, microcline and quartz as associates. The species page, petalite, carries the sheet data and the lithium ledger; they are not repeated here.
The 12-gram line, worked
The 12-gram line: a clean 20.00 g piece of petalite reads 11.71 to 11.74 g in water, and none of the ten feldspar sheets reads below 12.13 g. Worked as 20 − 20/D from each sheet’s measured density: microcline 12.13 to 12.22 g, orthoclase 12.16 to 12.40 g, sanidine 12.19 to 12.37 g, anorthoclase 12.22 to 12.31 g, albite 12.31 to 12.45 g and the other five plagioclase sheets 12.40 to 12.75 g. The gap from petalite’s heaviest to microcline’s lightest is nearly 0.4 g.
Against the other sources. The International Gem Society gives petalite specific gravity 2.3 to 2.5: 11.30 to 12.00 g, still under the lightest feldspar. Quartz reads 12.45 g and spodumene 13.40 to 13.81 g, the figures on quartz and spodumene. The live is spodumene a pyroxene page gives petalite the same 11.71 to 11.74 g. The method, and why a clean single piece matters, is on how to measure specific gravity at home.
Where the line fails. Leucite reads 11.84 to 12.00 g and cancrinite 11.74 to 12.03 g on their sheets, so a light reading does not prove petalite; both are feldspathoids of alkalic rocks, and neither is on petalite’s association list. A crystal on matrix, or a mass intergrown with quartz or feldspar, weighs as a mixture and the line says nothing about it.
Is petalite a quartz?
Petalite is not quartz: different formula, different optics, different weight. The quartz sheet gives SiO2, hexagonal, hardness 7, density 2.65, uniaxial (+) with ω = 1.544 and ε = 1.553, and cleavage rarely observable. Petalite is LiAlSi4O10, monoclinic, hardness 6.5, biaxial (+), with its highest index, γ, at 1.523: 0.021 below quartz’s lowest. On the balance quartz reads 12.45 g against petalite’s 11.71 to 11.74 g.
Where the two meet. Petalite is 78.48% SiO2 by mass, worked, and its formula has the composition of spodumene plus two SiO2, but that makes it a silicate, not a silica mineral. Quartz is one of the associates its sheet lists. Whether quartz is a mineral at all, and its tests, are on is quartz a mineral.
A documented disagreement. The gem society describes petalite as just below quartz in hardness; its own figure is 6 to 6.5, the sheet’s 6.5, against quartz’s 7.
Cleavage and optics: what they can and cannot choose
Cleavage chooses on a cleavable mass; the refractometer does not choose against potassium feldspar. The petalite sheet gives one perfect cleavage, on {001}, a poor one on {201}, and the angle between them, 38.5°. Every feldspar sheet gives a perfect cleavage on {001} and a second on {010}, from perfect to distinct; the microcline sheet says they intersect at about 90°, the oligoclase sheet about 94°. Two bright planes near a right angle point away from petalite. The flash test for feldspar is on how to identify feldspar.
The refractometer. Petalite reads 1.504 to 1.523. Orthoclase, 1.518 to 1.525, microcline, 1.514 to 1.539, and sanidine, 1.518 to 1.530, overlap it; albite starts at 1.526, only 0.003 above. Petalite is biaxial (+), as is low albite; its 2V of 82° to 84° sits just below low albite’s 85° to 90°. None of that separates them reliably at the bench.
Birefringence. Worked from the ranges, petalite’s is 0.012 to 0.016; the gem society gives 0.012 to 0.014. The feldspars run lower, 0.004 to 0.011 for orthoclase, microcline and albite, but measuring birefringence needs a cut stone or a thin section, not a hand specimen.
Who each test is wrong for
Each test fails on some specimen, so choose the one that fits the piece. The 12-gram line is right for a clean, loose, single-species piece and wrong for matrix specimens, intergrowths and the two light feldspathoids. Cleavage is right for a cleavable mass and wrong for a tumbled or cut stone. The refractometer is right against quartz and spodumene and wrong against orthoclase, microcline and sanidine. Hardness is wrong for all of them.
The label. A white, glassy, cleavable pegmatite mineral labelled feldspar may be petalite, and the balance is the cheapest test that settles it on a clean piece. The nepheline question, a different reason for not being a feldspar, is on is nepheline a feldspar; the labradorite, sunstone and amazonite questions are on is labradorite a feldspar, is sunstone a feldspar and is amazonite a feldspar.
On this site, the lithium neighbours are spodumene and lepidolite; the classification questions are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.