Is labradorite a feldspar?
Yes. Labradorite is plagioclase feldspar, An50 to An70, hardness 6 to 6.5. Worked from its Handbook analysis, San Marcos labradorite is An 62.7. No IMA row.
Yes. Labradorite is a feldspar: the Handbook files it in the feldspar group, plagioclase series, hardness 6 to 6.5, and GIA defines it as plagioclase between An50 and An70. It is not an IMA species, though; the list has rows only for albite and anorthite, the series ends. The sheet's San Marcos analysis works out at An 62.7.
In short
- Yes, on every reference read for this page. The Handbook of Mineralogy sheet says feldspar group, plagioclase series. GIA’s 2025 review of phenomenal gems says labradorite without modification is any plagioclase between An50 and An70. The International Gem Society gives its formula as a variable composition of NaAlSi3O8 and CaAl2Si2O8, the albite and anorthite ends.
- The An check, worked. An = 100 × Ca / (Ca + Na + K). The labradorite sheet’s analysis from the San Marcos Mountains, California, prints CaO 12.59, Na2O 3.97, K2O 0.26 weight per cent. Per 100 g: Ca = 12.59 / 56.077 = 0.2245 mol; Na = 2 × 3.97 / 61.979 = 0.1281; K = 2 × 0.26 / 94.195 = 0.0055. An = 22.45 / 0.3581 = 62.7, inside An50 to An70.
- Six of six. Run on the first analysis printed on each of the six Handbook plagioclase sheets, the same arithmetic gives albite An 1.0, oligoclase 12.5, andesine 40.9, labradorite 62.7, bytownite 76.0 and anorthite 91.9: each inside its own sheet’s band. The controls, the sheets’ ideal end-member columns, return exactly 50.0, 70.0, 10.0 and 90.0.
- Why aluminium rises with calcium. Eight oxygens carry 16 negative charges. NaAlSi3O8 balances them as 1 + 3 + 12; CaAl2Si2O8 as 2 + 6 + 8. Every calcium that replaces a sodium must bring an aluminium in place of a silicon, so Al = 1 + Ca: the sheet’s Al1.5 to Al1.7 matches its Ca0.5 to Ca0.7. A formula that breaks this is a typo or a bad analysis.
- A feldspar, but not a species. The September 2026 IMA-CNMNC Master List has no line containing labradorite, plagioclase or even feldspar: it lists species, and those are series and group names. A formula match does not make a feldspar either: Ca(Al2Si2O8) has three rows, and the anorthite sheet calls anorthite trimorphous with dmisteinbergite and svyatoslavite.
| Sheet | Analysis (1), locality | CaO | Na2O | K2O | An, worked | Band from the sheet formula | Inside its band? |
|---|---|---|---|---|---|---|---|
| Albite | Alp Rischuna, Switzerland | 0.22 | 11.72 | 0.03 | 1.0 | An0 to An10 | Yes |
| Oligoclase | Monteagle, Ontario, Canada | 2.64 | 9.72 | 0.68 | 12.5 | An10 to An30 | Yes |
| Andesine | Beaver Bay, Lake Co., Minnesota, USA | 8.45 | 6.39 | 0.55 | 40.9 | An30 to An50 | Yes |
| Labradorite | San Marcos Mountains, California, USA | 12.59 | 3.97 | 0.26 | 62.7 | An50 to An70 | Yes |
| Bytownite | Rustenburg mine, Transvaal, South Africa | 15.38 | 2.57 | 0.17 | 76.0 | An70 to An90 | Yes |
| Anorthite | Crookdene, Northumberland, England | 18.28 | 0.82 | 0.11 | 91.9 | An90 to An100 | Yes |
Is labradorite a feldspar? The definition in one sentence
Labradorite is a plagioclase feldspar whose composition lies between 50 and 70 per cent anorthite. The Handbook of Mineralogy labradorite sheet files it in the feldspar group, plagioclase series, with the formula Na0.5-0.3Ca0.5-0.7Al1.5-1.7Si2.5-2.3O8, triclinic, hardness 6 to 6.5, measured density 2.68 to 2.72. Jin, Renfro, Palke and Shigley, Gems & Gemology, Summer 2025 defines labradorite, without modification, as any plagioclase between An50 and An70. The species page, labradorite, carries the full sheet and localities.
The boundary: feldspars that are not labradorite. Below An50 the sheet name is andesine, An30 to An50; above An70 it is bytownite, An70 to An90. Peristerite, which GIA describes as sodic plagioclase between An2 and An16, is a feldspar at the other end of the series. Moonstone is an alkali feldspar, orthoclase layered with albite in GIA’s description, and the comparison is on is labradorite a moonstone. The iridescent feldspar in larvikite, which GIA says is often misidentified as labradorite, is anorthoclase, filed by its Handbook sheet in the alkali feldspar group: a feldspar, but not a plagioclase.
And things that are not feldspar at all. GIA names Nuummite as an iridescent amphibolite that looks very similar to labradorite. An amphibolite is a rock of amphibole, not feldspar. How to separate the look-alikes in the hand is on how to tell if labradorite is real, and is not repeated here.
How to check a labradorite analysis: the An check, worked
An analysis names a plagioclase; its number is An = 100 × Ca / (Ca + Na + K). Convert each oxide weight per cent to moles of the metal: CaO by 56.077 g/mol; Na2O by 61.979 and multiply by 2, for two sodium atoms; K2O by 94.195, times 2. On the San Marcos Mountains analysis printed on the labradorite sheet that is Ca 0.2245, Na 0.1281 and K 0.0055 mol per 100 g, and An = 100 × 0.2245 / 0.3581 = 62.7. Leave potassium out and it reads 63.7, still inside the band. Substitute your own label’s analysis in the same three divisions.
Method and controls. The arithmetic was scripted on standard atomic weights and run on the first analysis printed on each of the six plagioclase sheets, from albite to anorthite. All six land inside their own sheet’s band, as the table shows. As positive controls the same script was run on the sheets’ ideal end-member columns: labradorite’s Na0.50Ca0.50 and Na0.30Ca0.70 columns give exactly An 50.0 and 70.0, albite’s Na0.90Ca0.10 gives 10.0 and bytownite’s Na0.10Ca0.90 gives 90.0. The International Gem Society’s Oregon labradorite, quoted as Ab32An68, is already in this form: An 68.
The charge check, stated once. Eight oxygens hold 16 negative charges, so the cations must total +16: Na + Al + 3Si is 1 + 3 + 12, and Ca + 2Al + 2Si is 2 + 6 + 8. Calcium for sodium is always coupled with aluminium for silicon, so in any plagioclase formula Al = 1 + Ca and Si = 3 − Ca. The labradorite sheet’s Al1.5-1.7 and Si2.5-2.3 pass. What weight in water can and cannot do across the six names is on the labradorite page.
Is labradorite a mineral species? What the IMA list records
Labradorite has no row on the IMA list, and neither does the word feldspar. The September 2026 IMA-CNMNC Master List was converted with pdftotext -layout and every line searched case-insensitively: 0 lines contain labradorite, oligoclase, andesine, bytownite, plagioclase, feldspar or anorthoclase. The list gives species, and these are series, group and composition names. Albite, Na(AlSi3O8), G, 1815, Sweden, and anorthite, Ca(Al2Si2O8), G, 1823, Italy, are the plagioclase rows; orthoclase, microcline and sanidine, all K(AlSi3O8), are the potassium ones.
Documented failure: matching the formula is not enough. A row parser over the list finds Ca(Al2Si2O8) three times, as anorthite, dmisteinbergite and svyatoslavite, and the anorthite sheet calls anorthite trimorphous with the other two. K(AlSi3O8) appears four times and Na(AlSi3O8) twice. A formula of the feldspar shape is a starting point; the structure, which the Handbook records as feldspar group, is what makes the feldspar.
So the label reads two ways, both correct. Feldspar, or plagioclase, is a sound determination; labradorite is a sound name for a plagioclase of An50 to An70. What a hand specimen can carry without an analysis is the ladder on how to identify feldspar. How the IMA list treats other common names is on is quartz a mineral.
When the answer changes: andesine that flashes, and rocks sold as labradorite
A flash does not certify the An number. GIA’s 2025 review puts the bulk composition of iridescent labradorites between An46 and An60, each give or take 2. On the sheet ladder and on GIA’s own An50 line, the lower end of that range is andesine. A stone sold as labradorite on the strength of its flash is a plagioclase feldspar either way; whether it is labradorite by number takes an analysis.
Rock, not crystal. GIA says anorthosite composed of iridescent labradorite is used for high-end countertops, and the gem society describes dark igneous building stone in which labradorite grains flash at many angles. A slab of that is a rock containing labradorite. Larvikite, whose iridescent feldspar GIA identifies as anorthoclase, is a rock of alkali feldspar. Both are feldspar-bearing rocks; only the first contains labradorite.
Sunstone. GIA says most Oregon sunstones are labradorites; the gem society says sunstone is also characteristic of oligoclase. So the name sunstone does not fix the feldspar, and neither does the name moonstone, which is on is moonstone a mineral. 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.