Oligoclase
Oligoclase is plagioclase feldspar, An10 to An30: triclinic, hardness 6 to 6.5, density 2.63 to 2.66. Why a balance and a refractometer can take it for quartz.
Oligoclase is the plagioclase feldspar between An10 and An30: triclinic, hardness 6 to 6.5, density 2.63 to 2.66, with no row of its own on the IMA list. Gem oligoclase can pass for quartz: a 20.00 g Hawk mine piece reads 12.46 g in water, quartz 12.45 g. Biaxial optics and two cleavages separate them.
In short
- A composition band, not a species. The Handbook of Mineralogy sheet gives Na0.9–0.7Ca0.1–0.3Al1.1–1.3Si2.9–2.7O8, feldspar group, plagioclase series: An10 to An30. A case-insensitive scan of every line of the September 2026 IMA-CNMNC Master List finds 0 containing oligoclase; albite and anorthite, the series ends, have rows.
- The Hawk mine trap, a documented failure. The International Gem Society reports facetable oligoclase from the Hawk mine, Bakersville, North Carolina, with indices 1.537 to 1.547 and density 2.651. A 20.00 g piece of it reads 12.46 g in water; quartz, at 2.65 on its sheet, reads 12.45 g. The balance cannot separate them, and quartz’s ω of 1.544 falls inside that index range.
- What does separate them. Oligoclase is biaxial and quartz uniaxial (+), on their sheets. Oligoclase has perfect {001} and less perfect {010} cleavage meeting at about 94°; quartz’s cleavage is rarely observable. Hardness, 6 to 6.5 against 7, is for rough only.
- Oligoclase is a sunstone feldspar. GIA’s Gems and Gemology review of phenomenal gems (Summer 2025) calls sodic sunstone mostly oligoclase, with Tvedestrand, Norway as the most studied example; Tvedestrand is on the oligoclase sheet’s Distribution. GIA’s sunstone facts give oligoclase sunstone 1.537 to 1.547 and SG 2.65.
- Two optic signs, one name. The sheet prints biaxial (+) or (−), with 2V from 84° through −87° to 90° in the low form, so the sign alone cannot name it. The International Gem Society’s plagioclase table prints (−) only. Both are printed here; neither is chosen.
| Material, source | Optical class | Refractive indices | Hardness | Density | 20.00 g piece in water | Cleavage |
|---|---|---|---|---|---|---|
| Oligoclase, Handbook sheet | Biaxial (+) or (−) | α 1.533 to γ 1.552 | 6 to 6.5 | 2.63 to 2.66 | 12.40 to 12.48 g | Perfect {001}, less perfect {010}, about 94° |
| Oligoclase, Hawk mine, North Carolina (IGS) | Not stated | 1.537 to 1.547 | Not stated | 2.651 | 12.46 g | Not stated |
| Oligoclase, Kenya (IGS) | Not stated | α 1.538 to γ 1.550 | Not stated | 2.64 | 12.42 g | Not stated |
| Oligoclase sunstone (GIA) | Not stated | 1.537 to 1.547 | 6.0 to 7.2 (all sunstone) | 2.65 | 12.45 g | Not stated |
| Quartz, Handbook sheet | Uniaxial (+) | ω 1.544, ε 1.553 | 7 | 2.65; 2.59 to 2.63 massive | 12.45 g; 12.28 to 12.40 g massive | Rarely observable, poor |
| Albite, Handbook sheet (An0 to An10) | Biaxial (+) low, (−) high | α 1.526 to γ 1.541 | 6 to 6.5 | 2.60 to 2.65 | 12.31 to 12.45 g | Perfect {001}, very good {010} |
| Andesine, Handbook sheet (An30 to An50) | Biaxial (+) | α 1.545 to γ 1.563 | 6 to 6.5 | 2.66 to 2.68 | 12.48 to 12.54 g | Perfect {001}, less perfect {010} |
What is oligoclase? The An10 to An30 plagioclase
Oligoclase is the name for a plagioclase feldspar that is 10 to 30 per cent anorthite. The Handbook sheet’s formula, Na0.9–0.7Ca0.1–0.3Al1.1–1.3Si2.9–2.7O8, says exactly that when read as An = 100 Ca / (Ca + Na). Its two ideal columns, Na0.90Ca0.10 and Na0.70Ca0.30, recompute on standard atomic weights to within 0.01 of the printed oxide figures: 66.05 SiO2, 21.26 Al2O3, 2.13 CaO and 10.57 Na2O at An10; 60.75, 24.82, 6.30 and 8.12 at An30. Below An10 the sheet name is albite; above An30, andesine.
What the IMA list records. The September 2026 IMA-CNMNC Master List was converted with pdftotext -layout and every line searched case-insensitively: 0 lines contain oligoclase, andesine, labradorite, plagioclase or feldspar. Albite, Na(AlSi3O8), G, 1815, Sweden, and anorthite, Ca(Al2Si2O8), G, 1823, Italy, have rows. The International Gem Society says faceted plagioclase is often identified only as a feldspar of the series, and that pinning down the name takes optical and X-ray analysis. How an analysis is checked against the band, worked end to end, is on is labradorite a feldspar; that page’s check puts the sheet’s Monteagle, Ontario analysis at An 12.5.
What the sheet records. Triclinic, point group -1; crystals uncommon, flattened and tabular along [010], to 6 cm; more commonly cleavable to compact or massive; typically twinned by the Albite, Carlsbad and Pericline laws. Cleavage perfect on {001}, less perfect on {010}, imperfect on {110}, with (001) and (010) meeting at about 94°. Hardness 6 to 6.5; density 2.63 to 2.66. White, tinted grey, green or red, and may be opalescent or iridescent; streak white; vitreous to pearly. It is common in granite, syenite, nepheline syenite and their pegmatites, in diorite, rhyolite and andesite, and in amphibolite-facies rocks, with quartz, orthoclase, sanidine, tourmaline and corundum. What a hand specimen can be labelled is on how to identify feldspar.
Is oligoclase quartz? The Hawk mine trap
The Hawk mine trap: a 20.00 g piece of gem oligoclase from the Hawk mine reads 12.46 g in water and a 20.00 g piece of quartz 12.45 g, so a balance cannot tell them apart. The International Gem Society oligoclase article reports colourless to pale green facetable crystals from the Hawk mine, Bakersville, North Carolina, with indices 1.537 to 1.547 and density 2.651. Worked as 20 − 20/D, that is 12.46 g; the quartz sheet’s 2.65 gives 12.45 g, a difference of 0.003 g before rounding. The oligoclase sheet’s whole range, 2.63 to 2.66, reads 12.40 to 12.48 g and contains quartz. The method is on how to measure specific gravity at home.
The refractometer does no better on one reading. The quartz sheet prints ω 1.544 and ε 1.553; the oligoclase sheet prints α 1.533 to γ 1.552. Quartz’s ω lies inside the Hawk mine range of 1.537 to 1.547, and quartz’s ε sits 0.001 above oligoclase’s highest γ. A spot reading near 1.54 to 1.55 fits both minerals.
What decides it. Optical character: quartz is uniaxial (+), and oligoclase biaxial, so a polariscope with a conoscope, or a refractometer read on several faces, separates them. On rough, the cleavage decides: oligoclase breaks along two cleavages at about 94°, and the quartz sheet calls quartz cleavage rarely observable, with a conchoidal fracture. Hardness, 6 to 6.5 against 7, damages the piece and is for rough only: how to test mineral hardness properly. The species page is quartz.
Oligoclase sunstone: Tvedestrand and hematite
Oligoclase is one of the feldspars sold as sunstone, and GIA’s review places the most studied examples at Tvedestrand, Norway, a locality on the Handbook oligoclase sheet. GIA’s Gems and Gemology review of phenomenal gems (Summer 2025) says hematite sunstones are mostly found in granites or pegmatites and divide into orthoclase and sodic plagioclase, oligoclase or albite; it calls sodic sunstone mostly oligoclase, and the examples from Tvedestrand the most studied. The oligoclase sheet’s Distribution names Tvedestrand, with Arendal and Tromöy Island, among its Norwegian localities.
The figures. GIA’s sunstone facts give oligoclase sunstone refractive indices of 1.537 to 1.547 and specific gravity 2.65: 12.45 g in water for 20.00 g, the quartz figure again. The International Gem Society’s sunstone article says sunstone may be oligoclase or labradorite. The same review says thin-film and platy iron-oxide inclusions are primarily found in sodic plagioclase and alkali feldspars, naming oligoclase, albite and orthoclase.
The sources disagree on Tvedestrand. The International Gem Society’s orthoclase article lists Tvedestrand for orthoclase sunstone. A locality name on a sunstone label therefore does not name the feldspar; the reasoning, and the three refractometer bands GIA prints, are on is sunstone a feldspar. The Handbook sheet itself does not use the word sunstone, by script.
Peristerite, moonstone and the meaning of the name
Peristerite is mostly albite with a little oligoclase, not oligoclase itself. GIA’s 2025 review defines peristerites as sodic plagioclases with bulk compositions between An2 and An16 and exsolution textures, in which low albite near An0 is the dominant phase, with intergrowths of minor oligoclase near An20. It says peristerite is nearly always misidentified as moonstone. Moonstone itself is on is moonstone a mineral, and the sheet’s note that oligoclase may be opalescent or iridescent is as far as the Handbook goes.
The name check. The sheet derives oligoclase from the Greek for little and fracture, for cleavages believed inferior to albite; the International Gem Society gives the same derivation. The two sheets bear the wording out: albite’s {010} cleavage is very good, oligoclase’s less perfect. Both print perfect {001}. Hardness, 6 to 6.5, is identical.
Not orthoclase. The International Gem Society warns readers to distinguish orthoclase, a species, from oligoclase, which is in the plagioclase series. Orthoclase is KAlSi3O8, monoclinic; the alkali feldspars are on orthoclase and microcline.
Where oligoclase is found
The Distribution field calls oligoclase a widespread mineral and names fine material from Danviken, near Stockholm, Sweden; Arendal, Tromöy Island at Alve, Tvedestrand and elsewhere in Norway; Ytterby, Sweden; Kemiö (Kimito) Island, Finland; Silberberg, near Bodenmais, Bavaria, Germany; Tenerife, Canary Islands; Fine and Macomb, St. Lawrence Co., New York; Corundum Hill, Chester Co., and Media, Delaware Co., Pennsylvania; near Hawk, east of Bakersville, Mitchell Co., North Carolina; and Lake Harbour, Baffin Island, Canada.
Gem material, cross-checked. The International Gem Society’s gem localities agree with the sheet twice: the Hawk mine near Bakersville, North Carolina, and Baffin Island, which it says has yielded cut gems up to about 5 carats. Its third, colourless grains from Kenya with indices α 1.538 to 1.540 and γ 1.549 to 1.550 and SG 2.64, is not on the sheet, and we add no Kenyan locality to it. No British locality is on the oligoclase sheet.
Related pages. The calcic neighbour two names up the series is labradorite. 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.