Anorthite is the calcium-aluminium silicate end-member of the plagioclase feldspar solid-solution series, with the idealised formula CaAl₂Si₂O₈. It occupies the calcic extreme of that series, standing opposite albite (NaAlSi₃O₈) at the sodic end, with a continuum of intermediate compositions — oligoclase, andesine, labradorite, and bytownite — filling the space between. In strict mineralogical usage, a plagioclase feldspar is classified as anorthite only when its anorthite component (An) exceeds 90 mol%, making truly end-member material uncommon even in geological settings. Transparent, facetable-quality anorthite is extraordinarily rare, and finished gemstones are essentially collector's curiosities rather than commercial articles. Nevertheless, the mineral holds genuine interest for the advanced gemmologist: it anchors the feldspar taxonomy, appears in some of the most ancient and exotic geological environments on Earth, and has even been recovered from lunar and meteoritic samples.

Crystal System and Physical Properties

Anorthite crystallises in the triclinic system — the name itself derives from the Greek anorthos, meaning "not straight" or "oblique", an allusion to the absence of right-angle axial intersections that characterises the triclinic class. Crystals typically display a tabular or prismatic habit and exhibit the characteristic polysynthetic twinning (albite and pericline laws) common across the plagioclase group, visible as fine parallel striations on cleavage faces — a useful diagnostic feature.

• Hardness (Mohs): 6–6½
• Cleavage: Perfect in two directions (001 and 010), intersecting at approximately 86°, with an additional less-perfect cleavage; the near-right-angle intersection distinguishes plagioclase from orthoclase (90°) and is a classical hand-specimen test
• Fracture: Uneven to conchoidal
• Specific gravity: approximately 2.74–2.76, the highest in the plagioclase series, reflecting the substitution of the heavier calcium ion for sodium
• Refractive indices: α ≈ 1.575, β ≈ 1.583, γ ≈ 1.588; biaxial negative, with a moderate 2V angle
• Birefringence: approximately 0.013
• Lustre: Vitreous on fresh surfaces; pearly on cleavage planes
• Colour: Typically white, grey, or colourless; rarely pale yellow or greenish in gem-quality material

The combination of moderate hardness, perfect cleavage in two directions, and brittle tenacity renders faceted anorthite extremely fragile. Even experienced lapidaries find the material challenging, and finished stones are prone to cleavage-related damage during both cutting and subsequent wear. Anorthite is emphatically not a stone suited to jewellery intended for regular use.

Geological Occurrence

Anorthite is a characteristic mineral of calcium-rich, silica-poor igneous and metamorphic environments. Its principal geological settings include:

• Mafic and ultramafic igneous rocks: Anorthite-rich plagioclase is a defining constituent of gabbros, norites, and troctolites, as well as of the layered intrusions — such as the Bushveld Complex of South Africa and the Stillwater Complex of Montana — where it forms thick cumulate layers known as anorthosite.
• Anorthosite massifs: Large bodies of rock composed predominantly of calcic plagioclase occur in Precambrian cratons worldwide, including the Adirondack Mountains of New York State, the Nain Province of Labrador, and the Rogaland region of Norway. These massifs are the primary source of the labradorite and spectrolite used commercially, though the plagioclase in many such bodies is bytownite or labradorite rather than true anorthite.
• High-grade metamorphic terranes: Anorthite-bearing assemblages (granulites, eclogites) form under conditions of high temperature and pressure in deep crustal settings.
• Skarn deposits: Contact metamorphism of calcareous rocks by silicic intrusions can produce anorthite in calc-silicate assemblages alongside grossular, wollastonite, and diopside.
• Extraterrestrial occurrences: Anorthite is a major constituent of the lunar highlands, forming the pale, heavily cratered terrain visible to the naked eye. Lunar anorthosite returned by the Apollo missions — including the celebrated "Genesis Rock" (Apollo 15, sample 15415) — is composed almost entirely of calcic plagioclase approaching end-member anorthite composition. Anorthite also occurs in calcium-aluminium-rich inclusions (CAIs) within carbonaceous chondrite meteorites, representing some of the oldest solid material in the solar system.

Gem-Quality Material and Localities

Transparent anorthite of facetable quality is among the rarest members of the feldspar family. The most frequently cited source of gem-quality material is Japan, particularly from skarns associated with contact-metamorphic aureoles; colourless to pale yellowish crystals from Japanese localities have been faceted into small collector's stones. Additional occurrences of transparent or translucent material have been documented from Italy (notably from Monte Somma and the Colli Albani volcanic complex in Lazio, where anorthite occurs in calcium-rich ejecta blocks) and from various skarn localities in the former Soviet Union.

Finished faceted anorthite gemstones are seldom encountered even at specialist mineral and gem shows. When they do appear, stones typically weigh well under one carat; examples exceeding a few carats are exceptional and of significant mineralogical interest. Colour is most commonly colourless or pale grey, with faint yellow tints occasionally present. The stones display no notable optical phenomena such as adularescence or labradorescence — those effects are associated with other plagioclase members (moonstone-type orthoclase and oligoclase, and labradorite respectively).

Distinction from Related Feldspars

In hand specimen and even under the loupe, distinguishing anorthite from other calcic plagioclases (bytownite, labradorite) is not straightforward without refractive index measurement or chemical analysis. The key practical indicators are:

• Refractive indices increase progressively from albite to anorthite across the plagioclase series; values above approximately 1.585 (mean) suggest a highly calcic composition.
• Specific gravity similarly increases toward the anorthite end; values approaching 2.76 are consistent with An90+ material.
• Polysynthetic twinning striations are present in all plagioclases and do not discriminate between members.
• Definitive compositional assignment requires electron microprobe analysis or X-ray diffraction, techniques available at gemmological and mineralogical laboratories.

Anorthite should not be confused with anorthoclase, a sodium-potassium feldspar of the alkali feldspar group (not the plagioclase group) that is sometimes encountered as a gem material — notably in the blue-schiller moonstone from Mount Kilimanjaro — and which has a distinctly different chemistry and optical character.

Gemmological and Collector Significance

From a purely commercial standpoint, anorthite occupies a negligible position in the gem trade. Its fragility, lack of optical phenomena, and restricted palette of colours offer little competitive advantage against more durable or visually compelling gem minerals. Its significance is instead mineralogical and taxonomic: as the calcic anchor of the plagioclase series, it provides the reference point against which all intermediate compositions are measured and named. For the systematic collector of feldspar gems — a group that might assemble specimens of orthoclase, sanidine, adularia moonstone, oligoclase sunstone, labradorite, and peristerite — a faceted anorthite represents the logical and satisfying completion of the plagioclase sequence. Its extraterrestrial occurrences lend it an additional dimension of interest that few gem minerals can claim.

No significant treatments are applied to anorthite, nor would any be commercially warranted. The material is assessed and traded strictly on the basis of natural clarity and crystal quality.

Further Reading

• GIA Gems & Gemology — Feldspar Gems
• International Gem Society — Feldspar Gemstone Information