Anorthite: Meaning, Properties & Symbolism

Anorthite is the calcium-rich end member of the plagioclase feldspar series, with the ideal composition CaAl₂Si₂O₈. Its most distinctive specimens are white, colorless or reddish triclinic crystals set in dark volcanic matrix, while transparent faceting material is exceptionally rare.

Anorthite at a Glance

Mindat classifies anorthite as the calcium-rich end member of the albite–anorthite plagioclase series and records Mount Somma in Campania, Italy, as its type locality.

What Is Anorthite?

Anorthite is a framework silicate built from linked silicon-oxygen and aluminum-oxygen tetrahedra, with calcium balancing the structural charge.

In strict mineralogical usage, anorthite represents plagioclase compositions containing approximately 90–100% of the calcium-rich end-member component. Pure CaAl₂Si₂O₈ is uncommon because natural plagioclase normally contains at least some sodium.

The opposite end member is Albite, which is sodium-rich. Intermediate plagioclase compositions include oligoclase, andesine, labradorite and bytownite.

The wider classification is explained in the Feldspar guide.

How Anorthite Forms

Anorthite-rich plagioclase crystallizes in calcium-rich, silica-poor igneous systems. It is particularly associated with basalt, gabbro and other mafic rocks containing pyroxene, olivine, magnetite or spinel.

It also occurs in granulite-grade metamorphic rocks, metamorphosed carbonate rocks and some corundum-bearing deposits.

Because calcium-rich feldspar weathers comparatively rapidly at Earth’s surface, fresh well-formed crystals are less common in exposed rocks than its abundance in primary igneous systems might suggest.

Anorthite Meaning and Symbolism

Traditional symbolism associates anorthite with foundations, adaptation and understanding how individual components support a larger structure. Its position at one end of a continuous mineral series is sometimes interpreted as a symbol of clear identity within gradual change.

These metaphysical associations are traditional beliefs rather than scientifically demonstrated emotional, psychological or medical effects.

A specimen can still serve as a personal reminder to examine the underlying structure of a project or understand how one part of a system affects the whole.

Anorthite and the Plagioclase Series

Plagioclase is not one fixed composition. It is a continuous solid-solution series ranging from sodium-rich albite to calcium-rich anorthite.

Mineralogists commonly express a plagioclase composition using an “An” number. An₉₅, for example, indicates a composition containing approximately 95% anorthite component and 5% albite component.

This system matters because many crystals marketed simply as anorthite may actually be highly calcium-rich plagioclase rather than chemically pure An₁₀₀.

Labradorite occupies an intermediate portion of this series. Its familiar iridescence is not a standard feature of pure anorthite.

Important Anorthite Localities

Mount Somma and Vesuvius, Italy

Mount Somma, part of the Somma–Vesuvius volcanic complex near Naples, is the type locality and one of the classic sources of collectible anorthite crystals.

Specimens commonly contain colorless or white crystals in cavities with augite, spinel and other volcanic minerals. Antique examples with institutional or historic labels can carry provenance value beyond their visual quality.

Miyake-jima, Japan

Miyake-jima in the Izu Archipelago has produced white to transparent anorthite crystals partly coated or enclosed by dark volcanic glass. The sharp contrast gives these specimens a distinctive appearance.

A 2025 mineralogical study described Miyake-jima anorthite as a useful analogue for calcium-rich plagioclase in the lunar crust.

Great Sitkin Island, Alaska

Great Sitkin Island is notable for rare transparent pale yellow material suitable for faceting. The International Gem Society reports that cut stones up to approximately eight carats have been obtained from this locality, unusually large for anorthite.

Other Sources

Additional documented occurrences include the Monzoni–Vallaccia Mountains in Trentino, Italy; Greenland; Finland; India; Sweden; California; Colorado and Nevada.

Locality should be supported by a specimen label or credible dealer documentation, because ordinary white plagioclase cannot be assigned an origin visually.

Anorthite Beyond Earth

Anorthite-rich plagioclase is a major component of lunar highland rocks. The bright lunar crust contains abundant ferroan anorthosite, a rock dominated by calcium-rich plagioclase.

Anorthite has also been reported in meteoritic materials and calcium-aluminum-rich inclusions. These extraterrestrial occurrences make it important in planetary geology even though terrestrial gem use is limited.

A commercial “moon anorthite” claim should not imply that the item came from the Moon. Genuine lunar material has strict provenance and legal considerations and is not equivalent to ordinary Earth-mined anorthite.

Is Anorthite a Gemstone?

Transparent anorthite is facetable, but it is among the rarest plagioclase gems. The mineral is widespread in rocks, yet clean transparent crystals large enough to cut are seldom found.

Faceted examples are commonly:

• Colorless, white or pale yellow.
• Below one carat.
• Cut as small squares, ovals, cushions or step cuts.
• Purchased by rare-species collectors rather than conventional jewelry buyers.

A documented Japanese example weighed only 0.23 carat, illustrating the small size of many faceted stones. Great Sitkin material is exceptional because much larger gems have been reported.

Hardness of 6–6.5 provides moderate scratch resistance, but multiple cleavage directions make anorthite vulnerable to sharp impact. Pendants, earrings and collector display settings are safer than exposed rings.

How Anorthite Is Sold

Most anorthite reaches the collector market as mineral specimens.

Typical formats include:

• Small white crystals in basalt cavities.
• Mount Somma specimens associated with augite and spinel.
• Miyake-jima crystals contrasted against black volcanic glass.
• Plagioclase-rich rock samples used for teaching or research.
• Rare transparent crystals and faceted species gems.

Anorthosite, a rock dominated by plagioclase, is not automatically pure anorthite. Polished anorthosite decor items should not be marketed as faceted anorthite gemstones.

Anorthite Price and Value

Indicative July 2026 retail asking-price ranges are:

The specimen market is thin, and comparable prices are difficult to establish. Current dealer records show anorthite sold as locality specimens and associations rather than by gram or carat, including Mount Vesuvius material with spinel and augite.

Faceted prices are especially variable because stones are extremely rare and usually small. Species collectors may pay a premium for confirmed identity, transparency and documented locality, while unidentified white feldspar has little comparable value.

How to Identify Anorthite

Anorthite cannot usually be identified by color alone. White, gray and colorless feldspars overlap visually across the plagioclase and potassium-feldspar groups.

Useful clues include:

• Triclinic crystals with fine striations.
• Hardness around 6–6.5.
• Two prominent cleavage directions.
• Higher refractive index and density than albite.
• Occurrence with mafic volcanic minerals.
• Calcium-rich composition confirmed by analysis.

Accurate separation from bytownite, labradorite and other plagioclase commonly requires refractive-index testing, optical microscopy, electron-microprobe analysis, Raman spectroscopy or X-ray diffraction.

Begin with the general process in How to Identify Crystals.

Anorthite vs. Albite

Anorthite is calcium-rich, whereas albite is sodium-rich. Their ideal formulas and physical properties differ, but natural crystals commonly contain a mixture of both end-member components.

Anorthite generally has greater density and higher refractive indices. Albite is more common in granites and pegmatites, while anorthite-rich plagioclase is more characteristic of mafic volcanic and plutonic rocks.

A visual label of “white feldspar” is insufficient to distinguish them.

Anorthite vs. Labradorite

Labradorite occupies an intermediate calcium-sodium range within the plagioclase series. It is widely used as a gemstone because some material displays labradorescence.

Anorthite normally lacks that broad blue, green or gold optical flash. Transparent faceting-grade anorthite is also much rarer than commercial labradorite cabochon material.

A feldspar with strong iridescence should not be called anorthite solely because it contains a high calcium component.

Anorthite Hardness and Durability

Anorthite has a Mohs hardness of 6–6.5. It is harder than calcite and fluorite but softer than quartz, topaz and corundum.

Perfect and good cleavage directions create structural weakness. A hard blow can separate the stone along a smooth plane even when the surface remains relatively unscratched.

The distinction between scratch resistance and breakage is explained in Gemstone Cleavage Explained.

How to Clean Anorthite

Clean stable loose stones with lukewarm water, mild soap and a soft brush. Rinse briefly and dry without applying pressure to facet junctions or crystal edges.

Avoid ultrasonic and steam cleaning because cleavage and hidden fractures may respond poorly to vibration or rapid heating. The International Gem Society similarly recommends gentle manual cleaning for anorthite.

Matrix specimens require separate evaluation because associated volcanic glass, calcite, spinel or altered rock may have different care needs.

How to Store Anorthite

Store faceted stones individually so that quartz, sapphire and diamond cannot scratch them. Protect sharp corners on square or step-cut gems.

Support mineral specimens from beneath the matrix. Do not hold a volcanic specimen by projecting feldspar crystals or allow dark matrix fragments to rub against their faces.

Preserve locality and collection labels. For a rare species such as anorthite, documentation can be as important as visual appearance.

Buying Anorthite

Ask whether the identification is based on laboratory analysis or only on appearance. This is especially important for loose faceted stones and white plagioclase crystals without matrix.

A seller should disclose:

• Exact locality.
• Whether the specimen has been repaired.
• Whether the material is anorthite-rich plagioclase or confirmed near-end-member anorthite.
• Treatments or coatings.
• Laboratory documentation for expensive faceted stones.

Beware of anorthosite, ordinary feldspar or pale labradorite being marketed as rare transparent anorthite without supporting evidence.

Frequently Asked Questions

Is anorthite a feldspar?

Yes. It is the calcium-rich end member of the plagioclase feldspar series.

Is anorthite rare?

It is widespread as a rock-forming component, but transparent crystals and faceted gemstones are extremely rare.

Is anorthite found on the Moon?

Anorthite-rich plagioclase is a major constituent of lunar highland anorthosite. Ordinary commercial specimens, however, are Earth-mined unless exceptional provenance proves otherwise.

Is anorthite the same as anorthosite?

No. Anorthite is a mineral, while anorthosite is a rock composed predominantly of plagioclase feldspar.

Can anorthite be faceted?

Yes. Transparent material can be faceted, but most stones are small and collected for mineral rarity.

Is anorthite suitable for rings?

A protected occasional-wear ring is possible, but cleavage makes earrings and pendants safer.

Find it among other A minerals in Crystals That Start With A or browse the Gemstone Guides collection.