Overview

Anhydrite (CaSO4) is the water-free counterpart of gypsum and a classic evaporite mineral. It crystallizes in the orthorhombic system and is best known for its perfect cleavage in three directions at right angles, yielding crisp rectangular fragments and blocky crystals. Colors range from colorless and white to bluish-gray, lilac, and pale pink; translucent to opaque habits predominate, though rare crystals can be surprisingly glassy. In pockets, anhydrite may appear as thick blades, stout prismatic blocks, rosettes, or radiating aggregates; in evaporite beds it commonly forms nodules or “chicken-wire” textures.

Collectors value anhydrite for its sharp form, color (notably soft blues known commercially as “angelite” when massive), and dramatic associations—especially with fluorite, calcite, sphalerite, and galena in Mississippi Valley-type (MVT) mines, and with halite/sylvite in potash deposits. A key challenge is stability: anhydrite is metastable at Earth-surface conditions and can slowly hydrate to gypsum with moisture exposure, altering luster and form. Fresh, unaltered, well-crystallized pieces from classic pockets are therefore both uncommon and prized.

Popularity

While not as ubiquitous in showcases as quartz or fluorite, anhydrite occupies a respected niche. Historic potash mines like Staßfurt (Germany) and Boulby (UK) supplied textbook material to museums. Mexico’s Naica Mine produced striking blocky crystals and important gypsum-after-anhydrite pseudomorphs that captured broad attention. Tennessee’s Elmwood district occasionally yielded attractive lavender-gray to blue-gray blades and blocks on sphalerite or calcite, making for aesthetic, contrasting specimens.

Massive blue anhydrite (“angelite”) from Peru popularized the species with lapidary enthusiasts in the late 20th century, indirectly increasing interest among mineral collectors. Today, choice anhydrite is still comparatively scarce in top specimen quality. The best pieces—sharp, lustrous crystals on contrasting matrix with minimal hydration—are genuinely hard to find and can command strong prices relative to the species’ general availability.

Top Collecting Localities

Below are several historically and currently important sources of collectible anhydrite, each known for distinct habits or associations.

Naica Mine, Mexico

Naica (Chihuahua) is famed for its world-class gypsum crystals, but it also produced excellent anhydrite. Specimens range from thick, blocky, translucent plates to coarse blades, often perched with calcite or set among sulfides. Many outstanding Naica specimens show alteration rims or partial pseudomorphs of gypsum after anhydrite, revealing sharp, geometric outlines with softer gypsum textures—a fascinating paragenetic story in one piece. Truly fresh anhydrite surfaces from Naica, with a satiny to vitreous luster, are scarce and coveted.

Collector’s Guide

Evaluating Specimen Quality

• Color and Transparency: Most anhydrite is opaque to translucent, with colorless, white, gray, bluish-gray, lilac, and pale pink tones. Subtle, even color with a light translucency is attractive; soft blues and lavenders are particularly prized. Strong internal alteration (hydration to gypsum) can turn surfaces chalky or patchy—fresh, uniform color is preferred.
• Luster and Surface Freshness: The best pieces show a satiny to vitreous sheen on cleavage faces and terminations. Dull, powdery surfaces often indicate hydration or handling wear. Fresh, crisp faces elevate a specimen substantially.
• Crystal Form: Look for blocky orthorhombic crystals, thick blades, or rosette-like aggregates with well-defined edges. Sharp, right-angle cleavage intersections are characteristic and desirable. Nodular “chicken-wire” textures and textural replacements (e.g., gypsum after anhydrite) can be interesting scientifically, but sharp free-standing crystals are generally more collectible.
• Condition: The mineral’s perfect cleavage makes it fragile. Chips on edges are common; prioritize intact corners and edges, as damage is conspicuous. Because anhydrite can hydrate to gypsum, inspect for alteration rims, powdery bloom, or soft, uneven surfaces—minor, stable alteration is acceptable, wholesale conversion is not.
• Associations and Aesthetics: Matrix pieces with contrasting species markedly increase appeal—think gray-blue anhydrite blades alongside purple fluorite or honey sphalerite (Elmwood), or blocky crystals amidst calcite (Naica). In evaporite assemblages, attractive intergrowths with halite, sylvite, or polyhalite are desirable, especially when the composition is balanced and displays clearly from a front angle.
• Size: While large chunks exist, fine, undamaged, display-worthy crystals are uncommon. Quality trumps size—smaller, sharp blocky crystals with fresh luster can be far more desirable than large but altered masses.

Detecting Repairs or Treatments

• Repairs: Due to perfect cleavage, reattachments are not uncommon. Examine edges and planar breaks carefully with a loupe for ultra-straight, glossy glue lines, small offsets, or epoxy seep. Check where crystals meet matrix—look for adhesive shine or color mismatch.
• Hydration/Alteration: The most prevalent “change” is natural hydration to gypsum. Signs include powdery white bloom, softening of sharp edges, translucency change, or fibrous gypsum overgrowths. Partial pseudomorphs can be both natural and collectible, but ongoing, active hydration is a red flag for stability.
• Coatings/Sealants: Some preparators may apply consolidants to inhibit hydration or enhance luster. Look for uneven gloss, tackiness, or dust trapped in a film. Such treatments should be disclosed; heavy coatings are generally discouraged.
• Fakes: Laboratory-grown anhydrite crystals are not an issue in the specimen market. Be more concerned with mislabeling (e.g., gypsum sold as anhydrite or vice versa). A quick test is cleavage and density; also, anhydrite does not scratch as easily as gypsum (gypsum is Mohs 2, anhydrite ~3–3.5).

Care and Storage

Anhydrite is inherently moisture-sensitive and may hydrate to gypsum over time. Proper environmental control is essential.

• Humidity Control: Keep anhydrite in a dry environment (ideally 30–45% RH). Use closed display cases with desiccant (silica gel) and a hygrometer. Avoid basements, bathrooms, and coastal humidity. Rotate or recharge desiccants regularly.
• Avoid Water and Wet Cleaning: Never soak or rinse anhydrite. Clean gently with a soft, dry brush, bulb blower, or carefully with slightly dampened swabs only if absolutely necessary—and dry immediately. Do not use ultrasonic cleaners, steam, acids, or aggressive chemicals.
• Temperature and Light: Normal room temperatures are fine. Avoid prolonged heat or strong sunlight that could accelerate microfracturing or drive humidity cycles in a case. LEDs are preferred over hot halogens.
• Handling: Support specimens from beneath; avoid gripping across edges or corners due to perfect cleavage. For transport, use soft foam and immobilize the piece to prevent vibration damage.
• Mounting and Display: Use minimal, inert mineral tack and ensure it does not trap moisture against the specimen. For matrix pieces, consider stands that allow air circulation. If a specimen shows ongoing hydration (new powdery spots), move it to a drier environment immediately.
• Long-Term Stability: Even well-kept anhydrite can slowly change over decades. Inspect annually for surface bloom or softness. If detected early, improved humidity control often halts or slows further alteration.

By prioritizing dryness, minimal handling, and gentle cleaning, collectors can preserve the crisp edges, fresh luster, and distinctive forms that make anhydrite a rewarding, if challenging, species to own.