Metavariscite

Chemical formula: AlPO<sub>4</sub>·2H<sub>2</sub>O

Metavariscite is a hydrated aluminum phosphate, a rarer polymorph of variscite, most often forming small crystals and crusts.

## Characteristics Metavariscite is a hydrated aluminum phosphate, chemically identical to variscite but crystallizing in a different crystallographic system (it is its polymorph). It occurs much less frequently than variscite. It usually forms very small, tabular or prismatic crystals, which rarely reach sizes visible to the naked eye. It is most commonly found in the form of fine-grained aggregates, crusts, veins, or spherulites (spherical aggregates) with a radial internal structure. Its appearance is often inconspicuous, and identification without advanced methods can be difficult. ## Physical Properties Metavariscite crystals exhibit a vitreous luster, while its aggregates can be dull or earthy. It is a relatively soft mineral, with a hardness of about 3.5 on the Mohs scale. It is brittle and translucent, rarely transparent. Its density is similar to that of variscite. ## Colors and Varieties Metavariscite is most often colorless, white, pale green, or blue-green. Its color is usually delicate and less intense than that of variscite. No named commercial or colored varieties are distinguished. ## History and Name The mineral was first described in 1937 by Duncan McConnell based on material from the Utahlite mine in Lucin, Box Elder County, Utah, USA. The name "metavariscite" refers to its relationship with variscite and its different crystal structure (the prefix "meta-" in mineralogy often denotes a compound with the same chemical composition but a different structure or lower degree of hydration, although in this case the degree of hydration is the same).

Properties

Mohs hardness
3.5
Luster
Vitreous
Streak
White
Density
2.54
Cleavage
Perfect on {010}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identifying metavariscite is difficult and often requires laboratory analysis (e.g., X-ray diffraction) to distinguish it from variscite and other similar phosphates. In the field, its distinguishing feature may be its occurrence as small, radial aggregates or crusts of a light, greenish color in a geological environment typical for aluminum phosphates. ## Distinguishing from Similar Minerals Metavariscite is most often confused with variscite. Macroscopically, they are practically indistinguishable. It can also be confused with other greenish secondary minerals, such as wavellite (which often forms larger spherulites) or faustite. Certain differentiation requires crystallographic studies. ## Crystal Forms It crystallizes in the monoclinic system. It forms small, tabular crystals, often elongated and striated. It most commonly occurs as radial or spherulitic aggregates, as well as massive, earthy, or cryptocrystalline crusts and veins.

Geological environment

## Genesis Metavariscite is a secondary mineral, forming in the oxidation zone of deposits rich in phosphorus and aluminum. It forms as a result of surface waters acting on aluminous and phosphatic rocks, often in voids and fissures. It can also form in caves as a result of reactions between phosphate-rich solutions (e.g., from bat guano) and clay rocks. ## Mineral Associations It co-occurs with minerals such as variscite (with which it often forms intergrowths), wavellite, strengite, phosphosiderite, crandallite, montgomeryite, as well as with clay minerals and iron oxides (limonite). ## Localities Important localities worldwide include Lucin and Fairfield in Utah, USA (type locality); the Tom mine in Yukon Territory, Canada; near Canberra in Australia; and some localities in Germany and Brazil. It is a much rarer and less widespread mineral than variscite.

Rarity

Rare

For collectors

## Quality Criteria As a rare mineral, metavariscite is valued by collectors specializing in mineral systematics or phosphate minerals. The most desirable specimens are those with well-formed, even microscopic, crystals of distinct color. Well-formed spherulites and crusts on a contrasting rock matrix are also attractive. However, certain identification, often confirmed by analysis, is key. ## Popular Localities The most classic and prized specimens come from localities in Utah, USA, especially from the Lucin and Fairfield areas, where this mineral was discovered and where it occurs in association with other rare phosphates.

Care and storage

## Cleaning Metavariscite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always dry the specimen thoroughly at room temperature. ## What to Avoid Avoid contact with chemicals, especially acids and bases, which can damage the mineral. Metavariscite is sensitive to high temperatures, which can lead to its dehydration and destruction of its structure. It should be protected from intense sunlight, which can cause color fading. ## Storage Specimens should be stored in stable conditions, away from sources of heat and moisture. It is best to keep them in sealed boxes or display cases to protect them from dust and mechanical damage. Due to its softness, it should not be stored in contact with harder minerals.

Sources

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