Evansite

Chemical formula: Al₃PO₄(OH)₆·8H₂O

Evansite is a rare, amorphous aluminum phosphate, forming vitreous, spherical or botryoidal aggregates, ranging in color from colorless to greenish-blue.

## Characteristics Evansite is a hydrated aluminum phosphate, which, due to its lack of an ordered internal structure, is classified as a mineraloid rather than a mineral in the strict sense. It occurs as botryoidal, reniform, spherical aggregates, or forms crusts and veins. Its appearance is often vitreous, resembling opal or hyalite. Specimens are usually small, and their surface can be smooth or bubbly (botryoidal). ## Physical Properties As an amorphous substance, evansite exhibits no cleavage, and its fracture is conchoidal, very similar to that of glass. Hardness on the Mohs scale is variable, ranging from 3-4. The luster is typically vitreous, sometimes waxy or resinous on fracture surfaces. It is a light mineral, with a density of approximately 1.8-2.2 g/cm³. ## Colors and Varieties Evansite can be colorless, white, yellowish, greenish, as well as blue or bluish-green. The color is often pale and delicate. No named varieties of this mineraloid are distinguished. ## History and Name The name evansite was given in honor of Brooke Evans (1797-1862), a British metallurgist from Birmingham, who first drew attention to this material. The mineral was first described in 1864 by David Forbes based on specimens found in the Zeleznik gold mine in Slovakia. ## Uses Evansite has no industrial application. It is solely an object of interest for collectors of rare and unusual minerals.

Properties

Mohs hardness
3.5-4
Color
Colourless, milk-white, lightly tinted blue, green or yellow at times; brown, reddish-brown, or red due to FeO inclusions; colourless to brown in transmitted light
Luster
Vitreous
Streak
White; lightly tinted.
Density
1.8
Cleavage
None
Fracture
Conchoidal
Transparency
Transparent,Translucent
Crystal system
Amorphous

Diagnostic features

## Identification Evansite is most easily recognized by its characteristic form of occurrence – vitreous, botryoidal or spherical aggregates. Key features also include conchoidal fracture, vitreous luster, and relatively low hardness. It feels light to the touch. As an amorphous substance, it does not form crystals. ## Distinguishing from Similar Minerals Evansite is sometimes confused with opal (hyalite), which is also amorphous and has a vitreous appearance. However, opal is harder (5.5-6.5). It can also be mistaken for variscite or turquoise, which are crystalline (variscite in the orthorhombic system, turquoise in the triclinic system) and rarely have such a distinctly vitreous, almost "wet" appearance. From similarly looking hemimorphite, it is distinguished by its lack of reaction with hydrochloric acid.

Geological environment

## Genesis Evansite is a secondary mineral, forming in weathering zones of aluminum-rich rocks, in the presence of phosphate-rich solutions. It often forms near coal deposits, in clay shales, and also as a product of low-temperature hydrothermal processes. It can also form in caves as a result of bat guano reacting with clay minerals. ## Mineral Associations It often co-occurs with minerals such as variscite, limonite, allophane, gibbsite, wavellite, and quartz. ## Locations The most important localities worldwide include the type locality in Zeleznik, Slovakia, as well as Gold-Chrome in Zimbabwe, where beautiful blue specimens were found. It also occurs in the Moab area of Utah (USA), in Bolivia, and in Ladispoli, Italy.

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are specimens with an intense, blue or greenish-blue color, resembling turquoise. The form is important – well-formed, spherical or botryoidal aggregates with a strong, vitreous luster are more desirable than shapeless crusts. The size of the aggregates and the absence of mechanical damage also significantly increase the value of the specimen. ## Popular Localities Historical specimens from Zeleznik, Slovakia, are considered classic and most sought after. Intensely blue evansites from the Gold-Chrome mine in Zimbabwe are also highly valued and are considered the best in the world in terms of color.

Care and storage

## Cleaning Evansite 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, as it is a hydrated mineral and may undergo changes. It should be thoroughly dried immediately. ## What to Avoid All acids and strong detergents should be strictly avoided. Evansite is sensitive to high temperatures, which can cause dehydration, leading to cracking and loss of luster. It should also not be exposed to direct sunlight, which can cause color fading. ## Storage It is recommended to store specimens in closed, stable conditions, away from heat sources and sudden temperature changes. Due to its low hardness, it should be stored separately to avoid scratches from harder minerals.

External references

Sources

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