Mcallisterite

Chemical formula: Mg₂[B₆O₇(OH)₆]₂·9H₂O

Mcallisterite is a rare, colorless magnesium borate, forming small, vitreous crystals and aggregates in borate deposits.

## Characteristics Mcallisterite is a hydrated magnesium borate, typically occurring as small, colorless or white crystals with a tabular or short prismatic habit. It also forms rosette-like aggregates, radial aggregates, or occurs as massive and earthy aggregates. It is a brittle mineral with a vitreous luster, often transparent. ## Physical Properties This mineral is characterized by low hardness, 2-2.5 on the Mohs scale, which means it is very soft. Its density is also low, approximately 1.88 g/cm³. It exhibits a white or colorless streak. ## Colors and Varieties It is usually colorless or white. No colored varieties or trade names are distinguished for it. ## History and Name The mineral is named in honor of James F. McAllister (1911-2000), a geologist with the United States Geological Survey, who made significant contributions to the study of the geology and borate deposits in Death Valley, California. The mineral was first described in 1965 based on samples discovered in the Furnace Creek area of Inyo County. ## Applications Due to its rarity and small crystal size, mcallisterite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals or borate minerals.

Properties

Mohs hardness
2.5
Color
Colourless, white
Luster
Vitreous
Streak
White
Density
1.867
Cleavage
<mi>on {0001} and {01_12}</mi>
Fracture
Conchoidal
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identification of mcallisterite is based on its physical characteristics: it is very soft (hardness 2-2.5), has low density, and a vitreous luster. However, the locality is key – it is almost always found in association with other borates in specific evaporitic environments. It reacts with acids. ## Distinguishing from Similar Minerals It can be confused with other colorless borates, such as colemanite or inderite. Differentiation requires chemical analysis (EDS) or X-ray diffraction (XRD). In field conditions, observing crystal habit and hardness can be helpful – mcallisterite is significantly softer than most similar minerals. ## Crystal Forms It crystallizes in the trigonal system, forming small, pseudohexagonal, tabular crystals. It often occurs as radial or rosette-like aggregates, as well as massive or earthy aggregates.

Geological environment

## Genesis Mcallisterite is a secondary mineral, formed by the alteration of other borate minerals, mainly colemanite, in a magnesium-rich environment. It forms in playa-type borate deposits, associated with volcanic activity and hot springs in arid, desert climates. ## Mineral Associations It co-occurs with other borates, such as colemanite, inderite, hydroboracite, ulexite, as well as gypsum and calcite. ## Localities The most important and classic localities for this mineral are in the USA, in the Furnace Creek area of Death Valley (Inyo County, California). It has also been reported in other localities with similar geology, including Salta Province in Argentina and the Bigadiç region in Turkey.

Rarity

Rare

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with well-formed, transparent crystals forming attractive, rosette-like or radial aggregates. Rich crystalline aggregates on a rock matrix, especially in association with other rare borates, are also highly valued. Due to the small size of the crystals, even small but well-formed specimens are valuable. ## Popular Localities The most prized specimens come from the type locality in Death Valley, California, USA. Specimens from Argentina and Turkey are also sought after by collectors specializing in mineral systematics.

Care and storage

## Cleaning Mcallisterite specimens should be cleaned very carefully, dry only, using a soft brush to remove dust. The mineral is partially soluble in water, so contact with it is absolutely inadvisable. ## What to Avoid Avoid all contact with water, moisture, and chemicals. Storage in a humid environment can lead to slow degradation of the specimen. It is a very soft and brittle mineral, sensitive to mechanical damage. ## Storage It is recommended to store specimens in sealed, dry containers (e.g., "perky box" type) away from sources of moisture and sudden temperature changes. It is best to place a desiccant, such as silica gel, in the container.

External references

Sources

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