Montebrasite

Chemical formula: LiAlPO<sub>4</sub>(OH)

Montebrasite is a lithium-rich phosphate, valued for its well-formed crystals with a vitreous luster, and is an important member of the amblygonite-montebrasite series.

## Characteristics Montebrasite is a phosphate mineral belonging to the amblygonite-montebrasite isomorphic series. It most often forms anhedral, massive aggregates, less commonly short-prismatic or thick-tabular crystals, which can reach considerable sizes, sometimes exceeding a meter. It usually occurs in translucent to opaque forms. It is brittle and exhibits an uneven, sometimes conchoidal fracture. ## Physical Properties This mineral is characterized by a hardness of 5.5 on the Mohs scale and a density of approximately 3.04 g/cm³. The luster is vitreous, transitioning to pearly on cleavage surfaces. It is a relatively brittle mineral. ## Colors and Varieties Montebrasite is most often colorless, white, or grayish-white. It can also take on pale yellow, greenish, and even pinkish or light brown hues. No named varieties are distinguished, and color variability results from impurities and crystallization conditions. ## History and Name The mineral's name comes from its discovery location – the Montebras mine in the Creuse region of France. It was first described in 1871 by the French mineralogist Alfred des Cloizeaux. It is the hydroxyl (OH) analogue of the more fluorine (F)-rich amblygonite. ## Uses Due to its high lithium content, montebrasite is locally used as an ore of lithium. It also has scientific significance as an indicator of geological processes in granitic pegmatites. For collectors, well-formed, clear, and large crystals are the most valuable.

Properties

Mohs hardness
5.5
Luster
Vitreous, pearly
Streak
White
Density
3.04
Cleavage
Perfect on {001}, good on {100}, distinct on {011}
Fracture
Uneven to conchoidal
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Montebrasite is most easily identified by its occurrence in granitic pegmatites, its relatively high density for a mineral with a vitreous luster, and its perfect cleavage in one direction. It often forms large, anhedral masses of a whitish color. It reacts with acids. ## Distinguishing from Similar Minerals Montebrasite is difficult to distinguish from amblygonite without advanced analyses (e.g., XRD, EDS), as they form a series. Generally, montebrasite is less dense than amblygonite. It can be confused with albite or other feldspars, from which it is distinguished by its higher density and a single direction of perfect cleavage. From quartz, it is distinguished by its lower hardness and the presence of cleavage. ## Crystal Forms Crystals are rare, usually short-prismatic, thick-tabular, or isometric. It is most commonly found as massive, granular, or anhedral aggregates, often of considerable size.

Geological environment

## Genesis Montebrasite is a mineral typical of lithium- and phosphorus-rich granitic pegmatites. It forms in the late stages of pegmatitic magma crystallization, under conditions of elevated water activity and other volatile components. ## Mineral Associations It often co-occurs with other pegmatitic minerals such as quartz, potassium feldspar, albite, lepidolite, spodumene, tourmaline (especially elbaite), cassiterite, apatite, and beryl. ## Localities The most important global localities include the Montebras mine in Creuse (France) – the type locality. Beautiful crystals come from Brazil (Minas Gerais and Paraíba states). It also occurs in the USA (e.g., Keystone mine in South Dakota, California, Maine), Namibia, Mozambique, Rwanda, and Spain.

Rarity

Not very common

For collectors

## Quality Criteria The most highly prized montebrasite specimens are those that form large, well-formed, and partially transparent crystals. A clear, light color (colorless, pale yellow) and the absence of fractures and inclusions enhance its attractiveness. Associations with other pegmatitic minerals, such as lepidolite or elbaite, also significantly increase its collector's value. ## Popular Localities Collectors particularly seek transparent, faceted crystals from Brazil, especially from the Linópolis and Governador Valadares regions in Minas Gerais. Specimens from the original locality in France have historical significance, and large crystals also come from mines in the Black Hills region of South Dakota (USA).

Care and storage

## Cleaning Montebrasite specimens should be cleaned carefully, using a soft brush and distilled water. Short soaks in ultrasonic cleaners can be used, but with great caution, especially for specimens with inclusions or fractures. ## What to Avoid The mineral is sensitive to acids, which can damage it. Avoid sudden temperature changes, which can cause internal fractures. Due to its relatively low hardness, it must be protected from scratching by harder minerals (e.g., quartz). ## Storage Montebrasite is best stored in separate, padded boxes or on a soft surface in display cases, away from minerals that could scratch it. It is not particularly sensitive to light, but prolonged exposure to direct sunlight is not recommended.

External references

Sources

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