Amblygonite-series

Chemical formula: LiAl[PO<sub>4</sub>](F,OH)

The amblygonite series is a group of lithium aluminum phosphates, most commonly forming large, whitish masses with perfect cleavage in granitic pegmatites.

## Characteristics The amblygonite series includes phosphate minerals, forming a continuous solid solution between fluorine-rich amblygonite and hydroxyl-rich montebrasite. In practice, most specimens have an intermediate composition, and identifying a specific end-member requires chemical analysis. These minerals most often form large, irregular or poorly developed tabular or short prismatic crystals, occurring as massive aggregates. Well-formed, single crystals are rare. A characteristic feature is their high density for a light-colored mineral. ## Physical Properties The hardness of minerals in this series is 5.5-6 on the Mohs scale, making them susceptible to scratching. The luster is vitreous, becoming pearly on perfect cleavage surfaces. They are transparent to translucent. The density ranges from 3.0-3.1 g/cm³, which is a helpful diagnostic feature. ## Colors and Varieties Most commonly, specimens are white, cream, gray, or yellowish. Pale green, bluish, or pink varieties are rarer. Transparent, clear, and colored varieties, mainly from Brazil, are valued as gemstones, although due to their cleavage and hardness, they are primarily of collector's interest. It should be noted that most gemological material historically called amblygonite is actually montebrasite. ## History and Name The name amblygonite was given in 1817 by August Breithaupt and comes from the Greek words *amblys* (blunt) and *gonia* (angle), referring to the angle between the cleavage planes, which is close to 90°. The name of the second end-member of the series, montebrasite, comes from its discovery locality – the Montebras mine in France. ## Applications Minerals from the amblygonite series are an important source of lithium, used in the production of batteries, ceramics, and specialized lubricants. They also constitute a phosphorus ore. Transparent and colored crystals are faceted for gemstone collectors.

Properties

Mohs hardness
5.5-6
Luster
Vitreous, Pearly on cleavages
Streak
White
Density
3.0-3.1
Cleavage
Perfect on {100}, Good on {010}
Fracture
Uneven to Subconchoidal
Transparency
Transparent to Translucent
Crystal system
Triclinic

Diagnostic features

## Identification A key diagnostic feature is the combination of perfect cleavage in one direction, pearly luster on its surfaces, and relatively high density (the specimen feels heavier than its appearance suggests). Occurrence in granitic pegmatites in association with other lithium minerals is also a strong indicator. ## Distinguishing from Similar Minerals Amblygonite is sometimes confused with feldspars (e.g., albite), which, however, have a lower density (approx. 2.6 g/cm³) and usually two distinct cleavage planes intersecting at an angle close to 90°. Spodumene, another lithium mineral, is harder (6.5-7) and has a different crystal habit. It is distinguished from quartz by its lower hardness and the presence of cleavage. A definitive distinction from very similar beryllonite often requires advanced studies. ## Crystal Forms Crystals are triclinic, often poorly developed, with a thick tabular or short prismatic habit. They typically form massive, granular, or fractured aggregates that can reach considerable sizes, sometimes weighing hundreds of kilograms.

Geological environment

## Genesis Minerals from the amblygonite series are typical components of lithium- and phosphorus-rich granitic pegmatites. They form in the late stages of pegmatitic magma crystallization, often in the cores or intermediate zones of these intrusions. ## Mineral Associations It co-occurs with other pegmatitic minerals, such as spodumene, lepidolite, tourmaline (especially elbaite), beryl, apatite, quartz, albite, and other lithium phosphates (e.g., lithiophilite). ## Localities The most important global localities include Brazil (Minas Gerais), from where large, transparent, gem-quality crystals originate (mainly montebrasite). In the USA, significant deposits are found in Keystone, South Dakota (Etta mine), Newry, Maine, and the Pala region, California. Historical localities include Saxony, Germany (type locality for amblygonite) and Montebras, France (type locality for montebrasite). It also occurs in Namibia, Spain, Sweden, and Australia.

Rarity

Not very common

For collectors

## Quality Criteria The collector's value of specimens depends on several factors. For gem material, the most important are transparency, clarity (lack of inclusions and fractures), intensity of color (yellow, greenish, and blue hues are particularly valued), and the size of the cut stone. For mineralogical specimens, the degree of crystal development (well-formed ones are rare), their size, color, and attractive associations with other pegmatitic minerals, such as tourmaline or lepidolite, are important. ## Popular Localities The most prized transparent crystals for faceting come from pegmatites in Minas Gerais, Brazil. Classic, large, opaque masses and poorly developed crystals are known from localities in South Dakota and Maine in the USA. Namibia provides specimens with good development and interesting color.

Care and storage

## Cleaning Specimens should only be cleaned with lukewarm distilled water and a soft brush. Ultrasonic and steam cleaners should be avoided, as they can cause fracturing along cleavage planes. ## What to Avoid The mineral is sensitive to impact and pressure due to its perfect cleavage. It should be protected from sudden temperature changes and contact with acids and other strong chemicals. ## Storage It is recommended to store specimens separately, in padded display boxes, to avoid scratching by harder minerals. Jewelry specimens are not suitable for everyday wear and should be handled with great care.

Sources

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