Milarite

Chemical formula: KCa<sub>2</sub>(Be<sub>2</sub>AlSi<sub>12</sub>O<sub>30</sub>)·H<sub>2</sub>O

A rare cyclosilicate forming characteristic hexagonal prismatic crystals with a vitreous luster.

## Characteristics Milarite is a complex cyclosilicate from the osumilite group, containing beryllium and aluminum. Its most typical and recognizable form consists of well-developed, hexagonal crystals with a prismatic or columnar habit. The crystals are often elongated and terminated by flat faces. The crystal surface has a characteristic, strong vitreous luster, and the crystals themselves can be transparent or translucent. ## Physical Properties Milarite's hardness on the Mohs scale is 5.5 to 6, placing it among minerals with moderate scratch resistance. It has a density in the range of 2.55-2.59 g/cm³, which is typical for many silicates. The luster is distinctly vitreous, and the mineral is brittle, with a conchoidal to uneven fracture. ## Colors and Varieties Milarite is most commonly colorless, white, pale green, or pale yellow. The intensity of the color is usually low, and the crystals often have a delicate, pastel hue. There are no named commercial or colored varieties. ## History and Name The mineral's name comes from its type locality – the Milar Valley (Val Milar) in the canton of Graubünden, Switzerland. It was first described as a distinct mineral species by the German mineralogist Gustav Adolf Kenngott in 1870. ## Uses Milarite has no industrial applications. However, it is a valued and sought-after collector's mineral due to its rarity, attractive crystal form, and delicate colors.

Properties

Mohs hardness
5.5-6
Luster
Vitreous
Streak
White
Density
2.55-2.59
Cleavage
Indistinct on {0001}
Fracture
Conchoidal to uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification The most important diagnostic feature of milarite is its crystal habit – it almost always occurs as hexagonal, elongated prisms. Its hardness (5.5-6) distinguishes it from softer apatite and much harder quartz or beryl. Its occurrence in paragenesis typical of miarolitic cavities in pegmatites or in alpine fissures is also a strong indicator. ## Distinguishing from Similar Minerals Milarite is sometimes confused with several minerals of similar appearance: - **Apatite**: Often forms hexagonal prisms, but is distinctly softer (Mohs hardness 5). - **Beryl (varieties goshenite, heliodor)**: Is much harder (7.5-8) and has a slightly different crystal habit. - **Quartz**: Is harder (7) and rarely forms such perfectly shaped, single hexagonal prisms with a flat termination. - **Phenakite**: Also much harder (7.5-8) and has a different, often rhombohedral or lenticular crystal habit. ## Crystal Forms Milarite crystallizes in the form of elongated, hexagonal columns and prisms. Crystals are often terminated by a flat face perpendicular to the main axis (basal pinacoid). They can occur as single crystals or form radial aggregates and groups.

Geological environment

## Genesis Milarite is a hydrothermal mineral, crystallizing at relatively low temperatures. It forms mainly in miarolitic cavities (voids) in granites and granitic pegmatites. It is also a typical mineral for alpine-type fissures, where it grows on the walls of open rock fractures. ## Mineral Associations This mineral often coexists with quartz (including smoky quartz), adularia (a variety of potassium feldspar), albite, chlorite, apatite, titanite, calcite, and epidote. ## Localities The most important global localities for milarite are: - **Switzerland**: Val Milar and other localities in the Alps, from which the mineral's name originates (type locality). - **Namibia**: Erongo Mountains, known for producing some of the largest and highest quality milarite crystals in the world. - **Brazil**: Minas Gerais and Rio Grande do Sul states, where transparent, light green crystals are found. - **Russia**: Kola Peninsula, in pegmatite massifs. - **Pakistan**: Skardu area, where it occurs in association with aquamarine and other pegmatite minerals.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized milarite specimens are characterized by large, sharply terminated, and undamaged crystals with high transparency. A distinct, even if pale, greenish or yellowish color is desirable. Specimens where milarite crystals aesthetically contrast with matrix minerals (e.g., with white albite or smoky quartz) fetch higher prices. The size of individual crystals is a key factor influencing value. ## Popular Localities The best specimens in the world are considered to come from the Erongo Mountains in Namibia, which yield large, well-formed, and often transparent crystals. Classic, though usually smaller, specimens from the Swiss Alps are valued by collectors for their historical significance. Gem-quality crystals also come from Brazil.

Care and storage

## Cleaning Milarite specimens are best cleaned with a soft brush and distilled water. A small amount of neutral soap can be used, followed by thorough rinsing. Ultrasonic cleaners should be avoided, as vibrations can cause fractures in brittle crystals. ## What to Avoid The mineral is sensitive to strong acids. Its hardness (5.5-6) means it can be scratched by harder minerals such as quartz (hardness 7) or beryl. Avoid sudden temperature changes, although it is stable in sunlight. ## Storage Collector's milarite specimens should be stored in separate boxes or display cases, away from harder minerals, to prevent scratching. Due to its brittleness, it should be protected from impacts and falls.

External references

Sources

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