Magnesiotaaffeite-6<i>N</i>'3<i>S</i>

Chemical formula: Mg<sub>2</sub>BeAl<sub>6</sub>O<sub>12</sub>

Magnesiotaaffeite-6N'3S is an extremely rare mineral from the taaffeite group, transparent, with a vitreous luster, found in metamorphic rocks and alluvium.

## Characteristics Magnesiotaaffeite-6N'3S, formerly known as musgravite, is a very rare beryllium, magnesium, and aluminum oxide. It belongs to the nigerite supergroup and the taaffeite group. It most often forms small, tabular or prismatic crystals, often with a hexagonal outline. It usually occurs as rounded grains in alluvial deposits. It is a hard, brittle, and transparent mineral, which, combined with its rarity, makes it a prized gemstone. ## Physical Properties This mineral is characterized by high hardness, ranging from 8 to 8.5 on the Mohs scale, placing it among the hardest minerals. It has a vitreous luster and is transparent to translucent. Its density ranges from 3.63 to 3.68 g/cm³. It does not exhibit fluorescence under ultraviolet light. ## Colors and Varieties Its color is variable, from colorless, through gray, grayish-green, green, to grayish-purple and purple. The most desired specimens in jewelry are those with an intense purple or green coloration. Some crystals may exhibit subtle pleochroism. The commercial name "musgravite" is still commonly used in reference to its gem varieties. ## History and Name The mineral was first identified in 1967 in material from the Musgrave Ranges in South Australia, from which it took its original name - musgravite. The name was changed to magnesiotaaffeite-6N'3S in 2002 by the International Mineralogical Association (IMA) to better reflect its chemical composition and crystal structure in relation to other minerals of the taaffeite group. Despite the name change, the name "musgravite" still dominates in the gemstone trade. ## Uses Due to its extreme rarity, magnesiotaaffeite-6N'3S is primarily a sought-after and very valuable collector's stone. The highest quality, transparent crystals with attractive color are faceted and used in exclusive jewelry, fetching very high prices.

Properties

Mohs hardness
8-8.5
Luster
Vitreous
Streak
White
Density
3.63-3.68
Cleavage
Imperfect on {0001}
Fracture
Conchoidal, Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Identifying magnesiotaaffeite-6N'3S is very difficult without advanced analysis. Key features include exceptionally high hardness (8-8.5), vitreous luster, density, and characteristic hexagonal crystal forms. In river sediments, it occurs as rounded grains. Final confirmation requires chemical analysis (EDS) or X-ray diffraction (XRD) studies to distinguish it from other polytypes of the taaffeite group. ## Distinguishing from Similar Minerals It can be easily confused with the very similar taaffeite (magnesiotaaffeite-2N'2S), from which it is virtually impossible to distinguish visually – this requires advanced laboratory techniques such as Raman spectroscopy or XRD. It may also resemble some varieties of spinel, sapphire, or chrysoberyl, from which it differs in optical properties and crystal structure. ## Crystal Forms Crystals are usually small, tabular or short-prismatic, often terminated by a double pyramid. They exhibit a hexagonal outline. It also occurs as rounded, irregular grains in alluvial deposits.

Geological environment

## Genesis It is a metamorphic mineral, forming under high-temperature conditions in rocks rich in aluminum and carbonates, such as skarns, dolomitic marbles, or gneisses. It is also found secondarily in alluvial deposits (river sands and gravels), where it ends up as a result of erosion and weathering of parent rocks. ## Mineral Associations It most often co-occurs with minerals such as spinel, phlogopite, pargasite, sapphirine, chrysoberyl, calcite, dolomite, and other minerals from the taaffeite group. ## Localities The most important locality, which is also the type locality, is the Musgrave Ranges in South Australia. Significant quantities of gem material come from alluvial deposits in the Tunduru region of Tanzania and Mogok in Myanmar (Burma). Occurrences have also been reported in Madagascar (Tranomaro region), Greenland (Kangerlussuaq Fjord), and Antarctica (Enderby Land).

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued specimens are those with gemstone status – fully transparent, without visible inclusions or fractures. Color is crucial: the most sought-after are intense shades of purple, purplish-pink, and deep greens. Size is of great importance; faceted stones over one carat are considered large and exceptionally valuable. Well-formed, sharp-edged crystals, even if not of gem quality, are also highly prized by mineral collectors. ## Market Prices The prices of magnesiotaaffeite-6N'3S (musgravite) are very high. Faceted stones of good quality can fetch prices from several to tens of thousands of dollars per carat, depending on color, clarity, and size. Prices for rough, well-formed crystals are also high and depend on their form and association with other minerals. ## Popular Localities The most famous and valued gem specimens come from alluvial deposits in Tunduru (Tanzania) and Mogok (Myanmar). Material from the type locality in Australia is rarely found on the market. Specimens from Madagascar and Greenland are mainly of scientific importance and are rare in collections.

Care and storage

## Cleaning Specimens should be cleaned only with lukewarm soapy water and a soft brush. Ultrasonic cleaners can be used, but with great caution, especially for specimens with inclusions or internal fractures. After cleaning, rinse thoroughly and dry with a soft cloth. ## What to Avoid The mineral is resistant to most chemicals, but contact with hydrofluoric acid should be avoided. Despite its high hardness, it is brittle and sensitive to impacts and sudden temperature changes, which can cause fractures. It should not be exposed to very high temperatures. ## Storage It is recommended to store specimens separately, in padded boxes or display cases, to avoid scratching by other minerals (e.g., diamond) or damaging softer stones. It should be protected from impacts and falls.

Sources

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