Milanriederite

Chemical formula: (Ca,REE)₁₉Fe₃+Al₄(Mg,Al,Fe₃+)₈Si₁₈O₆₈(OH,O)₁₀

Milanriederite is an extremely rare, complex silicate known from only one locality worldwide, forming small, prismatic, brownish-red crystals.

## Characteristics Milanriederite is a very rare mineral from the silicate group, belonging to the enigmatite group. It occurs as small, well-formed, prismatic or tabular crystals, rarely exceeding 0.5 mm in length. Crystals often form radial or randomly oriented aggregates. Its most characteristic visual feature is an intense, brownish-red to brown color. ## Physical Properties This mineral has a Mohs hardness of 6.5. It has a vitreous luster and is transparent to translucent. Its density, calculated based on the chemical formula and unit cell parameters, is approximately 3.65 g/cm³. ## Colors and Varieties The dominant and only known color of milanriederite is brownish-red, transitioning to shades of brown. No color varieties or commercial varieties have been distinguished. ## History and Name The mineral was recognized by the International Mineralogical Association (IMA) in 2010. Its name honors Milan Rieder (1940-2007), a Czech mineralogist and crystallographer from Masaryk University in Brno, for his contributions to the study of layered silicates. It was discovered in a calc-silicate skarn in Sierra de Tlayacapan, Mexico, which remains its only known locality. ## Applications Due to its extreme rarity and microscopic crystal sizes, milanriederite has no industrial applications. It is solely an object of scientific interest and a valuable acquisition for specialized collectors of rare minerals.

Properties

Mohs hardness: 6.5
Luster: Vitreous
Streak: Light brown
Density: 3.65
Cleavage: Good on {010}
Fracture: Uneven
Transparency: Translucent to transparent
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of milanriederite is extremely difficult and practically impossible without advanced analytical methods. Preliminary identification is based on its unique locality (Sierra de Tlayacapan), co-occurrence with other skarn minerals, brownish-red color, and the form of small, prismatic crystals. Final confirmation requires chemical (EDS) and X-ray (XRD) analysis. ## Distinguishing from Similar Minerals This mineral can be confused with other brown silicates found in skarns, such as vesuvianite or garnet (andradite). Differentiation based on visual characteristics is practically impossible. It differs from them in chemical composition and crystal structure. ## Crystal Forms Milanriederite forms elongated, prismatic crystals with a nearly rectangular cross-section, as well as tabular crystals. They often occur as radial aggregates or disordered clusters on the surface of other minerals.

Geological environment

## Genesis Milanriederite is a mineral of metamorphic origin. It forms in contact zones, within calc-silicate skarns, which developed as a result of the interaction of intruding magma with carbonate rocks (limestones). ## Mineral Associations This mineral occurs in association with other minerals typical of a skarn environment. These most commonly include vesuvianite, wollastonite, andradite, diopside, titanite, apatite, fluorite, and minerals from the epidote group. ## Localities The only confirmed occurrence of milanriederite in the world is its type locality - Sierra de Tlayacapan, in the state of Morelos, Mexico.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of milanriederite specimens, which are typically micromounts, is assessed based on several criteria. Specimens with sharp, well-formed, and undamaged crystals are most highly valued. The intensity and attractiveness of the brownish-red color are also important. The abundance of crystals on the rock matrix and aesthetic composition with other associated minerals (e.g., white wollastonite) significantly increase the collectible value of the specimen. ## Popular Localities All milanriederite specimens available on the collector's market come from the same single location - its type locality in Sierra de Tlayacapan, Mexico.

Care and storage

## Cleaning Milanriederite specimens should be cleaned with utmost care, preferably using only compressed air to remove dust. Due to the small size of the crystals, mechanical cleaning, including with brushes, is not recommended. Washing in water, even distilled, is risky and should be avoided. ## What to Avoid Contact with acids and other chemicals should be strictly avoided. The mineral should not be subjected to ultrasonic cleaning or rapid temperature changes. Prolonged exposure to direct sunlight may not be harmful, but caution is advised. ## Storage Collectible milanriederite specimens, typically micromounts, should be stored in stable conditions, in sealed "micromount" boxes that protect them from dust, mechanical damage, and humidity.

Sources

IMA Mineral List (CNMNC)