Rustumite
Chemical formula: Ca₁₀(Si₂O₇)₂(SiO₄)(OH)₂Cl₂
Rustumite is a rare calcium silicate, occurring as colorless, fine grains in skarns and metamorphic rocks.
Properties
- Mohs hardness
- 4-5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.94
- Cleavage
- (100) poor, (010) and (001) very poor
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Amateur identification of rustumite is practically impossible due to its small size and lack of characteristic macroscopic features. It usually occurs as colorless, vitreous grains embedded in the rock. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals Macroscopically, it can be confused with many other colorless, granular silicate minerals, such as quartz, calcite, or wollastonite, with which it often co-occurs. Differentiation relies solely on laboratory analysis. ## Crystal Forms Rustumite primarily forms anhedral, irregular grains. Rarely, poorly developed, tabular crystals of small size, usually less than 1 mm, are observed.
Geological environment
## Genesis Rustumite is a product of high-temperature contact metamorphism. It forms in skarns and calcareous xenoliths that have been subjected to basaltic magma. It forms under low-pressure and high-temperature conditions, in an environment rich in calcium and silica, with the involvement of chlorine and hydroxyl groups. ## Mineral Associations This mineral co-occurs with other minerals typical of skarns, such as larnite, rankinite, wollastonite, gehlenite, spurrite, hillebrandite, as well as calcite and portlandite. ## Localities The most important and classic locality for rustumite is its type locality in Kilchoan, Ardnamurchan, Scotland (United Kingdom). It has also been reported in several other locations worldwide, including the Hatrurim Complex in Israel, the Bellerberg Mountains in Germany, and Okayama Prefecture in Japan.
Rarity
Very rare
For collectors
## Quality Criteria As a "micromount" type mineral, the collector's value of rustumite primarily depends on confirmed identification and precise locality. Specimens where rustumite grains are relatively large (even fractions of a millimeter) and clearly visible against a contrasting rock matrix are more highly prized. However, the scientific value and connection to a classic locality are paramount. ## Popular Localities Almost all collector specimens come from the type locality in Kilchoan, Scotland. Material from other confirmed localities, such as the Hatrurim Complex, is extremely rare on the market.
Care and storage
## Cleaning Specimens containing rustumite should be cleaned very carefully, preferably using compressed air to remove dust. Due to its small size and occurrence within the parent rock, wet cleaning is avoided, as it could damage co-occurring minerals. ## What to Avoid Contact with acids and other chemicals should be avoided. The mineral is brittle, so it should be protected from impacts and vibrations. There are no specific recommendations regarding exposure to light or temperature, but standard protection against extreme conditions is advisable. ## Storage Specimens should be stored under stable conditions, preferably in sealed "micromount" boxes, which protect against dust and mechanical damage. A label with precise locality information is crucial for the scientific value of the specimen.