Monticellite
Chemical formula: CaMgSiO<sub>4</sub>
Monticellite is a rare calcium magnesium silicate from the olivine group, forming small, vitreous crystals that are gray, greenish, or colorless.
Properties
- Mohs hardness
- 5
- Luster
- Vitreous
- Streak
- White
- Density
- 3.2
- Cleavage
- Weak on {010} and {100}
- Fracture
- Conchoidal to uneven
- Transparency
- Transparent to translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Monticellite, in typical occurrences, forms granular aggregates or small, poorly formed crystals with a vitreous luster, most often in skarns or marbles. Its hardness (5) and lack of distinct cleavage are helpful in identification. However, this often requires advanced research methods, such as chemical analysis or X-ray diffraction. ## Distinguishing from Similar Minerals Monticellite can be confused with other minerals from the olivine group, such as forsterite, which, however, usually has a greener color. It can also be confused with some garnets (e.g., grossular) or vesuvianite, which are significantly harder than monticellite (hardness 6.5-7.5). Diopside, often co-occurring, has a different crystal form and distinct cleavage. ## Crystal Forms Monticellite crystals, when well-formed, take the shape of short, prismatic columns, often flattened, with a cross-section similar to a square or rectangle. Most often, however, it occurs as grains without distinct crystal faces or in the form of granular aggregates.
Geological environment
## Genesis Monticellite is a characteristic mineral of contact metamorphism processes. It forms at high temperatures as a result of the alteration of impure dolomites and siliceous limestones at the contact with magmatic intrusions. It forms in metamorphic rocks called skarns and marbles. Less commonly, it is found in some ultramafic igneous rocks (kimberlites) and in meteorites. ## Mineral Associations This mineral often co-occurs with other contact metamorphic minerals, such as calcite, vesuvianite, diopside, wollastonite, garnets (mainly andradite and grossular), melilite, åkermanite, as well as forsterite and spinel. ## Localities The most important and classic locality for monticellite is the Monte Somma – Vesuvius volcanic complex in Italy. Other known localities include Crestmore in California (USA), where well-formed crystals have been found, as well as areas in Arkansas (USA), on the Isle of Skye in Scotland, and in South Africa (in kimberlites).
Rarity
Rare
For collectors
## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and transparent crystals, which is a great rarity for this mineral. The size of the crystals is also important – the larger, the more valuable. The attractiveness is enhanced by a contrasting background of the host rock (e.g., white marble) and the co-occurrence with other rare minerals, such as vesuvianite or melilite. ## Popular Localities Specimens of collecting significance mainly come from the classic locality of Monte Somma – Vesuvius in Italy and from quarries in Crestmore, California, USA. Less common, but also valued, are specimens from Magnet Cove in Arkansas, USA.
Care and storage
## Cleaning Monticellite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always thoroughly dry the specimen after cleaning. ## What to Avoid Monticellite is sensitive to acids, which can damage or completely dissolve it. Contact with household chemicals and mineral acids should be avoided. It should also not be heated or subjected to sudden temperature changes. Due to its moderate hardness, it is susceptible to scratching by harder minerals (e.g., quartz). ## Storage Monticellite specimens are best stored in separate, padded boxes or display cases to avoid scratches and abrasions. Protect from dust and moisture. It does not require special protection from light.