Tilleyite
Chemical formula: Ca<sub>5</sub>Si<sub>2</sub>O<sub>7</sub>(CO<sub>3</sub>)<sub>2</sub>
Tilleyite is a rare calcium silicate that stands out for its occurrence in contact metamorphic zones of limestones and its ability to fluoresce under UV light.
Properties
- Mohs hardness
- 5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.84
- Cleavage
- Good on {100}, poor on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Tilleyite is difficult to identify visually. A key diagnostic feature is its vigorous reaction with hydrochloric acid, which distinguishes it from many other silicates. Its occurrence in a characteristic geological environment – skarns – is also an important clue. Identification is usually confirmed by analytical methods, such as X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other white or colorless skarn minerals, such as wollastonite, diopside, or vesuvianite. Wollastonite does not react as vigorously with acids. From calcite, with which it often co-occurs, it is distinguished by its higher hardness (5 compared to 3 for calcite). Spurrite is very similar but has different optical properties and crystal structure, requiring advanced studies for differentiation. ## Crystal Forms Tilleyite crystals are rare and usually small, with a tabular or short-prismatic habit. Most often, it forms compact, granular, fibrous, or radial aggregates embedded in the host rock.
Geological environment
## Genesis Tilleyite is a mineral typical of high-temperature contact metamorphic environments. It forms in contact zones of magmatic intrusions (e.g., gabbro, diorite) with calcium carbonate-rich sedimentary rocks, such as limestones and marls. It forms in the sanidinite facies, under conditions of high temperature and relatively low pressure. ## Mineral Associations This mineral often co-occurs with other rare skarn minerals. Its typical associations include: calcite, vesuvianite, grossular (garnet), wollastonite, spurrite, gehlenite, merwinite, and larnite. ## Localities The most important and classic tilleyite localities worldwide are: - Crestmore, Riverside County, California, USA - type locality (localitas typica). - Carneal, County Antrim, Northern Ireland. - Isle of Muck, Scotland. - Vesuvius region, Italy. - Hatrurim Formation in Israel. - Okayama Mine in Japan.
Rarity
Rare
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, even if small, tilleyite crystals, which is a great rarity. Rich aggregates of this mineral, clearly visible on the rock matrix, are also highly valued. Contrast with other associated minerals, especially those of a different color, enhances the attractiveness of the specimen. A confirmed locality is also important, especially if it is one of the classic sites. ## Popular Localities Specimens from the type locality, Crestmore in California, have particular historical value. Those from Northern Ireland (Carneal) and Scotland (Isle of Muck), which have provided well-studied materials, are also sought after.
Care and storage
## Cleaning Tilleyite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, compressed air can be used. Due to its possible reactivity with acids, contact with water, especially tap water which may contain dissolved carbon dioxide forming weak carbonic acid, should be avoided. Dry cleaning is safer. ## What to Avoid Tilleyite is sensitive to acids, in which it reacts vigorously, releasing carbon dioxide. It should be protected from all chemicals, including cleaning agents. Rapid temperature changes and prolonged exposure to moisture should also be avoided. ## Storage Specimens should be stored in stable, dry conditions, away from direct sunlight. It is best to place them in a closed display box or cabinet to protect them from dust and mechanical damage. Due to its brittleness, it should not be stored in contact with harder minerals.