Delhayelite
Chemical formula: K₇Na₃Ca₅Al₂Si₁₄O₃₈F₄Cl₂
Delhayelite is a rare silicate with a complex chemical composition, forming colorless to white, tabular or fibrous crystals in phonolitic lavas.
Properties
- Mohs hardness
- 5
- Color
- Colourless; Silver-grey, Greenish-grey
- Luster
- Vitreous, Pearly on cleavage surfaces
- Streak
- White
- Density
- 2.60
- Cleavage
- Parallel to (010)
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Characteristic features of delhayelite include its habit (tabular crystals, often in radial aggregates), perfect cleavage in one direction with a pearly luster, and its specific occurrence environment – voids in phonolitic lavas. It reacts with hydrochloric acid, gelatinizing. ## Distinguishing from Similar Minerals Delhayelite can be confused with other white silicates occurring in similar conditions, such as some zeolites (e.g., natrolite) or microcline. It is often distinguished from zeolites by its habit and the lack of the characteristic fibrous structure of many of them. From feldspars (e.g., microcline), it differs by its single, perfect cleavage (feldspars have two) and lower hardness. ## Crystal Forms Crystals are typically thin-tabular or lath-like, exhibiting a pseudohexagonal outline. They often form fan-shaped, radial, or spherulitic aggregates. Single, well-formed crystals are less common.
Geological environment
## Genesis Delhayelite is a mineral of volcanic origin. It forms in the late stages of alkaline magma crystallization, crystallizing in vesicles and voids within phonolitic and nephelinitic lavas. It is a product of hydrothermal activity associated with volcanism. ## Mineral Associations This mineral co-occurs with other rare alkaline minerals. Typical associated minerals include nepheline, leucite, augite, melilite, kalsilite, götzenite, and aphthitalite (sulfate). ## Localities The most important and classic localities for this mineral are in the Democratic Republic of Congo, in the Nyiragongo and Nyamuragira volcanoes (North Kivu province) – this is the type locality. It is also known from the Weilsberg volcanic complex in the Eifel Mountains in Germany and from Khibiny on the Kola Peninsula in Russia.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp crystals forming aesthetic, radial aggregates. The clarity and transparency of the crystals and the absence of mechanical damage are important. A contrasting matrix (rock substrate) also enhances the specimen's attractiveness. The size of the aggregates is significant, and specimens with crystals exceeding a few millimeters are rare. ## Popular Localities The best quality specimens, considered classic, come from the type locality in the Democratic Republic of Congo (Nyiragongo and Nyamuragira volcanoes). Material from the Eifel Mountains in Germany is also sought after by collectors specializing in minerals from that region.
Care and storage
## Cleaning Delhayelite specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Due to its perfect cleavage, ultrasonic cleaners and mechanical cleaning should be avoided. ## What to Avoid Contact with acids and other chemicals that may react with the mineral's complex composition should be avoided. Delhayelite is brittle, so it must be protected from impacts and falls. It should not be heated or exposed to sudden temperature changes. ## Storage Specimens are best stored in individual, padded collector's boxes to prevent scratches and mechanical damage. Protect from dust and moisture. It does not require special protection from light.