Siwaqaite
Chemical formula: Ca₆Al₂(Cr⁶⁺O₄)₃(OH)₁₂·26H₂O
Siwaqaite is a hydrated calcium aluminum chromate, forming characteristic, bright yellow, hexagonal crystals with a tabular habit.
Properties
- Mohs hardness
- 2
- Color
- Canary yellow
- Luster
- Vitreous
- Streak
- Yellowish-gray
- Density
- 1.819
- Cleavage
- <mi>(10_10)</mi>
- Fracture
- Irregular/Uneven
- Transparency
- Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Key diagnostic features include its bright, lemon-yellow color, hexagonal, tabular crystal habit, and very low hardness. The mineral is very light and brittle. Its strong, greenish-yellow fluorescence under UV light is also characteristic. ## Distinguishing from Similar Minerals - **Autunite/meta-autunite**: They have similar color and fluorescence but form square, not hexagonal, crystals. They are also significantly harder and denser. Autunite is a uranium mineral and is radioactive, whereas siwaqaite exhibits no radioactivity. - **Bentorite**: It is structurally and chemically very similar (it is a sulfate analogue) but has a purple color. - **Native sulfur**: It can have a similar color but forms different crystal habits (orthorhombic), has a characteristic odor when heated, and is not water-soluble. ## Crystal Forms Siwaqaite forms thin, tabular crystals with a hexagonal outline. These crystals often combine into rosette-like, fan-shaped, or spherical aggregates, growing on the surface of the host rock.
Geological environment
## Genesis Siwaqaite is a mineral of hydrothermal origin. It forms under low-temperature conditions in calcite veins cutting through altered (burnt) bituminous marls. Its formation is associated with the oxidation of sulfides in the presence of calcium- and aluminum-rich solutions. ## Mineral Associations This mineral co-occurs with calcite, ettringite, afwillite, portlandite, and other rare minerals from the ettringite group, such as bentorite and thaumasite. ## Localities The only confirmed locality for siwaqaite worldwide is its type locality: the Daba-Siwaqa area in the Hashemite Kingdom of Jordan. This is a unique geological environment, known for the occurrence of many rare minerals formed as a result of the burning of bituminous rocks.
Rarity
Very rare
For collectors
## Quality Criteria The most prized siwaqaite specimens are those with well-formed, sharply terminated crystals of intense, bright yellow color. The abundance of crystals on the rock matrix and their size (though they are usually small, up to a few millimeters) enhance their appeal. Contrast with a white, calcitic matrix is also desirable. The absence of damage is important, which is difficult to achieve due to the mineral's extreme brittleness. ## Popular Localities The only source of collector specimens is the type locality in Jordan. Specimens from this location are highly sought after by collectors specializing in rare minerals and minerals from the ettringite group.
Care and storage
## Cleaning Utmost care must be taken. Siwaqaite is very soft, brittle, and water-soluble. Wet cleaning is absolutely forbidden, as it will lead to the destruction of the specimen. Only very gentle dust removal with a soft brush or a stream of compressed air from a distance is permissible. ## What to Avoid - Water and all liquids: the mineral is soluble and will be destroyed. - Chemicals and acids: it reacts violently, dissolving. - High temperatures and direct sunlight: can cause dehydration and loss of color. - Vibrations and impacts: it is extremely brittle. - Humid air: can cause slow degradation of the specimen. ## Storage Siwaqaite specimens must be stored in dry conditions, preferably in a sealed, lidded box (a "perky box") away from light sources and moisture. Due to its brittleness, avoid placing it in contact with harder minerals.