Dickite
Chemical formula: Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>
Dickite is a clay mineral of the kaolinite group, forming compact, earthy aggregates or rare, pearly crystals in the form of pseudohexagonal plates.
Properties
- Mohs hardness
- 2-2.5
- Luster
- Pearly to dull
- Streak
- White
- Density
- 2.6
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque, transparent in thin flakes
- Crystal system
- Monoclinic
Diagnostic features
## Identification Helpful identification features include very low hardness (can be scratched with a fingernail), perfect cleavage in one direction, pearly luster on cleavage planes, and a white streak. Occurrence in hydrothermal environments is also an important clue. ## Distinguishing from Similar Minerals Macroscopic differentiation of dickite from other minerals of the kaolinite group (kaolinite, nacrite) is practically impossible without advanced studies, such as X-ray diffraction (XRD). Talc is similarly soft but feels distinctly greasy (soapy) to the touch, whereas dickite is rough or earthy. Brucite, also soft and platy, occurs under different geological conditions (metamorphic rocks). ## Crystal Forms Dickite crystallizes in the monoclinic system, but its crystals almost always appear as hexagonal (pseudohexagonal) plates. These microscopic plates often combine into larger aggregates, resembling stacks of sheets, books, or fans. Most often, however, it is found in the form of compact, microcrystalline clay masses.
Geological environment
## Genesis Dickite is a mineral of hydrothermal origin, forming at low to moderate temperatures. It forms as a result of the alteration of other aluminosilicate minerals (e.g., feldspars) by aqueous solutions circulating through rocks. It is a typical component of ore veins, fractures, and cavities in sandstones and volcanic rocks. ## Mineral Associations It often co-occurs with quartz, kaolinite, pyrite, marcasite, barite, fluorite, dolomite, and calcite. ## Localities The most important and historical localities from which well-formed dickite crystals originate include the San Juan Mountains region in Colorado (USA), St. George in Utah (USA), and numerous sites in China, from which transparent specimens originate. The type locality is Pant-y-Gaseg, Anglesey Island, Wales (UK). It also occurs in many mining regions worldwide as an associated mineral.
Rarity
Not very common
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp crystals with a pseudohexagonal outline. Aesthetic aggregates in the form of "books" or "fans" are also highly prized, especially when set on a contrasting rock matrix. Desirable features include high luster, purity (snow-white color or attractive transparency), and crystal size. ## Popular Localities The highest quality collector specimens primarily come from China (Hunan and Sichuan provinces), from which transparent, yellowish crystals are known, and from classic localities in the USA, especially in Colorado, which have yielded many museum-quality aggregates of white, pearly crystals.
Care and storage
## Cleaning Dickite is a very delicate mineral and sensitive to water. To remove dust, use only a soft, dry brush. Avoid contact with water, as it can penetrate the mineral's structure, causing it to swell or disintegrate, especially in the case of loosely structured specimens. ## What to Avoid Ultrasonic cleaners, detergents, acids, and all other chemicals should be absolutely avoided. The mineral is very soft, so it must be protected from impacts and scratching. It should also not be exposed to high temperatures or prolonged humidity. ## Storage Dickite specimens are best stored in closed, padded boxes to protect them from dust, moisture, and mechanical damage. It is not sensitive to sunlight, but stable environmental conditions are crucial for its preservation.