Crookesite

Chemical formula: Cu¹⁺₇Tl¹⁺Se²⁻₄

Crookesite is a rare copper thallium selenide with a lead-gray color and metallic luster, occurring in massive aggregates.

## Characteristics Crookesite is a rare mineral from the sulfide group, specifically a copper thallium selenide. It forms compact, massive, or granular aggregates, rarely exhibiting any crystalline forms visible to the naked eye. Its appearance is typically metallic, with a characteristic lead-gray color, which on a fresh surface may have a bluish or brownish tint. It is an opaque and rather heavy mineral. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5-3, meaning it can be scratched by a copper wire. It is characterized by a strong, metallic luster. Its density is significant, approximately 6.9 g/cm³, which makes even small specimens noticeably heavy in hand. ## Colors and Varieties Crookesite is a mineral of constant color, always lead-gray. It does not form color varieties or commercial types. ## History and Name The mineral's name honors the British chemist and physicist, Sir William Crookes (1832-1919), who discovered the element thallium (Tl) in 1861, a key component of crookesite. The mineral was first described in 1866 based on specimens found at its type locality – the Skrikerum mine in Sweden. ## Uses Due to its extreme rarity, crookesite has no industrial application. It is solely an object of scientific and collecting interest as a mineral containing the rare element thallium.

Properties

Mohs hardness: 2.5-3
Luster: Metallic
Streak: Black
Density: 6.90
Cleavage: Two at right angles
Fracture: Uneven
Transparency: Opaque
Crystal system: Tetragonal

Diagnostic features

## Identification Crookesite is identified by its lead-gray color, strong metallic luster, high density, and softness. It occurs in massive, compact aggregates, which is its typical characteristic. ## Distinguishing from Similar Minerals It can be confused with other gray, metallic sulfides or selenides, such as galena, chalcocite, or acanthite. It differs from galena by the absence of characteristic, perfect cleavage in three directions. From chalcocite and acanthite, it is usually difficult to distinguish without advanced chemical analyses (EDS/XRD) that confirm the presence of thallium and selenium. ## Crystal Forms Well-formed crystals are practically nonexistent. Crookesite forms almost exclusively massive, amorphous, or fine-grained aggregates embedded in the host rock.

Geological environment

## Genesis Crookesite is a hydrothermal mineral. It forms in low-temperature calcite veins cutting through metamorphic or sedimentary rocks. Its crystallization is associated with processes where solutions rich in selenium, copper, and thallium migrate through rock fissures. ## Mineral Associations It often co-occurs with other selenides, such as berzelianite, eucairite, umangite, and also with calcite, which is the main vein mineral. ## Localities The most important and historical occurrence is its type locality: the Skrikerum mine in Valdemarsvik, Sweden. Besides this, it is known from a few other localities worldwide, including Třídec in the Czech Republic and several places in Argentina and Russia.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of crookesite does not lie in its aesthetics but in its extreme rarity. The most prized specimens are those in which crookesite forms rich, clearly visible aggregates within associated minerals, especially in calcite. Confirmed locality is also important, particularly specimens from the type locality (Skrikerum). ## Popular Localities By far the most sought-after and classic specimens come from Sweden (Skrikerum). Material from other, rare localities is also valuable for specialized collectors of rare minerals.

Care and storage

## Cleaning Crookesite specimens should be cleaned only dry, using a soft brush to remove dust. Contact with water, and especially with chemicals, is not recommended. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral. As a thallium-bearing mineral, caution should be exercised, dust inhalation should be avoided, and hands should be washed after contact with the specimen. It should not be heated or subjected to ultrasound. ## Storage Crookesite is best stored in a closed, dry container or display cabinet, away from moisture and contaminants. Due to its softness, it should be isolated from harder minerals to prevent scratching.