Thometzekite

Chemical formula: Pb₂+Cu₂+₂(As₅+O₄)₂·₂H₂O

Thometzekite is a rare lead-copper arsenate, forming characteristic blue-green, spherical aggregates and crusts.

## Characteristics Thometzekite is a hydrated lead-copper arsenate belonging to the tsumcorite group. It most commonly occurs as microcrystalline crusts, spherical or reniform aggregates, and rosette-like clusters. Individual crystals are very small, usually bladed or tabular, and rarely visible to the naked eye. Its structure and chemical composition are closely related to other minerals from the same group, such as tsumcorite and gartrellite. ## Physical Properties This mineral is characterized by a hardness of 4.5 on the Mohs scale. It has a vitreous luster and is translucent to opaque. Its density is high, approximately 5.56 g/cm³, which is typical for lead-bearing minerals. ## Colors and Varieties Thometzekite occurs in shades from blue to blue-green and green. Its color is a characteristic visual feature. There are no named color varieties or commercial varieties. ## History and Name The mineral was first described in 1985 by Karl Schmetzer, Gerd Tremmel, and Wolfgang Bartelke. The name honors the German mineral collector, Walter Thometzek (1860-1940), who first found samples of this mineral in the Tsumeb mine in Namibia around 1920. These specimens were misidentified and deposited in the mineralogical collection of the University of Heidelberg, where they were re-examined and correctly identified several decades later. ## Uses Thometzekite has no industrial applications. It is solely a mineral of scientific and collector's interest, valued for its rarity and attractive appearance.

Properties

Mohs hardness: 4.5
Luster: Vitreous
Streak: Pale blue-green
Density: 5.56
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Thometzekite is identified by its characteristic blue-green color, spherical or rosette-like aggregates, and high density. It occurs in the oxidation zones of ore deposits, often in association with other rare arsenates. ## Distinguishing from Similar Minerals It can be confused with other secondary copper and lead minerals, such as tsumcorite, gartrellite, azurite, or malachite. - **Tsumcorite**: It is very similar, often co-occurs, and forms series with thometzekite. Differentiation without chemical analysis (EDS/XRD) is practically impossible. - **Gartrellite**: Also belongs to the same group and is visually indistinguishable without advanced analysis. - **Azurite**: Usually has a more intense, azure color and often forms larger, better-formed crystals. - **Malachite**: Always has a green color and often forms fibrous or banded aggregates. ## Crystal Forms Thometzekite crystals are very small, tabular or bladed, with dimensions not exceeding a few tens of micrometers. Most often, they form spherical, radial, or rosette-like aggregates and thin crusts on the host rock.

Geological environment

## Genesis Thometzekite is a secondary mineral, formed in the oxidation zones (gossans) of polymetallic hydrothermal deposits rich in arsenic, lead, and copper. It forms as a result of the weathering of primary ore minerals, such as tennantite, galena, or chalcopyrite. ## Mineral Associations This mineral co-occurs with other secondary arsenates and sulfates. It is most commonly associated with: tsumcorite, gartrellite, anglesite, azurite, malachite, beudantite, cerussite, quartz, tennantite, and galena. ## Localities The most important and classic locality for thometzekite is the Tsumeb mine in Namibia, from which the best-known specimens originate. It has also been found in several other places worldwide, including: - Clara Mine, Wolfach, Black Forest, Germany. - Cap Garonne Mine, Le Pradet, Var, France. - Gara-Djebilet, Tindouf Province, Algeria.

Rarity

Very rare

For collectors

## Quality Criteria The most prized thometzekite specimens are characterized by an intense, blue-green color and well-formed, spherical or rosette-like aggregates. The aesthetic composition with associated minerals (e.g., white cerussite or quartz) and lack of damage are also important. Due to the microscopic size of the crystals, the size of the aggregates and the coverage of the matrix are key factors influencing value. ## Popular Localities Specimens from the type locality – the Tsumeb mine in Namibia – are by far the most sought after by collectors. Material from this mine is considered the global standard for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided, as they can damage delicate aggregates. ## What to Avoid The mineral is sensitive to acids and strong chemicals. As an arsenate containing lead and copper, it is toxic – avoid inhaling dust and wash hands after any contact with the specimen. It should not be heated or exposed to prolonged moisture. ## Storage Thometzekite specimens should be stored in a dry place, in sealed boxes or display cases, to protect them from dust and mechanical damage. They should be kept away from direct sunlight, which can cause fading over time. Due to its toxicity, it should be stored out of reach of children and pets.

Sources

IMA Mineral List (CNMNC)