Markwelchite

Chemical formula: Tl₁+Pb₂+Sb₃+S₂-₃

Markwelchyte is a rare lead, thallium, and antimony sulfosalt, forming acicular crystals with a metallic luster.

## Characteristics Markwelchyte is a rare mineral from the sulfosalt group, a chemically complex sulfide of lead, thallium, and antimony. It occurs as very small, acicular or hair-like crystals, which can form tangled aggregates. Individual crystals are elongated and striated along their long axis. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties This mineral is characterized by a metallic luster and is opaque. Its hardness and density have not been precisely determined due to the small size and rarity of samples suitable for standard tests. ## Colors and Varieties Markwelchyte has a steel-gray to black color. No varieties have been distinguished. ## History and Name The mineral's name honors Mark Welch (born 1956), an American mineral collector and philanthropist from Scottsdale, Arizona, in recognition of his contributions to sulfosalt research. Markwelchyte was approved as a new mineral species by the International Mineralogical Association (IMA) in 2017 (IMA number 2017-039). It was discovered in the Lengenbach quarry in the Binn Valley (Binntal) in Switzerland. ## Uses Markwelchyte has no industrial applications. It is solely an object of scientific and collecting interest, sought after by specialists collecting rare minerals and sulfosalts, especially those from the classic Lengenbach locality.

Properties

Luster: Metallic
Streak: Black
Cleavage: Good on {10-2}
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of markwelchyte is primarily based on chemical analysis (EDS) to confirm the presence of lead, thallium, and antimony. Visually, it is extremely difficult due to its resemblance to many other acicular sulfosalts. Identification requires specialized laboratory equipment. ## Distinguishing from Similar Minerals Markwelchyte is practically impossible to distinguish with the naked eye from other sulfosalts found in Lengenbach, such as hutchinsonite, wallisite, imhofite, or other rare lead and thallium minerals. Definitive differentiation is only possible using advanced analytical methods, such as X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). ## Crystal Forms It forms very fine, acicular or hair-like crystals, usually not exceeding 1 mm in length. These crystals often occur as tangled or radial clusters and aggregates, growing on other minerals, most often on realgar or in dolomite vugs.

Geological environment

## Genesis Markwelchyte forms as a result of hydrothermal processes in ore deposits. In its type locality (Lengenbach), it crystallizes under low-temperature conditions in veins cutting dolomitic metamorphic rocks (dolomitic marbles). ## Mineral Associations This mineral co-occurs with other sulfosalts and sulfides. In the Lengenbach quarry, it has been found in association with realgar, orpiment, hutchinsonite, edenharterite, jentschite, and other rare thallium and lead minerals. ## Localities The only confirmed and described occurrence of markwelchyte in the world is its type locality: the Lengenbach quarry in the Binn Valley (Binntal), in the canton of Valais in Switzerland. This site is known for the occurrence of many unique and rare sulfosalts.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a markwelchyte specimen depends on the richness and aesthetics of the acicular crystal aggregates. The most prized specimens are those where the crystals are well-formed, create dense but not tangled aggregates, and are set on a contrasting matrix, for example, on red realgar or white dolomite. Due to the microscopic nature of the mineral, the quality of the photograph taken through a microscope is crucial. ## Popular Localities The only source of specimens is the Lengenbach quarry in Switzerland. Material from this location is highly valued by collectors specializing in systematic minerals and so-called "micromounts."

Care and storage

## Cleaning Due to the extreme delicacy of the acicular crystals, mechanical cleaning is not recommended. If necessary, compressed air can be used from a safe distance to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Avoid ultrasonic cleaners, steam cleaners, acids, solvents, and sudden temperature changes. The mineral is very brittle and sensitive to mechanical damage. As a sulfide, it may be susceptible to oxidation in humid environments. ## Storage Specimens should be stored under stable conditions, in a closed "micromount" box, to protect them from dust, moisture, and physical damage. Avoid vibrations and shocks.

Sources

IMA Mineral List (CNMNC)