Spaltiite

Chemical formula: Tl¹⁺₂Cu¹⁺₂As³⁺₂S²⁻₅

Spaltiite is a rare thallium, copper, and arsenic sulfosalt, forming opaque, grayish-black crystals with a metallic luster.

## Characteristics Spaltiite is a complex thallium, copper, and arsenic sulfosalt. It occurs as very small, tabular or bladed crystals, reaching sizes up to several hundred micrometers. It is an opaque mineral with a metallic luster and a grayish-black color. It typically forms encrusting aggregates on other minerals. ## Physical Properties This mineral is very soft, with a Mohs hardness of 1.5-2, meaning it can be scratched with a fingernail. It exhibits perfect cleavage in one direction. Its luster is described as metallic. ## Colors and Varieties Spaltiite occurs in uniform colors from gray to black. No color varieties or commercial varieties are known. ## History and Name The mineral's name comes from the Swiss mineral collector, Dr. Christoph A. Spalti (born 1961), in recognition of his contribution to the study of minerals from the Lengenbach quarry. Spaltiite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2020, and its discovery was published in 2022. The type locality is the Lengenbach quarry in Switzerland. ## Uses Due to its extreme rarity and microscopic crystal size, spaltiite has no industrial applications. It is solely an object of scientific interest and a collector's item for specialized micromineral collectors.

Properties

Mohs hardness: 1.5-2
Luster: Metallic
Cleavage: perfect cleavage parallel to {100}
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of spaltiite is only possible using advanced analytical methods, such as energy-dispersive spectroscopy (EDS) or X-ray diffraction (XRD), due to its microscopic size and similarity to other sulfosalts. Under collecting conditions, identification relies on a label with confirmed analysis. ## Distinguishing from Similar Minerals Spaltiite is visually indistinguishable from many other rare thallium sulfosalts found in Lengenbach, such as hutchinsonite, wallisite, imhofite, or bernardite. Certain differentiation requires specialized laboratory equipment. ## Crystal Forms It forms very small, tabular or bladed crystals, often grouped into small aggregates. The crystals tend to grow on other minerals, mainly realgar.

Geological environment

## Genesis Spaltiite forms as a result of hydrothermal processes in dolomitic deposits. In its type locality (Lengenbach), it crystallizes in small fissures and cavities within sugary (granular) dolomite. ## Mineral Associations This mineral co-occurs with other rare arsenic and thallium sulfosalts. It is most often found in association with realgar, smithite, hutchinsonite, edenharterite, and jentschite. ## Localities The only confirmed occurrence of spaltiite in the world is its type locality – the Lengenbach quarry, in the Binn Valley, Valais canton, Switzerland. This site is known for the occurrence of many unique and rare minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a spaltiite specimen is determined by the size and development of the microscopic crystals and the abundance of their occurrence on the rock matrix. The most prized specimens are those with clearly visible, though still very small, crystals, preferably in association with other rare minerals from this locality. Contrast with a colorful matrix (e.g., red realgar) also enhances aesthetic value. ## Popular Localities All known spaltiite specimens come from a single location in the world: the Lengenbach quarry in Switzerland. This is the sole source of this mineral for collectors and scientific institutions.

Care and storage

## Cleaning Spaltiite specimens, due to their microscopic size and fragility, should generally not be cleaned mechanically or chemically. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust particles. ## What to Avoid Avoid contact with all chemicals, acids, and detergents. The mineral is very soft, so it should be protected from scratching and any physical contact. It should not be heated or cleaned ultrasonically. ## Storage Microscopic specimens should be stored in specialized "micromount" boxes, which protect them from dust, shocks, and mechanical damage. Avoid exposure to moisture and extreme temperatures.