Thalliumpharmacosiderite

Chemical formula: Tl₁+Fe₃+₄[(As₅+O₄)₃(OH)₄]·₄H₂O

A very rare mineral from the pharmacosiderite group, which is a hydrated thallium and iron arsenate, forming microscopic cubic crystals.

## Characteristics Thalliumpharmacosiderite is a thallium analog of pharmacosiderite, belonging to the same mineral group. It is a hydrated iron and thallium arsenate. It occurs as very small, well-formed cubic crystals, rarely exceeding 0.2 mm in size. These crystals sometimes form small, drusy aggregates on the surface of the host rock. Its appearance is characteristic of minerals from the pharmacosiderite group, but due to the microscopic size of the specimens, magnification is necessary for observation. ## Physical Properties This mineral is characterized by a adamantine luster, which gives its crystals a vibrant appearance despite their small size. It is transparent to translucent. Its density, calculated based on chemical composition and structure, is 3.35 g/cm³. Hardness has not been precisely measured due to the small size of the crystals. ## Colors and Varieties Thalliumpharmacosiderite occurs in shades from greenish-yellow to yellow. No color or commercial varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – the dominance of thallium (Latin: *thallium*) – and its structural similarity to pharmacosiderite. It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2012. It was described by a team of mineralogists: Jakub Plášil, Jiří Sejkora, Jiří Čejka, Radek Škoda, and Vladimír Goliáš. The type locality is the Allchar deposit in North Macedonia. ## Applications Due to its extreme rarity and microscopic size, thalliumpharmacosiderite has no industrial application. Its significance is purely scientific and collectible, being an object of interest for specialized collectors of microscopic minerals (so-called micromounts).

Properties

Luster: Adamantine
Streak: Pale yellow
Density: 3.35
Cleavage: Indistinct on {001}
Transparency: Transparent to translucent
Crystal system: Isometric

Diagnostic features

## Identification Helpful features for identifying thalliumpharmacosiderite include its characteristic cubic crystal form, greenish-yellow color, adamantine luster, and, above all, its locality. Finding a mineral with these characteristics in the Allchar deposit strongly suggests that it may be thalliumpharmacosiderite. ## Distinguishing from Similar Minerals Visual differentiation of thalliumpharmacosiderite from other minerals of the pharmacosiderite group (e.g., pharmacosiderite, barytopharmacosiderite) is impossible without advanced studies. Certain identification is only provided by chemical analysis, e.g., using an electron microprobe (EDS/WDS), which confirms the presence of thallium as the dominant element in the appropriate structural position. ## Crystal Forms It forms exclusively simple, single cubic crystals. These crystals can occur individually or form small, encrusting aggregates and druses.

Geological environment

## Genesis Thalliumpharmacosiderite is a secondary mineral. It forms in the oxidation zone of low-temperature, hydrothermal polymetallic deposits rich in arsenic, antimony, and thallium. ## Mineral Associations At the type locality (Allchar), it occurs in association with other, often rare, thallium-bearing minerals. Its most common associations include lorandite, orpiment, realgar, barite, quartz, and calcite. ## Localities The most important and historically first occurrence of thalliumpharmacosiderite is the Allchar deposit, near the village of Majdan, in the municipality of Kavadarci in North Macedonia. This is the only locality from which well-documented specimens of this mineral originate.

Rarity

Very rare

For collectors

## Quality Criteria The quality of thalliumpharmacosiderite specimens, which are exclusively microminerals, is assessed based on several criteria. Samples with sharp, well-formed, undamaged cubic crystals are most highly valued. The intensity and purity of the color, as well as the richness of the specimen (i.e., the number of crystals on a given surface), are also important. An additional advantage is its association with other rare thallium minerals, such as lorandite. ## Popular Localities For collectors, the only significant source of specimens is the type locality – the Allchar deposit in North Macedonia. Specimens from this locality are the standard for this mineral.

Care and storage

## Cleaning Thalliumpharmacosiderite specimens consist of microscopic and fragile crystals, so mechanical cleaning, including with brushes, should be avoided. Dust can be removed very carefully using a photographic air blower or a can of compressed air, held at an appropriate distance. Cleaning with water or ultrasonics is not recommended. ## What to Avoid The mineral is a compound of thallium and arsenic, which makes it toxic. Direct skin contact should be avoided, and hands should be thoroughly washed after any contact with the specimen. Ingestion and inhalation of dust are strictly prohibited. It should be protected from high temperatures, which could damage its hydrated structure, and from contact with acids and other chemicals. ## Storage It is recommended to store specimens only in specialized, tightly sealed micromineral boxes (so-called micromount boxes). The box should be clearly labeled with the mineral name and information about its toxicity. Protect from dust, moisture, and mechanical damage.

Sources

IMA Mineral List (CNMNC)