Talmessite

Chemical formula: Ca₂Mg(As⁵⁺O₄)₂·₂H₂O

Talmessite is a rare calcium and magnesium arsenate, forming small, vitreous crystals, valued by collectors for its variety of colors.

## Characteristics Talmessite is a hydrated calcium and magnesium arsenate belonging to the collinsite group. It most commonly occurs as small, tabular or prismatic crystals, which can form radial aggregates, rosette-like clusters, or crusts. Specimens typically have a vitreous luster and are translucent. ## Physical Properties Its Mohs hardness is 5, and its density is approximately 3.42 g/cm³. The mineral is characterized by a vitreous luster and a white streak. It is translucent. ## Colors and Varieties Talmessite is most often colorless or white. The presence of impurities of other elements leads to the formation of colored varieties. Small amounts of nickel give it a pale green color, and cobalt imparts a pinkish to brownish-pink hue. The cobalt-bearing variety is sometimes referred to as cobaltian talmessite. ## History and Name The mineral was first described in 1960. Its name comes from its discovery locality – the Talmessi mine in the Anarak district, Iran, which is its type locality. ## Uses Talmessite has no industrial applications. It is a sought-after mineral valued by collectors specializing in rare or arsenate minerals.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 3.421
Cleavage: Imperfect/poor
Fracture: Uneven
Transparency: Translucent
Crystal system: Triclinic

Diagnostic features

## Identification Talmessite can be identified by its characteristic crystal forms – tabular or prismatic, often forming radial aggregates. Its vitreous luster, hardness (5 on the Mohs scale), and occurrence in association with other arsenates are key features. Colored varieties (greenish from nickel, pinkish from cobalt) are very characteristic. ## Distinguishing from Similar Minerals It can be confused with other minerals from the collinsite group, such as collinsite or gaiite, from which differentiation requires advanced chemical analyses (EDS/XRD). The pink variety may resemble erythrite, but erythrite is significantly softer (1.5-2.5) and has perfect cleavage. The greenish variety is sometimes confused with annabergite, which is also softer. ## Crystal Forms Crystals are usually small, tabular or short-prismatic. They often form fan-shaped, radial, or spherulitic aggregates, as well as massive forms and crusts on the host rock.

Geological environment

## Genesis Talmessite is a secondary mineral, forming in the oxidation zones (weathering) of ore deposits rich in arsenic, especially in carbonate rocks (dolomites, limestones). It forms as a result of reactions between arsenic-bearing solutions and calcium and magnesium minerals. ## Mineral Associations It often co-occurs with other arsenates, such as annabergite, erythrite, austinite, pharmacolite, as well as with calcite, dolomite, and iron oxides (e.g., goethite). ## Localities The most important and type locality is the Talmessi mine in Iran. It is also known from the Aghbar mine in the Bou Azzer region of Morocco, where prized pink, cobaltian varieties occur. Other reported localities include Jáchymov in the Czech Republic, and some deposits in Germany (Saxony) and the USA (Nevada).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp crystals creating aesthetic aggregates, especially radial rosettes. The intensity and purity of color significantly increase value – particularly sought after are vivid pink cobaltian varieties from Morocco and greenish nickelian varieties. Contrast with the matrix rock and lack of damage are also important. ## Popular Localities The most famous and desirable specimens come from two locations: the Talmessi mine in Iran (type locality) and the Bou Azzer region in Morocco, which yields the world's best cobaltian talmessites.

Care and storage

## Cleaning Talmessite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. Always dry the specimen thoroughly after contact with water. ## What to Avoid As an arsenate, talmessite is toxic – avoid inhaling dust and wash hands after any contact with the mineral. It should not be heated or exposed to acids and other chemicals. Protect it from prolonged exposure to direct sunlight, which can cause colored varieties to fade. ## Storage It is recommended to store specimens in closed, stable containers or display cases, away from dust and moisture. Due to its toxicity, it should be kept out of reach of children and pets. It is advisable to include a label with a warning about its arsenic content.