Machatschkiite

Chemical formula: Ca₆(As₅+O₄)(As₅+O₃OH)₃PO₄·₁₅H₂O

Machatschkiite is a very rare, hydrated calcium arsenate-phosphate, forming colorless or white, tabular crystals with a pearly luster.

## Characteristics Machatschkiite is a very rare mineral from the arsenate and phosphate group. It occurs as small, tabular or bladed crystals, often grouped into rosette-like or radial aggregates. It is usually colorless to white, with a characteristic pearly luster on cleavage surfaces and vitreous on other faces. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5. It is transparent to translucent. Its density is slightly greater than that of quartz, approximately 2.9 g/cm³. It exhibits perfect cleavage in one direction, which is the cause of its pearly luster. ## Colors and Varieties Machatschkiite is usually colorless or white. No colored or commercial varieties have been identified. ## History and Name The mineral was discovered in the Anton mine in Heubachtal (Heubach Valley) in the Black Forest, Germany. It was described in 1964 by Hugo Strunz. The name was given in honor of Felix Karl Ludwig Machatschki (1895-1970), an Austrian mineralogist and crystallographer, professor at the University of Vienna, for his contributions to the crystal chemistry of arsenates and phosphates. ## Uses Machatschkiite has no industrial application. It is of purely scientific and collector's interest due to its extreme rarity.

Properties

Mohs hardness: 2.5
Luster: Pearly, Vitreous
Streak: White
Density: 2.9
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Hexagonal

Diagnostic features

## Identification Machatschkiite is identified by its appearance – small, colorless or white, tabular crystals with a pearly luster, often forming rosette-like aggregates. Key features include low hardness, perfect cleavage, and occurrence in paragenesis with other rare arsenates. ## Distinguishing from Similar Minerals It can be confused with other colorless secondary minerals with a pearly luster, such as gypsum or some micas. It is distinguished from them by its crystal habit (thin tablets in rosettes), occurrence environment (oxidation zones of ore deposits rich in arsenic), and co-occurrence with other arsenates, e.g., pharmacolite. ## Crystal Forms Crystals are thin, tabular or bladed, with a hexagonal outline. They usually occur as small, radial or rosette-like clusters and aggregates, as well as thin coatings and efflorescences.

Geological environment

## Genesis Machatschkiite is a secondary mineral, formed in the oxidation (weathering) zones of hydrothermal ore deposits rich in arsenic, especially in veins containing skutterudite-chloanthite and other nickel and cobalt arsenides. ## Mineral Associations It most commonly co-occurs with pharmacolite, picropharmacolite, sainfeldite, gypsum, erythrite, and annabergite. ## Localities This is an extremely rare mineral. Besides its type locality – the Anton mine in the Heubach Valley in the Black Forest (Germany) – its occurrence has been confirmed in only a few other places worldwide, including Jáchymov (Czech Republic) and the Sterling Hill mine in Ogdensburg, New Jersey (USA).

Rarity

Extremely rare

For collectors

## Quality Criteria The most prized specimens are those with well-formed, sharp crystals forming aesthetic, rosette-like aggregates on a contrasting rock matrix. Crystal size is also important – specimens with crystals exceeding a few millimeters are exceptionally rare and sought after. Purity (lack of damage) and good association with other rare minerals, such as sainfeldite, significantly increase the value of the specimen. ## Popular Localities The best and most well-known specimens come from the type locality in the Black Forest, Germany. These are considered classic and serve as a benchmark for this mineral.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its low hardness and perfect cleavage, washing in water, and especially mechanical and ultrasonic cleaning, should be avoided. ## What to Avoid Avoid contact with water and chemicals that can damage the delicate crystals. The mineral is sensitive to heat and can dehydrate (lose water from its crystal lattice), leading to its destruction. It should be protected from high temperatures and direct sunlight. ## Storage It is recommended to store specimens in stable conditions, in closed, padded boxes or display cases, to protect them from mechanical damage, dust, and humidity changes. It should not be stored together with harder minerals.

Sources

IMA Mineral List (CNMNC)