Irhtemite

Chemical formula: Ca₄Mg(As₅+O₄)₂(As₅+O₃OH)₂·₄H₂O

A rare, hydrated calcium and magnesium arsenate, forming characteristic spherical aggregates of white or colorless, bladed crystals.

## Characteristics Irhtemite is a rare mineral from the arsenate group, chemically classified as a hydrated calcium and magnesium arsenate. Its most typical and recognizable form consists of small, spherical aggregates, up to several millimeters in diameter. These aggregates are composed of radially divergent, tiny crystals with a bladed or platy habit. Individual crystals are usually too small to be observed with the naked eye, and the entire clusters have a delicate, hedgehog-like texture. ## Physical Properties Irhtemite crystals exhibit a vitreous luster, and a pearly luster may appear on cleavage surfaces. It is a transparent to translucent mineral. Its Mohs hardness is approximately 3, making it relatively soft and susceptible to scratching. The density is estimated at 3.29 g/cm³. ## Colors and Varieties Irhtemite is most often colorless or white. Some specimens may show a delicate, pinkish tint, likely caused by cobalt impurities, which is common in minerals from its type locality. No named varieties are distinguished. ## History and Name The mineral was first described in 1980 by P. J. Schubnel and M. Pierrot. Its name comes from the discovery site – the Irhtem (Ightem) mine in the Bou Azzer district, Morocco, which is its type locality. ## Uses Due to its rarity, small crystal size, and toxic chemical composition (arsenic content), irhtemite has no industrial application. It is of purely scientific and collector's interest.

Properties

Mohs hardness: 3
Luster: Vitreous, Pearly
Streak: White
Density: 3.29
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Triclinic

Diagnostic features

## Identification The most important diagnostic feature of irhtemite is its characteristic spherical aggregates composed of radially arranged, white or colorless bladed crystals. The origin locality (Bou Azzer, Morocco) is a strong indicator. It reacts with hydrochloric acid. ## Distinguishing from Similar Minerals Irhtemite is sometimes confused with other white, secondary arsenates found in the same environment. - **Talmessite**: Forms similar aggregates, but its crystals are usually better formed, thicker, and often larger. - **Picropharmacolite**: Occurs more often as silky, fibrous aggregates or small needles, less frequently forming spherical aggregates. - **Pharmacolite**: Usually forms acicular and fibrous aggregates, often silky in luster. Final distinction of these minerals often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are very small, flattened, with a bladed or platy habit. They almost always occur as radial or spherical aggregates, which rarely exceed a few millimeters in diameter.

Geological environment

## Genesis Irhtemite is a secondary mineral, formed in the oxidation (weathering) zones of hydrothermal cobalt, nickel, and arsenic ore deposits. It crystallizes from solutions rich in arsenic, calcium, and magnesium under low temperature and pressure conditions. ## Mineral Associations This mineral co-occurs with other secondary arsenates, typical of the Bou Azzer deposit. The most common associated minerals include erythrite, roselite, wendwilsonite, talmessite, as well as calcite, dolomite, and remnants of primary arsenide ores (e.g., skutterudite). ## Localities The most important and practically sole source of well-formed irhtemite specimens is its type locality – the Irhtem mine and other nearby occurrences in the Bou Azzer district, Morocco. This is a classic locality for many rare cobalt and nickel arsenates.

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens by collectors are those presenting well-formed, perfectly spherical aggregates with a distinct, radial structure. Clusters embedded in a contrasting rock matrix are highly prized, especially when accompanied by other colorful minerals such as pink erythrite or roselite. The size of the spheres and the aesthetics of the overall composition are key factors influencing the value of the specimen. ## Popular Localities The only source of specimens of collector's significance is the Bou Azzer district in Morocco, particularly the Irhtem mine, from which the mineral takes its name.

Care and storage

## Cleaning Irhtemite specimens are very delicate and should be cleaned with the utmost care. The safest method is to use a soft brush to remove dust. Compressed air can also be used from a safe distance. Since it is a hydrated mineral, prolonged contact with water should be avoided. If washing is necessary, use distilled water and immediately dry the specimen thoroughly. ## What to Avoid Irhtemite is an arsenate, and thus a toxic mineral. Inhalation of dust should be avoided, and hands should be thoroughly washed after any contact with the specimen. It is soft and brittle, so it should be protected from impacts and scratches. As a hydrated mineral, it is sensitive to high temperatures, which can cause dehydration and destruction of its structure. All chemical agents and acids should be avoided. ## Storage It is recommended to store specimens in closed, padded "micromount" boxes to protect them from dust, mechanical damage, and sudden changes in humidity. Exposure to direct sunlight and temperature fluctuations should be avoided.

Sources

IMA Mineral List (CNMNC)