Geminite

Chemical formula: Cu₂+(As₅+O₃OH)·H₂O

A hydrated copper arsenate, a rare secondary mineral of intense emerald-green color, forming characteristic twinned crystals.

## Characteristics Geminite is a hydrated copper arsenate, belonging to the group of secondary minerals. It occurs as small, slender crystals with a prismatic or acicular habit. These crystals often form radial or divergent aggregates, as well as small crusts. A distinguishing feature of geminite is its vivid, intense color, most often emerald-green, sometimes with a bluish tint. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3. It is characterized by a vitreous luster and is transparent to translucent. It is a brittle mineral, exhibiting perfect cleavage in one direction. ## Colors and Varieties The color of geminite is its most characteristic feature – it ranges from bright, emerald green to greenish-blue. No named color varieties or commercial varieties are distinguished. ## History and Name Geminite was first described in 1990 by Halil Sarp and Pierre Perroud. Its name comes from the Latin word *gemini*, meaning "twins," which refers to the very common and characteristic twinning of crystals for this mineral. The type locality (locus typicus) is the famous Cap Garonne mine in the Var department of France. ## Uses Due to its rarity and occurrence as small crystals, geminite has no industrial application. It is of purely scientific importance and is a valued acquisition for advanced systematic mineral and micromount collections.

Properties

Mohs hardness: 3
Luster: Vitreous
Streak: Pale green
Density: 3.65
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Triclinic

Diagnostic features

## Identification Key diagnostic features of geminite are its intensely emerald-green color, acicular or slender prismatic crystal habit, vitreous luster, and frequent occurrence in radial aggregates. The environment of occurrence (oxidation zone of copper and arsenic ore deposits) and associated minerals are also important clues. ## Distinguishing from Similar Minerals Geminite is sometimes confused with other green, secondary copper minerals: - **Olivenite**: Usually has a more olive-green hue and a different crystal habit (short prismatic or fibrous). - **Euchroite**: Has a similar color but crystallizes in the orthorhombic system, forming thicker, prismatic crystals. It is also slightly harder (3.5-4). - **Tyrolite**: Often exhibits a pearly luster on cleavage surfaces and forms characteristic fan-shaped aggregates. - **Cornwallite**: Usually occurs as radially fibrous aggregates or amorphous forms, rarely forming distinct crystals. ## Crystal Forms Geminite crystals are slender, prismatic, or acicular, often elongated along one axis. They form radial, divergent aggregates, as well as sheaf-like forms. Twinning, from which the mineral takes its name, is very common and characteristic.

Geological environment

## Genesis Geminite is a rare secondary mineral, formed in the oxidation (weathering) zones of polymetallic hydrothermal deposits. It forms as a result of the alteration of primary copper and arsenic sulfides, such as tennantite, under the influence of oxygen-rich surface waters. ## Mineral Associations It most often co-occurs with other rare copper arsenates, especially at the type locality. Its typical associated minerals include: olivenite, cornwallite, cornubite, chalcophyllite, chenevixite, as well as azurite, malachite, and tennantite. ## Localities The most important and best-known worldwide locality for geminite, from which the best specimens originate, is the Cap Garonne mine in Le Pradet (Var, France). Apart from this locality, its presence has been recorded in only a few other places in the world, including the Jote mine in Chile, but specimens from these sites are extremely rare.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized geminite specimens are characterized by sharp, well-formed, and transparent crystals with a deep, emerald-green color. The value of a specimen is enhanced by the abundance of crystals on the rock matrix and visually attractive associations with other rare, colorful minerals, such as chalcophyllite or cyanotrichite. ## Popular Localities The Cap Garonne mine in France is undoubtedly the most important source of geminite for collectors. Specimens from this location are considered the global standard for this mineral.

Care and storage

## Cleaning Geminite specimens should be cleaned very carefully, preferably dry using a soft brush to remove dust. If necessary, distilled water can be used, avoiding a strong stream. Ultrasonic cleaners should not be used, as they can damage brittle crystals. ## What to Avoid As an arsenate, geminite is a toxic mineral. Avoid inhaling dust and always wash hands after contact with a specimen. It is soft and susceptible to scratches. It should be protected from contact with acids and other aggressive chemicals. Sudden temperature changes can be harmful. ## Storage It is recommended to store specimens in closed, padded boxes (e.g., "micromount" type) to protect them from mechanical damage, dust, and moisture. Ensure the specimen is clearly labeled with information about its toxicity.

Sources

IMA Mineral List (CNMNC)