Chalcophyllite

Chemical formula: Cu²⁺₁₈Al₂(As⁵⁺O₄)₄(S⁶⁺O₄)₃(OH)₂₄·₃₆H₂O

Chalcophyllite is a hydrated copper and aluminum arsenate and sulfate, distinguished by its intense emerald-green color and platy, micaceous crystals.

## Characteristics Chalcophyllite is a secondary mineral from the arsenate group, valued for its striking, emerald-green or bluish-green color. It forms characteristic thin, tabular crystals with a hexagonal outline, which often arrange themselves into rosette-like or stellate aggregates. Specimens typically have the appearance of delicate, micaceous flakes, which is related to its perfect cleavage. The luster is vitreous, and on perfectly flat crystal faces, it approaches subadamantine. ## Physical Properties It is a very soft mineral, with a Mohs hardness of just 2, making it extremely susceptible to mechanical damage. Its density is approximately 2.67 g/cm³, so it is relatively light. Crystals are transparent to translucent. Due to its delicacy, it is exclusively a collector's mineral. ## Colors and Varieties The dominant color of chalcophyllite is an intense green in shades from emerald to grass-green and bluish-green. In transmitted light, it also remains green. No named varieties are distinguished, and its collector's value is directly related to the intensity and quality of its color. ## History and Name The mineral's name, given in 1847 by August Breithaupt, comes from the Greek words *chalkos* (χαλκός) meaning "copper" and *phyllon* (φύλλον) meaning "leaf," which is a direct reference to its chemical composition and its platy, leafy form. It was described based on specimens from Cornwall, England, which is its type locality. ## Uses Chalcophyllite has no industrial applications. It is a prized and sought-after collector's mineral due to its beauty and the rarity of well-formed crystals.

Properties

Mohs hardness: 2
Luster: Vitreous to Sub-adamantine
Streak: Pale green to bluish green
Density: 2.67
Cleavage: Perfect on {0001}, micaceous; in traces on <mi>{10_11}</mi>.
Fracture: Micaceous
Transparency: Transparent,Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Chalcophyllite is most easily recognized by its characteristic appearance: thin, tabular crystals with a hexagonal outline, often gathered in rosette-like aggregates. Key features also include its intense emerald-green color, very low hardness (can be scratched with a fingernail), and perfect, micaceous cleavage in one direction. The streak is pale green or bluish-green. ## Distinguishing from Similar Minerals Chalcophyllite is sometimes confused with other secondary copper minerals, such as tyrolite, which has a similar form and color but usually a lighter shade and pearly luster. It is distinguished from torbernite by its lack of radioactivity. In rare cases, it may resemble micaceous varieties of annabergite or erythrite, but these have different colors (green and pink, respectively) and chemical compositions. ## Crystal Forms Crystals are thin-tabular, with a hexagonal base, resembling mica flakes. They usually occur as fan-shaped, rosette-like, or stellate aggregates. More rarely, it forms druzes of small, glistening crystals or massive, platy aggregates.

Geological environment

## Genesis Chalcophyllite is a secondary mineral, formed in the oxidation (weathering) zones of copper ore deposits, especially those that also contain arsenic. It forms as a result of surface waters acting on primary copper sulfides and arsenides in the presence of aluminum minerals. ## Mineral Associations It often co-occurs with other secondary copper minerals. Its typical associated minerals include: azurite, malachite, brochantite, cuprite, chrysocolla, as well as cornwallite, olivenite, tyrolite, spangolite, and tenorite. ## Localities The most important historical and classic localities are in Cornwall (England, United Kingdom), which is its type locality. Beautiful specimens also come from the Majuba Hill mine in Nevada and the Tintic district in Utah (USA). Other known localities include mines near Copiapó in Chile, and in Europe, the Ore Mountains (both on the German and Czech sides) and the Lavrion area in Greece.

Rarity

Rare

For collectors

## Quality Criteria The most highly prized chalcophyllite specimens are characterized by sharply defined, well-formed crystals of large (for this mineral) size, forming aesthetic, rosette-like or stellate aggregates. An intense, emerald-green color without discoloration is crucial. The matrix is also important – specimens on a contrasting rock matrix are more desirable. Purity, meaning the absence of damage to the delicate flakes, significantly increases the value of the specimen. ## Popular Localities Specimens from the historic mines in Cornwall, England, are considered classic and most highly valued. In recent decades, excellent specimens from the Majuba Hill mine in Pershing County, Nevada (USA), have dominated the market, often setting the quality standards for this mineral.

Care and storage

## Cleaning Chalcophyllite specimens are extremely delicate and must be handled with the utmost care. Cleaning should be kept to an absolute minimum. Only gentle brushing with a soft brush or careful use of compressed air from a safe distance is permissible. Contact with water, especially warm water, is inadvisable, as the mineral is partially soluble in it. ## What to Avoid Contact with water, acids, detergents, and all chemicals must be strictly avoided. The mineral is very soft (hardness 2), so any harder material can scratch it. It is sensitive to heat, which can lead to its dehydration and destruction. It should be protected from direct sunlight, which can cause fading. ## Storage Chalcophyllite requires storage in stable conditions, away from sources of heat, moisture, and light. It is best kept in a closed, padded display box or cabinet to protect it from dust and mechanical damage. Due to its fragility, it should not be stored in contact with other, harder minerals.