Barlowite

Chemical formula: Cu₂+₄BrF(OH)₆

Barlowite is an extremely rare copper mineral, forming characteristic, tabular crystals with a hexagonal outline and a dark green color.

## Characteristics Barlowite is a rare copper hydroxyhalide, prized for its perfectly formed, hexagonal crystals. It typically forms thin to thick, tabular crystals with sharp edges, which can occur singly, in parallel intergrowths, or form attractive, rosette-like aggregates. Its color is intensely dark green, often so dark that it appears almost black. This mineral is brittle and requires careful handling. ## Physical Properties The Mohs hardness of barlowite is approximately 2.5, making it a soft mineral. It has a vitreous luster, transitioning to submetallic on crystal faces. It is opaque, and only in very thin fragments does it become translucent, revealing its green color. Its density, calculated from the formula and cell parameters, is 4.21 g/cm³. It exhibits perfect cleavage in one plane, parallel to the base of the hexagonal crystals. ## Colors and Varieties This mineral occurs in a uniform, dark green color, which, depending on the lighting and crystal thickness, can appear black. No named color varieties or commercial varieties are distinguished. ## History and Name Barlowite was described as a new mineral species in 2009 by a team of mineralogists led by Anthony R. Kampf. Its name honors Alfred "Fred" Barlow (1940-2016), a renowned American mineral collector from Arizona, who specialized in minerals from that region and provided samples for research. The type locality (locus typicus) is Otto Mountain in San Bernardino County, California, USA. ## Applications Due to its extreme rarity and small crystal sizes, barlowite has no industrial applications. It is solely an object of scientific interest and a highly valued specimen in advanced systematic and regional mineral collections.

Properties

Mohs hardness: 2.5
Luster: Vitreous to Sub-Metallic
Streak: Light green
Density: 4.21
Cleavage: Perfect on {0001}
Transparency: Translucent to Opaque
Crystal system: Hexagonal

Diagnostic features

## Identification The most important diagnostic feature of barlowite is its characteristic habit in the form of sharply defined, hexagonal tablets. The dark green color, vitreous to submetallic luster, and perfect cleavage in one direction are also key. Occurrence in a specific environment of secondary copper minerals is a strong indicator. ## Distinguishing from Similar Minerals Barlowite can be confused with other tabular, green copper minerals, such as claringbullite, with which it often co-occurs. However, definitive differentiation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS), due to subtle differences in composition and structure. It is distinguished from dark mica varieties by its significantly higher density and hexagonal (rather than pseudohexagonal) habit. ## Crystal Forms Barlowite crystallizes in the form of thin to thick, hexagonal plates and tablets. These crystals often arrange into rosette-like or fan-like aggregates, and also form parallel intergrowths, where smaller crystals grow on larger ones while maintaining the same crystallographic orientation.

Geological environment

## Genesis Barlowite is a secondary mineral, forming in the oxidation zone of copper deposits. Its formation requires specific geochemical conditions, namely the presence of bromine and fluorine. At the type locality, it formed in fractures of trachyte and andesite as a result of the weathering of primary copper sulfides under the influence of saline, halide-rich waters. ## Mineral Associations This mineral co-occurs with other rare, secondary copper minerals. At the type locality on Otto Mountain, its most important associated minerals are claringbullite, atacamite, hematite, goethite, and other, often yet undescribed, copper halides. ## Localities The most important and practically sole source of well-formed barlowite specimens is its type locality – Otto Mountain in the Silver Lake Mining District, San Bernardino County, California, USA. This site is known for the occurrence of many rare secondary minerals.

Rarity

Extremely rare

For collectors

## Quality Criteria The most highly valued barlowite specimens are those with sharply formed, undamaged crystals with a distinct hexagonal outline and good luster. Visually attractive, rosette-like aggregates of crystals embedded on a small rock matrix are particularly sought after. Crystal size is a key factor – specimens with tablets exceeding a few millimeters are already considered exceptional. The purity of the specimen and the absence of mechanical damage (chipping resulting from cleavage) significantly increase its value. ## Popular Localities The only locality from which barlowite specimens of collector's significance originate is Otto Mountain in California, USA. Specimens from this locality are considered the global standard for this mineral.

Care and storage

## Cleaning Barlowite is an extremely delicate, soft mineral with perfect cleavage. It should only be dry-cleaned, using a soft brush or a gentle stream of compressed air to remove dust. Any contact with water or cleaning fluids is highly inadvisable. ## What to Avoid Ultrasonic cleaners, all chemicals (acids, solvents), and contact with water, which can initiate chemical processes on the crystal surfaces, should be absolutely avoided. The mineral is sensitive to impact and scratches—even a fingernail can damage it. It should be protected from sudden temperature changes. ## Storage Barlowite specimens are best stored in separate, padded "micromount" boxes to prevent contact with other minerals and protect them from mechanical damage. Storage in stable conditions with low humidity is recommended.

Sources

IMA Mineral List (CNMNC)