Mcconnellite
Chemical formula: Cu<sup>1+</sup>Cr<sup>3+</sup>O<sub>2</sub>
Mcconnellite is a very rare copper chromium oxide, forming characteristic, thin, hexagonal plates with a metallic luster.
Description
## Characteristics Mcconnellite is a copper chromium oxide belonging to the delafossite group. Its most typical and recognizable form consists of extremely thin, tabular crystals with a distinct hexagonal outline. Specimens are intensely black and exhibit a strong metallic luster on crystal surfaces. It usually occurs as small, individual flakes or small aggregates grown on the host rock. Due to its habit and small size, it is primarily a mineral of interest to collectors specializing in microminerals. ## Physical Properties The hardness of mcconnellite on the Mohs scale is 5.5, making it a relatively hard mineral. It is characterized by a strong metallic luster and is completely opaque. Its density, calculated based on its chemical composition and crystal structure, is approximately 5.46 g/cm³. ## Colors and Varieties This mineral is monochromatic and occurs exclusively in black. It does not form color varieties or commercial grades. ## History and Name Mcconnellite was first described in 1967 by S. A. Williams and Paul Brian Moore. The mineral is named in honor of Richard B. McConnell (1903-1990), director of the Geological Survey of British Guiana (now Guyana), in recognition of his contributions to the geology of the region. ## Applications Due to its extreme rarity and small crystal size, mcconnellite has no industrial applications. It holds only scientific significance and is a prized acquisition in advanced collections of systematic minerals and microminerals.
Diagnostic features
## Identification The key diagnostic feature of mcconnellite is its form – thin, tabular crystals with a perfectly hexagonal outline. Other important features include black color, strong metallic luster, black streak, and perfect cleavage in one plane. Its occurrence in association with other copper and chromium minerals is also an important clue. ## Distinguishing from Similar Minerals Mcconnellite can be confused with other black, platy minerals: - **Graphite**: It is much softer (hardness 1-2) and leaves a mark on paper. - **Molybdenite**: It has a similar habit but is much softer, flexible, and has a grayish-green streak. - **Hematite** (in tabular variety): Usually has a reddish-brown streak and forms thicker crystals. ## Crystal Forms Mcconnellite crystallizes in the form of thin, flat tablets with a hexagonal outline. Crystals rarely exceed a few millimeters in diameter. They can occur individually or form small, loose aggregates on the surface of the host rock.
Geological environment
## Genesis Mcconnellite is a secondary mineral, formed under specific conditions. It forms in the oxidation zones of ore deposits, especially in quartz-gold veins, where copper and chromium are concentrated. It is also found in altered (metamorphosed) massive sulfide deposits. ## Mineral Associations This mineral often co-occurs with other rare and common minerals of the oxidation zone. In the type locality, it was found in association with native gold, wauquelinite, fornacite, malachite, and derriksite. ## Localities It is a very rare mineral, known from only a few localities worldwide. The most important of these are: - **Mina da Passagem**, Mariana, Minas Gerais, Brazil – type locality. - **Bisbee**, Cochise County, Arizona, USA – known for excellent microcrystals. - **Tsumeb Mine**, Namibia. - **Kambalda**, Western Australia.
Rarity
Very rare
Collector aspects
## Quality Criteria The most highly prized mcconnellite specimens are those with sharp, well-formed, hexagonal crystals with a mirror-like luster. Crystal size is important, although they rarely exceed a few millimeters. The attractiveness of a specimen is enhanced by its placement on a contrasting rock matrix and aesthetic associations with other colorful secondary minerals. ## Popular Localities In the collector's market, specimens from the historic locality in Bisbee (Arizona, USA) and from the discovery site in Brazil are most sought after. Specimens from these localities are considered classic for this species.
Care and storage
## Cleaning Mcconnellite specimens are usually small and delicate. For dust removal, it is best to use compressed air from a safe distance or a very soft brush (e.g., squirrel hair). Wet cleaning, and especially the use of any chemical agents, should be avoided. ## What to Avoid The mineral is sensitive to acids. Ultrasonic cleaners should be absolutely avoided, as they could damage the fragile flakes. Due to its perfect cleavage, specimens are susceptible to mechanical damage – they should be protected from impacts, scratches, and falls. ## Storage The safest storage method is to place the specimen in a sealed "micromount" box. This protects the mineral from dust, moisture, and, most importantly, from mechanical damage during handling.