Fingerite

Chemical formula: Cu²⁺₁₁O₂(V⁵⁺O₄)₆

An extremely rare copper vanadate, known exclusively from one locality worldwide – Izalco volcano in El Salvador.

## Characteristics Fingerite is a mineral of exceptional rarity, occurring as very small, black, tabular or prismatic crystals, not exceeding 0.15 mm. It forms aggregates and crusts on the surface of volcanic rocks. Its crystals exhibit strong pleochroism, changing color from reddish-brown to almost black depending on the observation direction. ## Physical Properties Fingerite crystals have a metallic to submetallic luster. Due to the microscopic size of the crystals, precise determination of hardness and density has been impossible. Density was calculated based on the chemical formula and unit cell parameters, and is approximately 4.65 g/cm³. ## Colors and Varieties This mineral is opaque and black in color. No varieties have been distinguished. ## History and Name Fingerite was discovered in samples collected from fumaroles of Izalco volcano in El Salvador, which erupted between 1964 and 1965. It was described and named in 1985 in honor of Larry W. Finger (1940-2024), an American crystallographer and mineralogist from the Carnegie Institution for Science in Washington, for his contributions to the development of mineral X-ray crystallography at high temperatures and pressures. ## Uses Fingerite has no commercial or industrial applications. Its significance is purely scientific and as a collector's item, as a unique and extremely rare mineral species.

Properties

Color: black
Luster: Sub-Metallic
Streak: dark reddish brown
Density: 0
Cleavage: Perfect on {001}
Transparency: Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Identification of fingerite is primarily based on its unique locality, black color, metallic luster, and association with other rare minerals from volcanic fumaroles. Identification requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical analysis (EDS), due to the microscopic size of the crystals. ## Differentiation from Similar Minerals It can be confused with other black, fine-grained volcanic minerals, such as tenorite. Definitive differentiation is only possible through chemical analysis, which will reveal the presence of vanadium, characteristic of fingerite. ## Crystal Forms Fingerite forms very small, tabular or short-prismatic crystals with a hexagonal cross-section. It occurs as single, isolated crystals or in the form of small clusters and crusts on lava.

Geological environment

## Genesis Fingerite is a mineral of volcanic origin, crystallizing in a high-temperature (approximately 800°C) exhalation (fumarole) environment. It forms as a result of the sublimation of volcanic gases rich in copper and vanadium, which react with atmospheric oxygen and deposit on the surface of volcanic rocks near the crater. ## Mineral Associations This mineral is associated with other rare products of volcanic exhalations, such as tenorite, shcherbinaite, ziesite, bannermanite, stoiberite, and mcbirneyite. ## Localities The only confirmed occurrence of fingerite in the world is its type locality – Izalco volcano in the department of Santa Ana, El Salvador. All known samples originate from this single location.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a fingerite specimen is determined solely by the presence and quantity of crystals visible under a microscope. Since all specimens are microscopic, the main criterion is the richness of clusters and the ability to identify individual crystals on the rock matrix. Every confirmed specimen is extremely valuable from a scientific point of view. ## Market Prices Fingerite practically does not appear on the collector's market. Specimens are found almost exclusively in institutional and research collections. If a microscopic specimen were to enter the market, its price would be very high due to its extreme rarity, but establishing a specific price range is impossible. ## Popular Localities The only source of fingerite specimens is Izalco volcano in El Salvador.

Care and storage

## Cleaning Specimens of fingerite are extremely delicate and sensitive. Mechanical cleaning is absolutely inadvisable. Do not use water or any chemicals, as the mineral is soluble in water and acids. ## What to Avoid Avoid contact with water, moisture, water vapor, and all liquids and chemicals. The mineral is hygroscopic and unstable under atmospheric conditions – contact with humid air leads to its decomposition. It should be protected from temperature changes and direct sunlight. ## Storage Specimens must be stored in airtight, tightly sealed containers (so-called "perky boxes") away from moisture. It is best to store them in a dry environment at a stable temperature. Due to the microscopic size of the crystals, a microscope is essential for observation.