Montroseite

Chemical formula: (V₃+,Fe₂+,V₄+)O(OH)

Montroseite is a rare vanadium and iron oxyhydroxide, forming black, submetallic aggregates in uranium-vanadium deposits.

## Characteristics Montroseite is a mineral with a black color and submetallic luster. It most commonly occurs as massive, granular, or earthy aggregates. Less frequently, it forms small, elongated or acicular crystals, which can create radial or fan-shaped aggregates. Due to its appearance, it is sometimes mistaken for other black ore minerals. ## Physical Properties The mineral's hardness on the Mohs scale is approximately 5.5. It is relatively dense, with a specific gravity ranging from 4.0-4.16 g/cm³. It is characterized by a submetallic luster and is opaque. ## Colors and Varieties The mineral is uniformly black. No color varieties are distinguished. In air, it readily oxidizes and transforms into paramontroseite, which is a pseudomorph after montroseite. This process does not always involve a change in color but alters the crystal structure. ## History and Name The name montroseite comes from its discovery locality – Montrose County in Colorado, USA. The mineral was first described in 1949 by Alice D. Weeks, E. A. Cisney, and A. M. Sherwood. ## Applications As a vanadium-bearing mineral, montroseite is significant as a local ore of this metal. It is also an object of interest for collectors specializing in rare minerals or minerals from uranium-vanadium deposits.

Properties

Mohs hardness: 5.5
Luster: Submetallic
Streak: Black to brownish-black
Density: 4.0-4.16
Cleavage: Perfect on {010}, good on {100}
Fracture: Uneven
Transparency: Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features include black color, submetallic luster, relatively high density, and occurrence in a specific geological environment – in unoxidized zones of uranium-vanadium deposits in sandstones. ## Distinguishing from similar minerals Montroseite is difficult to distinguish from its alteration product – paramontroseite – without X-ray diffraction (XRD) analysis. It can also be confused with other black ore minerals, such as uraninite or corvusite. Differentiation requires analysis of associated minerals and geological context. ## Crystal forms It crystallizes in the orthorhombic system. It forms very small, prismatic or acicular crystals, often gathered in radial aggregates. However, it is most commonly found as massive and granular aggregates.

Geological environment

## Genesis Montroseite is a primary vanadium mineral, forming under reducing conditions. It forms in unoxidized parts of roll-front type uranium-vanadium deposits found in sandstones. It precipitates from low-temperature hydrothermal solutions. ## Mineral associations It co-occurs with other vanadium and uranium minerals, such as carnotite, tyuyamunite, corvusite, hewettite, as well as uraninite, coffinite, pyrite, and clay minerals from the montmorillonite group. ## Localities The main and best-known occurrences of montroseite are in the USA, on the Colorado Plateau (states of Colorado, Utah, Arizona, New Mexico), especially in the Uravan Mineral Belt. Outside the USA, its occurrences have been reported in Argentina and Gabon, among others.

Rarity

Rare

For collectors

## Quality criteria The most valued by collectors are specimens with well-formed, even microscopic, crystals forming radial aggregates on a rock matrix. The attractiveness of a specimen is also enhanced by association with color-contrasting minerals, e.g., yellow carnotite. ## Popular localities Classic and best-quality specimens come from mines in the Uravan district in Montrose and Mesa counties in Colorado and San Juan in Utah, USA.

Care and storage

## Cleaning Specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, especially with chemicals, can accelerate the oxidation process. ## What to avoid Moisture, high temperatures, and contact with acids and other chemicals should be avoided. The mineral is susceptible to oxidation and transformation into paramontroseite, so it should be protected from changes in atmospheric conditions. It is also brittle and sensitive to mechanical damage. ## Storage It is recommended to store specimens in a dry place, preferably in closed, padded boxes or display cases, to limit air and moisture access and protect against damage.

Sources

IMA Mineral List (CNMNC)