Postite

Chemical formula: MgAl₂(OH)₂[V₅+₁₀O₂₈]·₂₇H₂O

Postite is a very rare, hydrated magnesium aluminum decavanadate, forming microscopic, yellow tabular crystals.

## Characteristics Postite is a mineral from the vanadate group, specifically a magnesium aluminum decavanadate. It occurs as extremely small, tabular or bladed crystals, rarely exceeding 0.2 mm in length. These crystals are typically transparent and have an intense yellow color. Due to their microscopic size, observing morphological features requires specialized equipment. ## Physical Properties Postite crystals exhibit a vitreous luster. Hardness is too low to be precisely measured due to the size and fragility of the crystals, but it is estimated to be around 2 on the Mohs scale. The density calculated based on the formula and unit cell parameters is 2.31 g/cm³. The mineral is brittle. ## Colors and Varieties Postite is characterized by a uniform, light yellow to intense yellow color. No color or commercial varieties are known. ## History and Name The mineral was named in honor of James (Jim) L. Post (1923–2013), an American mineralogist and chemist specializing in clay mineralogy and X-ray analysis. His research on vanadium minerals, including pascoite and other decavanadates, made a significant contribution to understanding this group of minerals. Postite was approved as a new mineral by the IMA in 2011 (IMA2011-088), and its description was published in 2012. ## Uses Postite has no industrial applications. Its significance is purely scientific and collectible, as a unique and rare representative of decavanadates.

Properties

Mohs hardness: 2
Luster: Vitreous
Streak: Light yellow
Density: 2.31
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent
Crystal system: Triclinic

Diagnostic features

## Identification Identifying postite in the field is impossible due to the microscopic size of the crystals. Identification requires advanced laboratory methods, such as X-ray diffraction (XRD) and chemical analysis (EDS/WDS). In a collection, it can be preliminarily recognized by its light yellow color, characteristic tabular crystal habit, and co-occurrence with other secondary vanadium minerals on sandstone. ## Distinguishing from Similar Minerals Postite can be confused with other yellow secondary vanadium minerals, such as carnotite, tyuyamunite, or pascoite. Visual distinction is practically impossible. Pascoite has a similar chemical composition but crystallizes in the monoclinic system and has a different ion ratio. Final differentiation requires structural and chemical analysis. ## Crystal Forms Postite forms very small, thin, tabular crystals with a bladed habit, elongated in one direction. These crystals occur as single, dispersed individuals or form small, loose clusters and aggregates on the surface of the host rock.

Geological environment

## Genesis Postite is a secondary mineral, formed in the oxidation zone of uranium-vanadium deposits located in sedimentary rocks, mainly sandstones. It forms as a result of weathering and alteration of primary vanadium minerals under low temperature and pressure conditions, in an oxygen- and water-rich environment. ## Mineral Associations This mineral co-occurs with other secondary vanadate minerals. In the type locality (Blue Cap mine), it was found in association with pascoite, sherwoodite, as well as gypsum and quartz, which is the main component of the host rock (sandstone). ## Localities The only confirmed occurrence of postite in the world is its type locality: the Blue Cap mine, located in the La Sal district, San Juan County, Utah, USA. This is an area of the Colorado Plateau, known for numerous uranium-vanadium deposits.

Rarity

Very rare

For collectors

## Quality Criteria The quality of postite specimens is primarily assessed based on the richness and abundance of crystals on the rock matrix. Specimens with numerous, well-formed, intensely yellow crystals, forming aggregates visible under a microscope, are most valued. Crystal size, although always microscopic, is also important – larger (up to 0.2 mm) and clearer crystals increase the value of the specimen. Contrast with the darker host rock (sandstone) also enhances visual appeal. ## Popular Localities The only source of postite specimens is the Blue Cap mine in Utah, USA. All specimens available on the collector's market come from this single locality, making them easily identifiable by origin.

Care and storage

## Cleaning Postite specimens are extremely delicate and fragile. Mechanical cleaning is not recommended. If absolutely necessary, compressed air can be carefully used from a safe distance to remove loose dust particles. Any contact with water or other liquids should be avoided. ## What to Avoid Contact with water, which can dissolve or damage the microscopic crystals, must be strictly avoided. The mineral is sensitive to changes in humidity and temperature. No chemicals, including acids and solvents, should be used. It should be protected from direct sunlight, which can cause fading and dehydration. ## Storage Specimens should only be stored in sealed, closed "micromount" containers that protect against dust, mechanical damage, and humidity fluctuations. Store in stable room conditions, away from heat and light sources.

Sources

IMA Mineral List (CNMNC)