Johnwalkite

Chemical formula: K(Mn²⁺,Fe³⁺)₂(Nb⁵⁺,Ta⁵⁺)O₂(PO₄)₂·2(H₂O,OH)

Johnwalkite is a rare, dark brown phosphate mineral from the phosphofibrite group, discovered at the Champion mine in South Dakota.

## Characteristics Johnwalkite is a rare mineral from the phosphofibrite group, being a hydrated potassium, manganese, iron, niobium, and tantalum phosphate. It occurs as small, bladed or fibrous crystals, forming aggregates and clusters. Its color is very dark, reddish-brown, which makes it difficult to identify at first glance without specialized equipment. ## Physical Properties This mineral is characterized by a hardness of 4 on the Mohs scale. It is opaque, and its density, calculated based on the formula and cell parameters, is 3.40 g/cm³. It exhibits good cleavage in two directions. ## Colors and Varieties Johnwalkite occurs in a uniform, very dark, reddish-brown color. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1985. Its name honors John Walker, an American mineral collector and curator at the Cleveland Museum of Natural History, who specialized in pegmatite minerals. The type locality, where the discovery was made, is the Champion mine in Pennington County, South Dakota, USA. ## Applications Due to its extreme rarity, johnwalkite has no industrial applications. It is solely an object of scientific interest and is valued by specialized collectors of rare minerals.

Properties

Mohs hardness
4
Streak
Medium greenish yellow
Density
3.40
Cleavage
Good on {001} and {100}; poor on {010}
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Johnwalkite is difficult to identify visually. Its characteristic features include a dark reddish-brown color, bladed or fibrous crystal habit, and a medium greenish-yellow streak. It occurs in a specific geological environment – granitic pegmatites. Final confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). ## Distinguishing from Similar Minerals It can be confused with other dark secondary minerals found in pegmatites, such as some manganese oxides or other rare phosphates. The key distinguishing feature is its unique chemical composition and characteristic greenish-yellow streak. ## Crystal Forms It crystallizes in the orthorhombic system. It forms elongated, bladed crystals, often gathered into radial or tangled aggregates with a fibrous appearance.

Geological environment

## Genesis Johnwalkite is a secondary mineral that forms in complex granitic pegmatites. It originates from hydrothermal processes or weathering, which lead to the alteration of earlier phosphate, niobium, and tantalum minerals. ## Mineral Associations At the type locality (Champion mine), it co-occurs with minerals such as quartz, microcline, muscovite, rockbridgeite, strengite, jahnsite-(CaMnMn), and other rare phosphates. ## Localities This is an extremely rare mineral. Apart from the type locality at the Champion mine in South Dakota, USA, its occurrence has not been confirmed anywhere else in the world.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a johnwalkite specimen primarily depends on its authenticity and well-documented provenance. The most valued specimens are rich clusters of well-formed, even microscopic, crystals on a rock matrix, accompanied by other rare minerals from that locality. Due to the nature of the mineral, clarity and transparency are not evaluation criteria. ## Popular Localities The only known and confirmed source of johnwalkite specimens is its type locality – the Champion mine in Pennington County, South Dakota, USA. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Johnwalkite specimens should be cleaned very carefully, using only a soft brush to remove dust. Due to its rarity and potential fragility, contact with water and any chemical agents should be avoided. ## What to Avoid Avoid ultrasonic cleaners, chemical agents (acids, bases, solvents), high temperatures, and prolonged exposure to moisture. The mineral is relatively soft, so it must be protected from scratching by harder minerals. ## Storage It is recommended to store specimens under stable conditions, in closed collector boxes or display cases, away from dust, moisture, and direct sunlight. Each specimen should be isolated to prevent abrasion and mechanical damage.

External references

Sources

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