Grantsite

Chemical formula: (Na,Ca)₂+x(V⁵⁺,V⁴⁺)₆O₁₆·4H₂O

Grantsite is a rare, dark green to black vanadium mineral, forming aggregates of fibrous crystals in sedimentary rocks.

## Characteristics Grantsite is a hydrated sodium calcium vanadate, belonging to the layered vanadate group. It typically occurs as aggregates of fine, fibrous or bladed crystals, which can form rosetted aggregates or coatings on rock surfaces. Specimens are most often dark green, greenish-black, to almost black in color. ## Physical Properties Grantsite crystals exhibit a silky to dull luster, depending on the form of occurrence. The mineral is translucent in thin fragments and opaque in larger masses. It is relatively soft, with a Mohs hardness of about 1-2. ## Colors and Varieties The dominant color of grantsite is dark green in various shades, often transitioning to black with a greenish or bluish tint. No named varieties of this mineral have been distinguished. ## History and Name Grantsite was first described in 1964 by Alice D. Weeks, Mary E. Thompson, and Richard B. Sherwood. The mineral's name comes from its discovery locality in the Grants area of Valencia County, New Mexico, USA, which was an important center for uranium and vanadium ore mining.

Properties

Mohs hardness
1-2
Color
Dark olive green to greenish black
Luster
Silky
Streak
Greenish black
Density
2.94
Cleavage
Perfect on {010}
Fracture
Splintery
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Grantsite can be identified by its characteristic dark green to black color, fibrous or bladed crystal habit, and silky luster. A key feature is its occurrence in uranium and vanadium-rich sandy sediments, often in association with other vanadium minerals. ## Distinguishing from similar minerals It can be confused with other dark, fibrous vanadate minerals, such as montroseite or paramontroseite. Differentiation often requires chemical analysis (XRD/EDS), although grantsite is distinguished by a slightly lighter, greenish hue compared to black montroseite. Unlike some of them, grantsite is water-soluble. ## Crystal forms Grantsite forms elongated, bladed or fibrous crystals, which often combine into radial or rosetted aggregates. It also occurs as felted masses and coatings filling spaces in the host rock (sandstone).

Geological environment

## Genesis Grantsite is a secondary mineral, forming in the oxidation zone of uranium-vanadium deposits in sedimentary rocks, mainly sandstones. It forms under low-temperature conditions as a result of chemical changes in groundwater acting on primary vanadium minerals, such as montroseite. ## Mineral associations It most commonly co-occurs with other vanadium and uranium minerals. Typical associated minerals include montroseite, paramontroseite, hewettite, as well as uranium minerals like carnotite and tyuyamunite. It often occurs in association with calcite and clay minerals. ## Localities The most important and classic localities for grantsite are in the USA, on the Colorado Plateau, in the states of New Mexico (Grants area), Arizona, Colorado, and Utah. These are the main places from which specimens of this mineral originate.

Rarity

Rare

For collectors

## Quality criteria The most valuable grantsite specimens are characterized by well-formed, rosetted aggregates of fibrous crystals with an intense, dark green color. Contrast with the light host rock (sandstone) and the size and richness of the aggregates are important. Specimens with a well-defined crystalline structure are much more valued than felted masses. ## Popular localities The best specimens are considered to be those from the historic mines in the Grants area of New Mexico, where the mineral was first described. Specimens from other mines on the Colorado Plateau are also sought after by collectors specializing in uranium and vanadium minerals.

Care and storage

## Cleaning Due to its softness and fibrous nature, grantsite should be cleaned very carefully. The safest method is to use compressed air to remove dust. Any contact with water or liquids is inadvisable, as the mineral is water-soluble. ## What to avoid Absolutely avoid contact with water and other liquids, which can dissolve or damage the delicate crystals. The mineral is very soft, so it should be protected from scratching, impacts, and abrasion. It should not be heated or exposed to chemicals. ## Storage Grantsite specimens should be stored in a dry place, preferably in a closed box or display case, to protect them from dust, moisture, and mechanical damage. Due to its fragility, it should not be stored in contact with harder minerals.

External references

Sources

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