HEWETTITE
Hewettite – CaV6O16·9H2O – is an extremely rare oxide and vanadate, typically occurring as a secondary mineral. It serves as a secondary vanadium ore mineral, as it contains 71.43% V₂O₅.
It is somewhat soluble in water and is neither fluorescent nor magnetic. Dehydration of hewettite yields the mineral metahewettite; both are classified within the Hewettite Group.
In the mineral assemblages where they occur, identifying hewettite and metahewettite is simplified by the fact that they are the only dark-red minerals present – a color that can resemble that of crocoite. Associated minerals are generally yellow, green, dark (black, dark brown, etc.), or blue (goldquarryite).
CAUTION! In the Colorado Plateau-type U-V deposits, hewettite is always associated with radioactive minerals, some of which are very dangerous! Vanadium deposits such as those in Peru, on the other hand, do not contain radioactive minerals.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Deep red. Changes to chocolate brown upon exposure to light.
Habit: Acicular, interlaced fibers. Nodular aggregates, fracture fillings, coatings.
Cleavage: No information available.
Tenacity: No information available.
Twinning: No information available.
Fracture: No information available.
Mohs Hardness: Extremely soft.
Parting: No information available.
Streak: Brownish red.
Lustre: Silky, adamantine.
Diaphaneity: Transparent.
Density (g/cm³): 2.51 – 2.61
2. Geology and Deposits
Hewettite occurs in Colorado Plateau-type U-V deposits, which are vanadium-rich deposits formed in sedimentary rocks. Hewettite forms through the alteration of patronite (VS4) (as at Minasragra, Peru) or through the alteration of other, less oxidized vanadium minerals (as in the Colorado Plateau-type U-V deposits).
There are few occurrences of hewettite: Kazakhstan, Peru (Pasco province, where it was discovered), and several dozen localities in the western United States.
3. Mineral Associations
Occurs with minerals typical of sedimentary rocks, such as quartz, opal, gypsum, and barite. This assemblage also includes phosphates such as fluorapatite, goldquarryite, and fluellite.
The specific paragenesis includes a series of vanadates – with or without uranium – such as carnotite, corvusite, fervantite, metahewettite, montroseite, navajoite, rauvite, steigerite, tyuyamunite, and kazakhstanite. Also present is roscoelite, the vanadium analogue of muscovite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.770 nβ: 2.180 nγ: 2.350 – 2.400
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Red to orange, with pleochroism between light orange-yellow (X=Y) and dark red (Z=c).
Relief: Very high.
Cleavage: No.
Habits: Acicular, fibrous, fracture-filling, nodular aggregates, crusts.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Extreme birefringence, ranging from 0.58 to 0.63, with interference colors masked by the mineral’s intense own color.
Extinction: No information available, probably oblique.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: Medium.
Alterations: It can dehydrate to form metahewettita.
May be confused with: Metahewettite, whose optical properties are extremely similar to those of hewettite.
5. Reflected Light Microscopy
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx