Cavansite

Cavansite, whose name is derived from its chemical composition, calcium vanadium silicate, is a deep blue hydrous calcium vanadium phyllosilicate mineral, occurring as a secondary mineral in basaltic and andesitic rocks along with a variety of zeolite minerals. Discovered in 1967 in Malheur County, Oregon, cavansite is a relatively rare mineral. It is polymorphic with the even rarer mineral, pentagonite. It is most frequently found in Pune, India and in the Deccan Traps, a large igneous province.

Cavansite
Cavansite: 2.8 cm cluster on a bed of microcrystalline stilbite
General
Category	Silicate mineral
Formula (repeating unit)	Ca(VO)Si4O10·4(H2O)
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H–M Symbol: (2/m 2/m 2/m)
Space group	Pcmn
Unit cell	a = 9.792(2) Å, b = 13.644(3) Å, c = 9.629(2) Å; Z = 4
Identification
Color	Brilliant sky-blue to greenish blue
Crystal habit	Radiating acicular prismatic crystals commonly as spherulitic rosettes
Cleavage	Good on {010}
Tenacity	Brittle
Mohs scale hardness	3 - 4
Luster	Vitreous, pearly
Streak	Bluish-white
Diaphaneity	Transparent
Specific gravity	2.25 - 2.33
Optical properties	Biaxial (+)
Refractive index	nα = 1.542(2) nβ = 1.544(2) nγ = 1.551(2)
Birefringence	δ = 0.009
Pleochroism	Visible: X=Z= colorless Y= blue
2V angle	Measured: 52°
References	[1][2][3][4]

Uses of cavansite

While cavansite does contain vanadium, and could thus be a possible ore source for the element, it is not generally considered an ore mineral. However, because of its rich color and relative rarity, cavansite is a sought after collector's mineral.

Associated minerals

Cavansite on stilbite from the type locality

• Members of the apophyllite group
• Members of the zeolite group, particularly stilbite.
• babingtonite, Ca₂Fe₂Si₅O₁₄OH
• quartz, SiO₂
• calcite, CaCO₃
• pentagonite, Ca(VO)Si₄O₁₀ · 4(H₂O)

Notes for identification

Stalactitic cavansite, 3.6 x 2.1 x 1.8 cm, Wagholi, Pune District, Maharashtra, India

Cavansite is a distinctive mineral. It tends to form crystal aggregates, generally in the form of balls, up to a couple centimeters in size. Sometimes the balls are coarse enough to allow the individual crystals to be seen. Rarely, cavansite forms bowtie shaped aggregates. The color of cavansite is distinctive, almost always a rich, bright blue. The color is the same as its dimorph, pentagonite, but the latter is generally much more spikey with bladed crystals. Finally, the associated minerals are useful for identification, as cavansite is frequently found sitting atop a matrix of zeolites or apophyllites.

References

1. Mineralienatlas
2. Handbook of Mineralogy
3. Mindat
4. Webmineral

• Mineral galleries
• Evans, H.T. Jr., "The crystal structure of cavansite and pentagonite", Americal MIneralogist, Vol. 58, pg. 412-424, 1973.
• Makki, M.F., "Collecting cavansite in the Wagholi quarry complex, Pune, Maharashtra, India", The Mineralogical Record, Vol. 36, No. 6, pg. 507-512, Nov-Dec 2005.
• Staples, L.W., Evans, H.T. Jr., and Lindsay, J.R., "Cavansite and pentagonite, new dimorphous calcium vanadium silicate minerals from Oregon", American Mineralogist, Vol. 58, pg 405-411, 1973. http://www.minsocam.org/ammin/AM58/AM58_405.pdf