Pentagonite

 Ca(VO)Si4O10·4(H2O). It was named for the unusual twinning which produces an apparent five-fold symmetry.[1] It is a dimorph of cavansite.

Pentagonite
Pentagonite-177796.jpg
General
CategorySilicate mineral
Formula
(repeating unit)		Ca(VO)Si4O10·4(H2O)
Strunz classification9.EA.55
Crystal systemOrthorhombic
Crystal classPyramidal (mm2)
(same H-M symbol)
Space groupCcm21
Unit cella = 10.386(4) Å,
b = 14.046(7) Å,
c = 8.975(2) Å; Z = 4
Identification
ColorGreenish blue
Crystal habitPrismatic crystals, often as radiating clusters
TwinningMultiple twins producing a pseudo pentagonal symmetry
CleavageGood on {010}
TenacityBrittle
Mohs scale hardness3 - 4
LusterVitreous
DiaphaneityTransparent
Specific gravity2.33
Optical propertiesBiaxial (-)
Refractive indexnα = 1.533 nβ = 1.544 nγ = 1.547
Birefringenceδ = 0.014
PleochroismVisible: X=Z= colorless Y= blue
2V angleMeasured: 50°
References[1][2][3]

Pentagonite was first described in 1973 for an occurrence in Lake Owyhee State Park, Malheur County, Oregon.[2] It has also been reported from the Pune district of India. It occurs as fracture and cavity fillings in tuff and basalt. It occurs with cavansite, heulandite, stilbite, analcime, apophyllite and calcite.[1]

This article uses material from the Wikipedia article
 Metasyntactic variable, which is released under the 
Creative Commons
Attribution-ShareAlike 3.0 Unported License

Note

This article uses material from the Wikipedia article
 Metasyntactic variable, which is released under the 
Creative Commons
Attribution-ShareAlike 3.0 Unported License.