Cavansite
Cavansite is Ca(V4+O)Si4O10·4H2O: sky-blue prisms to 1 mm and rosettes to 5 mm, hardness 3 to 4, in zeolite cavities. Type in Oregon; fine specimens from Poona.
Cavansite is a calcium vanadium silicate, Ca(V4+O)Si4O10·4H2O, named for CAlcium, VANadium and SIlicon. It forms brilliant sky-blue to greenish-blue prisms to 1 mm and rosettes to 5 mm, hardness 3 to 4, density 2.21 to 2.31, in cavities in basalt and tuff. Type locality: Oregon; remarkable specimens: Wagholi, Poona district, India.
In short
- A vanadium silicate, named for its three cations. The Handbook of Mineralogy gives Ca(V4+O)Si4O10·4H2O; the September 2026 IMA-CNMNC Master List writes Ca(V4+O)(Si4O10)(H2O)4, status A, IMA 1967-019, USA. Worked on standard atomic weights, the formula is 11.29% vanadium by mass and reproduces the sheet’s ideal column to within 0.01%.
- Pentagonite is its dimorph. Both sheets say dimorphous, and both IMA formulas carry the same atoms. Pentagonite is greenish blue, biaxial (−) where cavansite is biaxial (+), and its cyclic twins give the pentagonal basal patterns it is named for. Its sheet lists one area: about three km south of Owyhee Dam, Oregon.
- The blue-in-the-pocket rule. Seven associates on the cavansite sheet have sheets in this site’s 176-sheet reference set: heulandite, stilbite, analcime, the apophyllites, calcite, copper and chalcocite. None prints blue in its colour field. In that pocket, the blue mineral is the one to examine first.
- Hardness and density do not separate it from the zeolites. Cavansite is 3 to 4 and 2.21 to 2.31; heulandite is 3.5 to 4 and 2.10 to 2.20, stilbite 3.5 to 4 and 2.19, analcime 5 to 5.5 and 2.24 to 2.29. Colour and habit do the work.
- Extreme dispersion is rare in the set. The cavansite sheet records dispersion r < v; extreme. Of the 176 sheets in the set, 93 print a dispersion and one calls it extreme: goethite.
| Species | Formula (Handbook) | Hardness | Density (measured) | Colour on the sheet | Optical sign | Prints blue? |
|---|---|---|---|---|---|---|
| Cavansite | Ca(V4+O)Si4O10·4H2O | 3 to 4 | 2.21 to 2.31 | Brilliant sky-blue to greenish blue | Biaxial (+) | Yes |
| Pentagonite | Ca(V4+O)Si4O10·4H2O | About 3 to 4 | Not determined; 2.33 calculated | Greenish blue | Biaxial (−) | Yes |
| Heulandite | Zeolite (sheet in the set) | 3.5 to 4 | 2.10 to 2.20 | Colourless, white, grey, yellow, red, pink, orange, brown, black | — | No |
| Stilbite-Ca | Zeolite (sheet in the set) | 3.5 to 4 | 2.19 | White, yellowish, grey, pink, reddish, orange, light to dark brown | — | No |
| Analcime | Zeolite (sheet in the set) | 5 to 5.5 | 2.24 to 2.29 | White, colourless, grey, pink, greenish, yellowish | — | No |
| Fluorapophyllite-(K) | Sheet in the set | 4.5 to 5 | 2.33 to 2.37 | Colourless, white, pink, pale, yellow, green | — | No |
| Calcite | CaCO3 | 3 | 2.7102 | Colourless or white, also grey, yellow, green, many others from inclusions | — | No |
What cavansite is: calcium, vanadium and silicon
The name is the formula. The sheet says it is named for CAlcium, VANadium, and SIlicon in the composition. The vanadium is tetravalent and bonded to one oxygen as a vanadyl group, written (V4+O). Worked on standard atomic weights, Ca(VO)Si4O10·4H2O has a formula mass of 451.41 and gives SiO2 53.24%, CaO 12.42% and H2O 15.96%, matching the sheet’s ideal column, and VO2 18.37% against the sheet’s 18.38. Vanadium is 11.29% of the mass.
It was described in 1973, with pentagonite, by Staples, Evans and Lindsay in American Mineralogist 58, 405–411, the paper the IMA list gives as first reference for both. The type material is in the National Museum of Natural History, Washington. The IMA number is 1967-019 and the country USA.
Crystals are small. The sheet gives prismatic crystals to 1 mm, elongated parallel to [001], and spherulitic rosettes to 5 mm. Single crystals on the sheet’s figures are therefore tiny, and even the rosettes stop at 5 mm; micromount scale is the natural scale for single crystals.
Cavansite or pentagonite? Same atoms, different symmetry
Both sheets say dimorphous, and both IMA formulas have one calcium, one vanadyl group, four silicons and four waters: Ca(V4+O)(Si4O10)(H2O)4 for cavansite (IMA 1967-019) and CaV4+OSi4O10(H2O)3·H2O for pentagonite (IMA 1971-039). The pentagonite sheet says its analysis showed Ca:V:Si as in cavansite, with the same water because their unit cells have the same volume.
What separates them on the sheets:
- Optical sign. Cavansite biaxial (+), pentagonite biaxial (−).
- Habit. Pentagonite twins are cyclical by reflection, with multiple twins having pentagonal basal patterns closely simulating five-fold symmetry; that is the source of its name. Cyclic twinning in general is on mineral twinning explained.
- Colour. Cavansite brilliant sky-blue to greenish blue; pentagonite greenish blue.
- Distribution. The pentagonite sheet lists only the area about three km south of Owyhee Dam, Malheur Co., Oregon. It names no Indian locality, where the cavansite sheet names Wagholi.
So the label rule is short. Blue rosettes from Poona are labelled cavansite, which is what the sheets support. A pentagonite label needs the twinned pentagonal habit or an optical determination.
Telling cavansite from the zeolites it sits on
The associates, from the sheet: pentagonite, calcite, heulandite, stilbite, analcime, apophyllite, thomsonite, copper in Oregon; stilbite, calcite, heulandite, mordenite, chalcocite in India. Seven of those have sheets in this site’s reference set: heulandite, stilbite (as stilbite-Ca), analcime, the two apophyllites, calcite, copper and chalcocite.
Method and result. We extracted the Color field from each of those sheets by script and searched it for blue. None prints blue. Heulandite runs from colourless to black, the apophyllites to pink, yellow and green, calcite to many other colors from included minerals. Natrolite, which is not on the cavansite sheet’s list, does print bluish, so the rule holds only for the listed associates.
The numbers do not help. Cavansite’s measured density of 2.21 to 2.31 falls inside the zeolite band: heulandite 2.10 to 2.20, stilbite 2.19, analcime 2.24 to 2.29, fluorapophyllite 2.33 to 2.37. Its hardness of 3 to 4 overlaps heulandite and stilbite at 3.5 to 4. On a mixed specimen the blue is diagnostic and the weight is not. The wider zeolite family is on which minerals are zeolites and the apophyllite question on is apophyllite a zeolite.
Where cavansite is found: Oregon and the Poona district
The Distribution field, in full: From near Owyhee Dam, Lake Owyhee State Park, Malheur Co., and the Chapman quarry, Columbia Co., Oregon, USA. Remarkable specimens from quarries around Wagholi, Poona district, Maharashtra, India. We add no other locality.
The settings: a brown tuff partly filling a fault fissure at Lake Owyhee; vesicular basalt and red tuff breccia, as cavity fillings and in calcite veinlets, at the Chapman quarry; and pores of altered basalt breccia and tuffaceous andesite in the Poona district. The sheet cites R. Kothavala’s 1991 article The Wagholi cavansite locality near Poona, India, in the Mineralogical Record 22, 415–420. How such cavity minerals grow is on how do mineral specimens form.
Against the 176 sheets in the set, neither cavansite nor pentagonite is mentioned. Both sheets used here are fetched comparators, not part of the set. This page gives no handling or health advice; the composition above is chemistry, not exposure. See toxic minerals in a collection, HSE EH40, COSHH 2002 or a clinician. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.