Pentagonite
Pentagonite has cavansite’s atoms, Ca(V4+O)Si4O10·4H2O, in a polar orthorhombic cell. It twins across {110} at 72.7°, so five crystals make 363.5°, not 360°.
Pentagonite is a calcium vanadium silicate, CaV4+OSi4O10(H2O)3·H2O on the IMA list (A, 1971-039, USA), dimorphous with cavansite: the same atoms in a different orthorhombic structure. It is greenish blue, hardness about 3 to 4, biaxial negative, and twins across {110} at 72.7°, so five crystals make a near-pentagon. Its sheet names one area: south of Owyhee Dam, Oregon.
In short
- The species. The Handbook of Mineralogy sheet gives Ca(V4+O)Si4O10·4H2O, orthorhombic, point group mm2, in prismatic crystals to 1 mm and spherulitic rosettes up to 5 mm; cleavage good on {010}; hardness about 3 to 4; greenish blue, vitreous; biaxial (−), α 1.533, β 1.544, γ 1.547. The September 2026 IMA-CNMNC Master List lists it as CaV4+OSi4O10(H2O)3·H2O, status A, IMA 1971-039, USA.
- The same-atoms check. Both sheets say dimorphous with the other. A script over the IMA list finds 17 rows with calcium, vanadium and silicon; parsed to atom counts, exactly 2 have Ca, V, Si4, O15, H8: cavansite and pentagonite. Staples et al. (1973) found the same Ca:V:Si in both by X-ray fluorescence and took the water to be the same because the cell volumes nearly match: 1286.45 and 1281.74 ų on the sheets, 0.37% apart.
- The 72.7° check. The sheet gives (110) ∧ (1-10) = 72.7°. From the cell, a 10.298 and b 13.999 Å, that angle is 2 × arctan(a/b) = 72.68°. A pattern that closed on five-fold symmetry would need 360 / 5 = 72.0°; five 72.7° angles add to 363.5°. Staples et al. tie the near-five-fold groups to that 72.7° angle; the sheet’s words are closely simulating five-fold symmetry.
- Documented failure: the balance and the scratch. The sheet prints D(meas.) n.d., and Staples et al. wrote that the specific gravity of pentagonite was not determined. Its calculated 2.33 is cavansite’s calculated 2.33. Their hardness, about 3 to 4, was not accurately determined because the crystals are small, brittle and well cleaved. Neither reading can confirm a pentagonite.
- Where it is found. The sheet names one area: about three km south of Owyhee Dam, Malheur Co., Oregon, in coatings on a tuff, with cavansite, heulandite, stilbite, analcime, apophyllite and calcite. It names no Indian locality. How to tell it from cavansite is on pentagonite vs cavansite.
| Property | Pentagonite | Cavansite |
|---|---|---|
| Formula (IMA) | CaV4+OSi4O10(H2O)3·H2O; A, 1971-039 | Ca(V4+O)(Si4O10)(H2O)4; A, 1967-019 |
| Atoms, by script | Ca V Si4 O15 H8 | Ca V Si4 O15 H8 |
| Point group, space group | mm2, Ccm21 | 2/m 2/m 2/m, Pcmn |
| Cell (Å), Z | a 10.298, b 13.999, c 8.891; Z 4 | a 9.792, b 13.644, c 9.629; Z 4 |
| Cell volume | 1281.74 ų | 1286.45 ų |
| Density from the cell / sheet | 2.339 / calc. 2.33, meas. n.d. | 2.331 / calc. 2.33, meas. 2.21–2.31 |
| Prism angle (110) ∧ (1-10), worked | 72.68° (sheet: 72.7°) | 71.33° (Staples et al. print 71.1° from their powder cell) |
| Five prism angles | 363.5°, 3.5° past 360° | 355.5° from 71.1°, 4.5° short |
| Twinning | Cyclical across {110}, common; nearly always twinned (Staples et al.) | None printed; none observed (Staples et al.) |
| Habit (sheet) | Prisms to 1 mm; rosettes up to 5 mm | Prisms to 1 mm; rosettes to 5 mm |
What is pentagonite? Cavansite’s dimorph
Pentagonite is Ca(V4+O)Si4O10·4H2O, the polar orthorhombic dimorph of cavansite. The Handbook of Mineralogy pentagonite sheet gives point group mm2 and space group Ccm21: crystals prismatic, to 1 mm, with dominant {110}, {201} and {00-1}, elongated parallel to [001], and commonly spherulitic rosettes up to 5 mm. Cleavage good on {010}, brittle, hardness about 3 to 4. It is greenish blue, vitreous and transparent; biaxial (−), α 1.533, β 1.544, γ 1.547, 2V measured 50°, dispersion r greater than v, strong, with pronounced pleochroism, Y blue. The sheet prints no streak.
How it was recognised. Staples, Evans and Lindsay (1973), American Mineralogist 58, 405–411 describe both minerals from Oregon. During the crystallographic study of cavansite, one small specimen from Owyhee Dam contained crystals that were clearly twinned; these proved different from cavansite, with very similar appearance and physical properties and the same chemical composition, and were named pentagonite in allusion to its striking twin aspect. The paper calls the dimorphs layer silicates whose silicate layers are linked differently: 4-fold and 8-fold rings in cavansite, only 6-fold rings in pentagonite.
What the IMA list records. CaV4+OSi4O10(H2O)3·H2O, status A, IMA 1971-039, USA, with the same 1973 paper as first reference as cavansite. The two IMA formulas are written differently but, parsed by script, carry identical atoms; the full comparison is on pentagonite vs cavansite. Whether either counts as a zeolite is taken up on is cavansite a zeolite.
Why is it called pentagonite? The 72.7° check
The name describes the twins, and the cell puts a number on them. The sheet derives the name from the unusual pseudosymmetrical five-fold or pentagonal habit of the twinned crystals. Twinning is cyclical by reflection across {110}, common, with (110) ∧ (1-10) = 72.7°. Staples et al. add that pentagonite is nearly always twinned by reflection across the prism (110), which is the composition plane, giving fivelings with star-shaped cross sections.
Worked from the cell. For an orthorhombic prism, the angle between (110) and (1-10) is 2 × arctan(a/b). Pentagonite’s a 10.298 and b 13.999 Å give 72.68°, the sheet’s 72.7° to the printed figure. Five-fold symmetry would need exactly 72.0°. Five 72.7° angles total 363.5°, 3.5° more than a full turn: close to five-fold, not exactly five-fold, and the sheet’s words are closely simulating. This page does not say how a real fiveling takes up the 3.5°; neither source does.
Twins in general. Cyclic twins and composition planes are explained on mineral twinning explained. Cavansite’s own prism angle, and why it does not twin the same way, is on pentagonite vs cavansite.
Can you weigh or scratch-test pentagonite? Not on the published data
Documented failure: no measured density. The sheet prints D(meas.) n.d. and D(calc.) 2.33, and Staples et al. say plainly that the specific gravity of pentagonite was not determined. From the sheet’s cell and the formula mass 451.41, the density works out at 2.339. Cavansite’s cell gives 2.331, and its sheet prints the same calculated 2.33. At 2.33 a 20.00 g piece would read 11.42 g in water; cavansite’s measured 2.21 to 2.31 reads 10.95 to 11.34 g. Staples et al. attribute cavansite’s measurements below the cell value probably to the zeolitic nature of the mineral.
Documented failure: hardness. Staples et al. write that the hardness has not been accurately determined because of the small size of the crystals, their brittleness and their good cleavage, and that it appears to be about 3–4 on the Mohs scale. The pentagonite sheet prints ~3–4 and cavansite’s 3–4. A scratch test cannot tell them apart, as how to test mineral hardness properly explains for any pair inside one range.
What does work. The twinned habit, and optics: pentagonite is biaxial negative, cavansite positive. The balance method itself is on how to measure specific gravity at home; on the published figures it cannot separate these two.
Where is pentagonite found? Owyhee Dam, Oregon
The Distribution field, in full. In the USA, about three km south of Owyhee Dam, Malheur Co., Oregon. Occurrence: in coatings on a tuff. Association: cavansite, heulandite, stilbite, analcime, apophyllite, calcite. The sheet names no other locality, and no Indian one; the cavansite sheet names Wagholi, Poona district, Maharashtra, for cavansite only. In 1973 Staples et al. wrote that pentagonite was found so far only at Owyhee Dam. We could not read a later source naming another locality, so this page names none.
The setting, from the 1973 paper. A brown tuff in a roadcut 2.1 miles by road south of Owyhee Dam, within Lake Owyhee State Park, partly filling a fault fissure, where a face 17 feet high carried a coating 2–3 mm thick of cavansite, pentagonite and associated minerals. Staples et al. give a sequence of deposition with pentagonite later than cavansite, and describe one specimen with both together, the pentagonite in long prisms penetrating heulandite crystals in all directions. They also note that heulandite and analcime enclosing the blue minerals turn deep blue themselves, giving the impression that more cavansite is present than is actually the case.
The rest. Pentagonite is a vanadium compound; for handling questions see toxic minerals in a collection, HSE EH40, COSHH 2002 or a clinician: this page is not advice. The wider cavity family is on which minerals are zeolites, and the reference section at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.