Pentagonite vs cavansite
Pentagonite and cavansite share Ca(V4+O)Si4O10·4H2O and β 1.544. Cavansite is biaxial positive, pentagonite negative and twinned at 72.7°. A fiveling decides.
Pentagonite and cavansite are two minerals with the same atoms, Ca(V4+O)Si4O10·4H2O: both orthorhombic, blue, hardness about 3 to 4, and both with β 1.544. Cavansite is biaxial positive (α 1.542, γ 1.551), pentagonite negative (α 1.533, γ 1.547). Pentagonite twins across {110} at 72.7° into fivelings; Staples et al. (1973) observed no cavansite twins.
In short
- Same atoms, two species. The September 2026 IMA-CNMNC Master List writes cavansite Ca(V4+O)(Si4O10)(H2O)4, A, 1967-019, and pentagonite CaV4+OSi4O10(H2O)3·H2O, A, 1971-039; parsed by script, both are Ca V Si4 O15 H8. Each Handbook of Mineralogy sheet calls it dimorphous with the other. Cavansite is centrosymmetric, Pcmn; pentagonite polar, Ccm21.
- The 1.544 pivot. Both sheets print β = 1.544. In cavansite, α sits 0.002 below and γ 0.007 above: γ is further away, so the crystal is biaxial positive. In pentagonite, α sits 0.011 below and γ 0.003 above: α is further, so it is negative. Staples et al. (1973) put it as the acute bisectrix shifting from c in cavansite to b in pentagonite, with the strong dispersion therefore reversed: r less than v, extreme, against r greater than v, strong.
- Documented failure: the indices alone. Each index is printed ±0.002. Varying all three by −0.002, 0 or +0.002 gives 27 combinations: pentagonite comes out negative in 24 and indeterminate in 3, but cavansite positive in only 15, negative in 3 and indeterminate in 9. Its printed sign rests on direct optical observation (Staples et al. used a spindle stage), not on index arithmetic.
- The prism-angle trap. Pentagonite twins across {110} with (110) ∧ (1-10) = 72.7°, near the 72° of a five-fold pattern. Cavansite’s prism angle, 71.1° per Staples et al. and 71.33° from its sheet cell, is almost as near, yet they observed no twinning in it, and report Evans (1973) as finding no twinning mechanism for its structure. On their account the structure, not the angle, decides. A fiveling says pentagonite.
- Documented failure: weight, scratch, colour, rosettes. Neither the sheet nor Staples et al. records a measured density for pentagonite; both minerals calculate at 2.33. Both are about 3 to 4. Staples et al. call pentagonite perhaps slightly less greenish, while the sheets give cavansite brilliant sky-blue to greenish blue and pentagonite greenish blue. Both grow as rosettes to 5 mm. The species pages are cavansite and pentagonite.
| Property | Cavansite | Pentagonite | Separates? |
|---|---|---|---|
| Atoms (IMA formula, by script) | Ca V Si4 O15 H8 | Ca V Si4 O15 H8 | No |
| Point group, space group | 2/m 2/m 2/m, Pcmn | mm2, Ccm21 | Yes, by X-ray |
| β | 1.544 | 1.544 | No: the pivot |
| α | 1.542 (0.002 below β) | 1.533 (0.011 below β) | Yes |
| γ | 1.551 (0.007 above β) | 1.547 (0.003 above β) | Yes |
| Optic sign | Biaxial (+) | Biaxial (−) | Yes |
| Birefringence, worked | 0.009 | 0.014 | Barely |
| 2V measured / from indices | 52° / 56.5° | 50° / 54.8° | No |
| Acute bisectrix (Staples et al.) | ∥ c | ∥ b | Yes |
| Dispersion | r less than v, extreme | r greater than v, strong | Yes: reversed |
| Prism angle (110) ∧ (1-10) | 71.1° (Staples et al.); 71.33° from the sheet cell | 72.7° (sheet); 72.68° from the cell | Barely |
| Twinning | None printed; none observed (Staples et al.) | Cyclical across {110}, common; fivelings | Yes |
| Habit (sheet) | Prisms to 1 mm; rosettes to 5 mm | Prisms to 1 mm; rosettes up to 5 mm | No |
| Hardness | 3–4 | ~3–4 | No |
| Density | 2.21–2.31 measured; 2.33 calc. | Not measured; 2.33 calc. | No |
| 20.00 g piece in water | 10.95–11.34 g | 11.42 g at 2.33 (calc. only) | No |
| Colour (sheet) | Brilliant sky-blue to greenish blue | Greenish blue | No: shared |
| Cleavage | Good on {010} | Good on {010} | No |
| Distribution (sheet) | Owyhee Dam and Chapman quarry, Oregon; Wagholi, Poona district, India | About three km south of Owyhee Dam, Oregon | A clue, not a test |
Are pentagonite and cavansite the same mineral?
No. They have the same atoms and are two species. A script that parses each IMA formula to atom counts finds Ca V Si4 O15 H8 for both cavansite, Ca(V4+O)(Si4O10)(H2O)4, and pentagonite, CaV4+OSi4O10(H2O)3·H2O; of 17 IMA rows carrying calcium, vanadium and silicon, only these two match. The Handbook cavansite sheet and the Handbook pentagonite sheet each call the mineral dimorphous with the other.
What makes them two. Staples, Evans and Lindsay (1973), American Mineralogist 58, 405–411 describe both as layer silicates whose layers are linked differently: 4-fold and 8-fold rings of silicate tetrahedra in cavansite, only 6-fold rings in pentagonite. The structure analyses showed a centrosymmetric space group for cavansite, Pcmn, and a noncentrosymmetric one for pentagonite, Ccm21. Their cells are nearly the same size, 1286.45 and 1281.74 ų on the sheets.
What this page adds. The cavansite page lists the optical sign, habit, colour and locality differences and gives a label rule; the pentagonite page works the 72.7° twin angle. This page asks which printed numbers actually separate the pair, and which do not.
How do you tell pentagonite from cavansite? The 1.544 pivot
Both have β = 1.544; the two other indices sit on opposite sides of it. Cavansite: α 1.542, γ 1.551, so β − α is 0.002 and γ − β 0.007, and the crystal is biaxial positive. Pentagonite: α 1.533, γ 1.547, so β − α is 0.011 and γ − β 0.003, and it is biaxial negative. Worked from the central indices by the exact formula, 2V is 56.5° for cavansite and 54.8° for pentagonite, against measured 52° and 50°.
The same fact, in Staples et al.’s words. Their Table 1 puts the acute bisectrix parallel to c in cavansite and parallel to b in pentagonite; the text says the indices of refraction are such that the acute bisectrix shifts from c in cavansite to b in pentagonite, and the strong dispersion is therefore reversed. The sheets print r less than v, extreme, for cavansite and r greater than v, strong, for pentagonite. Staples et al. measured the indices on a spindle stage.
Documented failure: the tolerance. Every index is printed ±0.002, and cavansite’s β − α is exactly 0.002. Shifting each index by −0.002, 0 or +0.002 gives 27 combinations. Pentagonite stays negative in 24, with 3 indeterminate. Cavansite is positive in only 15, negative in 3 and indeterminate in 9, where α and β meet or cross. A β reading alone cannot tell them apart, and the indices alone barely fix cavansite’s sign; the sign as observed and printed does.
Why does only pentagonite form fivelings? The prism-angle trap
A fiveling, a star-shaped cluster of five twinned prisms, is pentagonite. The pentagonite sheet prints cyclical twinning by reflection across {110}, common, with (110) ∧ (1-10) = 72.7°. Staples et al. write that pentagonite is nearly always twinned, and that the special geometry of its crystal structure readily permits twinning at an angle near 72°. The cavansite sheet prints no twinning.
Documented failure: the angle does not explain it. Staples et al. give cavansite’s prism angle as 71.1°, close to the 72.7° of pentagonite, and report that no twinning had been observed in cavansite. From the sheets’ cells, cavansite’s angle is 71.33°, 0.67° below 72°, and pentagonite’s 72.68°, 0.68° above: the same distance from a five-fold pattern on opposite sides. Staples et al. report that the structure study (Evans, 1973) found no such twinning mechanism for the cavansite structure. So an angle near 72° is not enough; the structure decides, and the fiveling is the practical test.
What a fiveling looks like. Staples et al. describe prismatic crystals twinned to form fivelings with star-shaped cross sections, and note that the twinning often looks polysynthetic in polarised light, with lamellae parallel to the crystals’ length. The general idea is on mineral twinning explained.
Where the easy tests fail, and what goes on the label
Documented failure: weight and scratch. Pentagonite’s density has not been measured in either source: the sheet prints n.d., Staples et al. say not determined, and its calculated 2.33 equals cavansite’s. A 20.00 g piece of cavansite reads 10.95 to 11.34 g in water on its measured 2.21 to 2.31; pentagonite would read 11.42 g at 2.33, a calculated figure only. Hardness is 3–4 on one sheet and ~3–4 on the other. The methods are on how to measure specific gravity at home and how to test mineral hardness properly.
Documented failure: colour and rosettes. The sheets give cavansite brilliant sky-blue to greenish blue and pentagonite greenish blue, so the words overlap; Staples et al. say pentagonite is perhaps slightly less greenish. Both sheets give spherulitic rosettes to 5 mm, so a rosette is not a cavansite test. Staples et al. also note that heulandite and analcime enclosing the blue crystals turn deep blue, making it look as if more cavansite is present than there is; the cavity minerals are on heulandite and analcime.
On the label. Fivelings or star-shaped prisms: pentagonite. Untwinned prisms or rosettes: cavansite, unless an optical determination says biaxial negative. Locality is a clue only: the pentagonite sheet names only Owyhee Dam, Oregon, and the cavansite sheet adds the Chapman quarry, Oregon, and Wagholi, India. Whether either is a zeolite is on is cavansite a zeolite. More identification questions are at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.