Cowlesite
Cowlesite is CaAl2Si3O10·5-6H2O, a zeolite approved in 1975 from Goble, Oregon: thin pearly laths to 2 mm in basalt vesicles, soft, density 2.14 measured.
Cowlesite is a calcium zeolite, Ca(Al2Si3)O10·5-6H2O, approved by the IMA in 1975 (1975-016) and first analysed from Goble, Oregon. It forms very thin, pearly, white to blue-grey laths to 2 mm in basalt vesicles: soft, density 2.14 measured. On paper it is scolecite's formula with 5 to 6 waters instead of 3.
In short
- Cowlesite is scolecite's formula with twice the water. Scolecite is CaAl2Si3O10·3H2O; cowlesite is CaAl2Si3O10·6H2O on its Handbook of Mineralogy sheet, and 5 to 6 H2O on the IMA list. Worked below: 13.78% water in scolecite, 21.03% to 24.21% in cowlesite.
- The two analyses on the sheet recalculate to 5.6 and 5.0 H2O per formula unit, from Goble, Oregon, and Superior, Arizona. That is why the IMA formula reads 5 to 6.
- The separating rule is softness plus habit. Cowlesite's hardness is printed only as Soft. It forms very thin pearly laths to 2 mm, and the sheet says it is typically the only zeolite within a single vesicle. Scolecite, natrolite and mesolite are 5 to 5.5 and grow as prisms and needles, some to 20 cm and more.
- Density will not separate it. Its measured 2.14 sits inside the 2.10 to 2.36 spread of the 9 zeolite-group sheets in this site's reference set, and inside heulandite's own 2.10 to 2.20.
- Two localities are in Britain and Ireland: Kingsburgh on the Isle of Skye, and Dunseverick, Ballyclare and elsewhere in Co. Antrim. The name honours John Cowles, 1907 to 1985, an amateur mineralogist of Rainier, Oregon.
| Species | Formula (Handbook) | H2O per formula | Hardness | Density (measured) | Habit on the sheet | Lustre |
|---|---|---|---|---|---|---|
| Cowlesite | CaAl2Si3O10·6H2O | 6 (IMA 5 to 6) | Soft | 2.14 | Very thin laths to 2 mm; radiating sprays, compact balls | Pearly |
| Scolecite | CaAl2Si3O10·3H2O | 3 | 5 to 5.5 | 2.25 to 2.29 | Slender prisms to 30 cm; radiating sprays | Vitreous, silky when fibrous |
| Mesolite | Na2Ca2Al6Si9O30·8H2O | 8 | 5 | 2.26 | Prisms to 20 cm; hairlike tufts | Vitreous, silky when fibrous |
| Natrolite | Na2Al2Si3O10·2H2O | 2 | 5 to 5.5 | 2.20 to 2.26 | Prisms to 1 m; stellate groups | Vitreous to pearly |
| Stilbite | NaCa2Al5Si13O36·14H2O | 14 | 3.5 to 4 | 2.19 | Thin tabular to 14 cm; sheaflike | Vitreous, pearly on cleavage |
| Heulandite | (Ca,Na2)Al2Si7O18·6H2O | 6 | 3.5 to 4 | 2.10 to 2.20 | Coffin-shaped tablets to 12 cm | Vitreous, pearly on {010} |
What is cowlesite?
Cowlesite is a calcium aluminium silicate zeolite. The September 2026 IMA-CNMNC Master List writes it Ca(Al2Si3)O10·5-6H2O, approved under number 1975-016 with the USA as type country. The Handbook header writes CaAl2Si3O10·6H2O and assigns it to the Zeolite group. Its first reference is the 1975 American Mineralogist description, Cowlesite, a new Ca-zeolite, by Wise and Tschernich. The powder pattern and the first analysis are of material from Goble, Oregon.
It is orthorhombic; the sheet prints the point group and space group as not determined. The crystals are very thin laths with few forms and sharp terminations, to 2 mm, in radiating sprays and compact balls. It is colourless, white, grey or blue-grey with a pearly lustre, brittle, with a perfect {010} cleavage. The measured density is 2.14(2) and the calculated 2.05.
The hardness is printed only as Soft. Among the 176 sheets in this site's reference set, berthierine and botallackite also print Soft rather than a number. For cowlesite that means no Mohs figure can be quoted, and this page does not supply one.
Cowlesite and scolecite: the same formula, twice the water
On paper, cowlesite and scolecite share the unit CaAl2Si3O10: one calcium, two aluminium and three silicon to ten oxygen. The Handbook gives scolecite 3 H2O and cowlesite 6. The comparison is between formulas only; neither sheet discusses the other.
A worked composition, on standard atomic weights:
- Scolecite, CaAl2Si3O10·3H2O, formula mass 392.33: H2O 13.78%, SiO2 45.94%, Al2O3 25.99%, CaO 14.29%.
- Cowlesite with 6 H2O, the Handbook formula, formula mass 446.38: H2O 24.21%, SiO2 40.38%, Al2O3 22.84%, CaO 12.56%.
- Cowlesite with 5 H2O, the IMA lower bound, formula mass 428.36: H2O 21.03%, SiO2 42.08%, Al2O3 23.80%, CaO 13.09%.
The analyses fall inside the IMA range. The Goble microprobe analysis reads H2O 22.8%, with water by weight loss to 600 °C, and the sheet recalculates it to 5.6 H2O. The Superior, Arizona, analysis reads 21.0%, recalculated to 5.0 H2O. The sheet prints totals of 103.41 and 109.34 for the two, so compare the raw percentages loosely. The recalculated water per formula unit is the figure to use, and it runs from 5.0 to 5.6: the IMA's 5 to 6.
How to tell cowlesite from other white zeolites
The rule: soft, pearly laths no more than 2 mm long, alone in their vesicle. The sheet records cowlesite in cavities in basalts and scoria, typically the only zeolite within a single vesicle. The table sets that against the sheets in the set:
- Scolecite, natrolite and mesolite are hardness 5 to 5.5. They grow as prisms and needles, to 30 cm, 1 m and 20 cm respectively on their sheets. Their lustre is vitreous or silky rather than pearly.
- Stilbite and heulandite are 3.5 to 4 and pearly on their cleavage, but they form tabular crystals, sheaves and coffin shapes, to 14 cm and 12 cm.
- Density does not help. The 9 zeolite-group sheets in the set print measured densities from 2.10 (heulandite, lower end) to 2.36 (gonnardite, upper end). Cowlesite's 2.14 is inside that spread and inside heulandite's own 2.10 to 2.20; barrerite and stellerite print 2.13. Method: the D(meas.) field parsed by script from all 176 sheets.
A blue-grey tint is recorded for cowlesite and not for scolecite. At 2 mm, this is a loupe and microscope species; see what is a micromount. The family is on which minerals are zeolites.
Where cowlesite is found, including Skye and Antrim
The distribution, in the sheet's order. In the USA: Goble, Columbia Co., Beech Creek, Grant Co., and Spray, Wheeler Co., Oregon; Capitol Peak, Thurston Co., Washington; eight km south of Superior, Pinal Co., Arizona; and Table Mountain, Jefferson Co., Colorado. In Canada, Monte Lake, British Columbia. Dunseverick, Ballyclare and elsewhere in Co. Antrim, Ireland. Kingsburgh, Isle of Skye, Scotland. Mjoädalsá Canyon near Hvammur, Iceland. Dalsnipa on Sandoy, and Satan on Streymoy, in the Faeroe Islands. Kuniga, Oki Islands, Japan. Flinders and Jindivick, Victoria, Australia. The sheet adds that a number of other localities are known.
Goble is also on three sheets in this site's reference set: heulandite, mesolite and stilbite (string search of each Distribution field). The associates on the cowlesite sheet are simply zeolites, apophyllite.
The rest of the zeolites are indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.