Cowlesite occurs in amygdules in basalts at Goble, Beech Creek and Spray, Oregon; near Superior, Arizona; Monte Lake, British Columbia; Capitol Peak, Washington; and Table Mountain, Colorado. It is associated with such zeolites as analcime, chabazite, garronite, heulandite, levyne, mordenite, phillipsite, stilbite, and thomsonite. Easily confused with thomsonite, cowlesite forms soft, exceedingly thin, pointed blades with one perfect cleavage, and white-to-gray crystal clusters. Microprobe analyses from the seven known localities gave compositions nearly identical with that from the type area, Goble, Oregon: SiO2 42.73, A12O3 24.32, CaO 12.86, Na2O 0.70, and H2O (by weight loss on heating to 600°C) 22.8 wt percent: giving the formula Ca0.96 Na0.09Al2Si3O10.5-6H2O.
The A1 + Si = 1/2 O, the water loss at low temperature, and the low density (d = 2.14 gm/cm3) suggest cowlesite is a zeolite. Optically the crystals have straight extinction, negative 2V near 50°, and α = 1.512, β= 1.515, γ= 1.517. Although single crystals have not been obtained, consideration of the optical properties and the X-ray powder diffraction data suggest that cowlesite is orthorhombic. Preliminary indexing of the powder data give a unit cell with a = 11.27(1), b= 15.25(1), and c = 12.61(3). The strongest lines of the X-ray powder pattern (d value in Å; relative intensity; indices) are: 15.2 100 010; 7.62 15 020; 508 17 030; 3.81 35 040; 3.75 15 300; 3.052 20 050; 2.964 35 051; 2.934 2 331. The name honors Mr. John Cowles of Rainier, Oregon.