Zeolites and associated minerals occur in a tholeiitic basaltic pillow lava sequence that makes up part of the Eocene Siletz River Volcanics in the central Coast Range, Oregon. Regional zoning of zeolite assemblages is not apparent; the zeolites formed in joints, fractures, and interstices, although most occur in central cavities of basalt pillows. The zeolites and associated minerals identified, in general order of paragenetic sequence, are smectite, pyrite, calcite (small spheres), thomsonite, natrolite, analcime, scolecite, mesolite, stilbite, heulandite, apophyllite, chabazite, mordenite, calcite (scalenohedra and twinned rhombohedra), laumontite, and amethystine quartz. Common three-mineral assemblages are: natrolite-analcime-stilbite, stilbite-heulandite-chabazite, stilbite-apophyllite-chabazite, and natrolite-mesolite-laumontite.

Alteration of basaltic glass, which was initially abundant, appears to have been an important factor in formation of the zeolites. Isotopic data suggest that zeolitization occurred during a low-temperature (60°–70°C) submarine hydrothermal event, or by reactions of cold (∼10°C) meteoric water with basalt over a long time. The occurrence of different mineral assemblages in cavities of adjacent basalt pillows indicates that these minerals crystallized in closed systems that were isolated as fractures and joints were sealed by deposition of smectite and early zeolites. Although the total chemical composition of the mineral assemblages in cavities is similar, different mineral species formed because of the sensitivity of zeolite minerals to slight variations in physical and chemical conditions within individual cavities.

Selected samples of large cavity filling and vein-type fibrous zeolites from Eocene volcanic rocks in the Kahrizak region, northern Iran, have been studied for their mineralogical and chemical characteristics. X-ray powder diffraction and electron microprobe analyses confirmed the presence of natrolite, mesolite and scolecite with compositions of [Na14.922Ca0.202K0.015Ba0.002] [Al15.697Si24.267O80]·nH2O, [Ca15.714Na14.224][Al46.431Si73.398O240]·nH2O and [Ca7.804Na0.142K0.024 Ba0.012Mg0.006][Al15.320Si24.437O80]·nH2O, respectively. In addition, examination of textural relationships in thin sections and back-scattered electron images reveals a paragenetic order in which the Ca-rich zeolites crystallized first. It is most probable that the fibrous zeolites of Kahrizak were formed during two pulses of hydrothermal activity in the area. Scolecite and mesolite were precipitated from Ca-rich solution, whereas the second stage Na-rich, low-temperature fluid crystallized natrolite and reacted with Ca-species.

A gonnardite-thomsonite-chabazite-calcite assemblage forms a cement in the Foveaux Formation, a fossiliferous gabbroic boulder bed that accumulated at the base of a sea cliff cut in a Permian igneous complex during late Oligocene±early Miocene time. Gonnardite was the earliest zeolite to form, locally following minor calcite. It was followed epitaxially by thomsonite, co-precipitating with chabazite. Crystal habits indicate a low-temperature origin. The maximum temperature to which the deposit may have been subjected is estimated as not more than ∽30°C. The chabazites are Ca-poor chabazite-K and chabazite-Na. Representative electron microprobe analyses are as follows, all + nH2O:

• thomsonite: Na3.77Ca7.73(Al19.39Si20.65)O80 and Na3.78K0.04Ca7.25Mg0.05(Al19.13Si21.05)O80;

• gonnardite: Na6.95K0.03Ca4.73(Al16.99Si23.15)O80 and Na8.56K0.03Ca4.05(Al17.32Si22.84)O80;

• chabazite-K: Na1.18K1.72Ca0.08Mg0.23(Al3.51Si8.49)O24 and Na1.67K1.92Ca0.18Mg0.17(Al4.11Si7.85)O24;

• chabazite-Na: Na2.51K1.13Ca0.17Mg0.02(Al4.08Si7.93)O24.

Such a Si-poor zeolite assemblage is unusual for marine sediments and is attributed to precipitation from marine water impoverished in silica in the gabbroic boulder bed and interacting with shell material and calcic plagioclase. In contrast, a dioritic clast in the boulder bed provides an example of less silica-poor zeolites originally formed in the parent igneous complex. Veinlets in the clast contain scolecite averaging Na1.19Ca7.36(Al15.84Si24.14)O80.nH2O, and mesolite averaging Na5.13K0.03Ca5.24 (Al15.93Si24.13)O80.nH2O, in part as sub-microscopic intergrowths. The composition of scolecite closely corresponds to the most Na-rich scolecite reported hitherto.

Pseudomesolite from Carlton Peak, described by Winchell (1900), is shown to be mesolite by means of chemical and X-ray data. A proposal to this effect has been accepted by the International Mineralogical Association's Commission on New Minerals and Mineral Names. Electron microprobe analysis revealed variations in the composition of pseudomesolite and showed the presence of faroelite. The X-ray powder diffraction pattern is similar to that of mesolite. Single-crystal Weissenberg photographs showed a twinning intergrowth which is explained by a 90° rotation of 50% of the unit cells about the c-axis, so that the a- and b-axes of rotated cells coincide with the b- and a-axes respectively of the unrotated cells. This twinning can not be detected optically. Mesolite has recently been proved to be orthorhombic, contrary to the long-held view that it is monoclinic.

Pseudomesolite from Oregon is also shown to be mesolite by single crystal Weissenberg photographs. A wet chemical analysis shows this material to be extremely silica-rich.