Geothermal fluid discharged by steam wells, which have been core drilled to depths as much as 4500 ft at Wairakei, New Zealand, has altered Pliocene to Pleistocene, silicic, mainly glassy volcanics and related aqueous tuffs and breccias. Measured temperatures (max. 265°C) indicate epithermal to mesothermal conditions in buried fault fissures, the locus of both the hydrothermal fluid and most intense alteration.

A supergene kaolinite alteration zone is distinguished from hypogene Ca-montmorillonite and combined micaceous and chloritic zones. The hypogene zones are usually wide and are temperature dependent and localized along fault fissures. Other common hypogene minerals are alkali feldspars, wairakite, epidote, quartz, calcite, laumontite, ptilolite, pyrite and pyrrhotite. Prehnite is rare.

The micaceous clay minerals include illite (about 10 Å) and a series of random mixed-layer illite-montmorillonites with d(001) values ranging from 10·28 to 12·45 Å. The amount of interstratified montmorillonite is related to temperature and fault fissures. Apart from rare mixed-layer swelling chlorite, the Fe-rich chloritic clay shows little or no variation in its composition. Both the micaceous clays and the chloritic clay result from alteration of earlier formed Ca-montmorillonite.

The stability of hydrothermal minerals is controlled by the temperature and chemical composition of the geothermal fluid ascending along the fault fissures. K-feldspar but not albite is deposited on fissure walls, but both alkali feldspars replace primary soda-lime plagioclase in the wall rock. The absence of albite on fissure walls is ascribed to low aqueous mNa+/mK+ ratio. Primary quartz is not affected by the altering solution but hydrothermal quartz is deposited on fissure walls and in the wall rock.

Plagioclase feldspar is one of the outermost minerals destroyed in the hydrothermal alteration of quartz monzonite of the Butte district, Montana. Single grains of andesine plagioclase showing varying stages of fringe alteration were selected to show the sequence of events leading from the fresh plagioclase tectosilicate to the phyllosilicate montmorillonite, and to evaluate the role of petrographically-identified allophane in the alteration process.

Replicas of fresh, incipiently, and thoroughly montmorillonitized grains reveal a complex sequence of events which may follow alternate equivalent paths. Quantitatively most important is the formation of a hobnail texture composed of sparse to dense ~0.5 μ discoids of amorphous material on feldspar cleavage surfaces. These discoids appear to fuse laterally to form amoeboid or lobate amorphous clusters. Some assume progressively more polygonal outlines characterizing the montmorillonite morphology, or their fringes become flake-like montmorillonite which peels away in wisps from the pitted feldspar surface. Wisps may persist singly, they may form minute ropy granules, they may develop into wispy ridges, or the entire surface may convert to ragged, wispy, or compact montmorillonite. Far less commonly, plagioclase appears to develop without a discernible amorphous stage. The alternative paths are probably influenced by the chemical activity of magnesium in the system.

A deposit of kaolin clay, the site of the General Zaragosa Mine, located about 50 km northwest of San Luis Potosi, S. L. P., Mexico, has produced more than 250,000 tons of refractory clay by room and pillar mining methods during the last 32 years. The clay was formed by hydrothermal argillation of part of a fault block of rhyolite flow-breccia and probably welded tuff, presumably lower Tertiary in age. A silica-rich gossan now exposed at the surface caps the clay deposit.

Samples collected sequentially from the fresh rock to the most highly altered kaolin and studied by optical, X-ray, DTA, and chemical methods, show progressively:

1. Bleaching of the breccia matrix accompanied by mobilization of much of the iron, which was concentrated locally as spots of reddish oxide in the least altered areas.

2. Mobilization, and removal of considerable alkali and alkaline earth metals, in excess of that required to form montmorillonite; mobilization of silica, which was redeposited as fine anhedral quartz crystals disseminated within the montmorillonite and associated kaolinite-halloysite, or developed tripolitic, argillized rock peripheral to the clay ore body.

3. Further desilication of the silica and clay yielding relatively pure, but poorly ordered, kaolinite-halloysite as the most intense end product of argillation. Much silica removed during intense argillation was reprecipitated as tripolitic clay, as minor cristobalite in microscopic globules and massive aggregates characteristic of colloform opal, and in vastly larger amounts as opal and chalcedony replacing the rock overlying the clay (forming a silica gossan).

4. Minor amounts of alunite are present at two places in the deposit, which are interpreted tentatively as being the most probable loci of rising solutions.

Hydroxy-Al and -Fe interlayers were prepared in “mono-mineralic” fine and coarse clay montmorillonite and coarse clay and silt vermiculite at 3°C and 21°C. The formation of hydroxy interlayers was evaluated by X-ray diffraction, cation exchange capacity, and chemical analyses.

At comparable particle size and regardless of temperature, the amounts of hydroxy-Al and -Fe interlayers in montmorillonite exceeded by far those formed in vermiculite. Likewise, the aluminum systems exhibited a higher degree of interlayering than iron systems.

Within montmorillonite, the amount of hydroxy-Al and -Fe interlayers increased as the particle size decreased, regardless of temperature. The aluminum interlayered montmorillonite equilibrated at 3°C was characterized by basal spacings of about 17 Å after Ca-saturation plus 54 per cent relative humidity. These spacings are larger than those normally observed for smectites.

Within vermiculite systems equilibrated at 3°C, more hydroxy-Al interlayers were recorded in coarse clay than in silt fraction, whereas at 21°C about equal amounts of interlayers were formed. By contrast, hydroxy-Fe interlayer was favored by the silt fraction at both temperature levels.

The formation of aluminum interlayers in both minerals increased with increasing temperature. The formation of hydroxy-Fe interlayers in montmorillonite was generally not temperature dependent, whereas the formation of such interlayers in vermiculite increased slightly with increasing temperature.

These data may partially explain the formation of chloritic intergrades in soils as a function of type of minerals, kind of ions, and thermal variations.

Electron microscopy of particles in size fractions (0·5–1·0 μ, 0·1–0·5 μ, 0·05–0·1 μ and < 0·05 μ e.s.d.) obtained by centrifugation of Na-saturated montmorillonites from Wyoming (two samples); Chambers, Arizona; and Fayette County, Texas, has shown details of particle morphology. The finest fraction of each montmorillonite consists predominantly of very thin, separate flakes whereas all coarser fractions, totalling 80 per cent or more by weight of the samples studied, are composed of microaggregates. For all the samples, preferred orientation is best developed in specimens formed from flakes of the finest fractions. Microaggregates are stable in dilute suspension although they swell to give large interlayer spacings, but can be disrupted into smaller, thinner flakes by ultrasonic vibration. Differences in dispersion behavior between separate flakes and microaggregates are not due to differences in interlayer charge or chemical composition, which are very small between fractions of each sample, but are thought to be due to the interlocking of flakes in microaggregates during crystal growth, which prevents their complete separation in dilute suspension.

The mechanism of boron uptake by clays, especially illitic clays, and the factors which control such uptake have long been debated issues. In an attempt to answer some of the questions of the controversy, three illites were treated in solutions containing boron. In the study, boron concentration, salinity, temperature, and time were varied independently over rather wide ranges.

For the illites studied, irreversible uptake of boron was increased by increasing boron concentration, salt concentration, temperature, and time of treatment. The amount of boron which was fixed also varied with the type of illite treated. The amount of fixation was controlled primarily by the specific surface area of the clay and also by the crystallinity, K content, and/or amount of mixed-layer material (“frayed-edge” development?), and apparently was independent of the original boron content of the clay.

A two-step mechanism is proposed for boron fixation by illite, consisting of rapid chemical adsorption of the tetrahedral B(OH)4− anion at the “frayed-edge” of the illite flake followed by much slower diffusion of boron into the tetrahedral part of the structure.

A kaolin bed in which many of the individual kaolinite platelets exceed 0·2 mm in size occurs in the immediate vicinity of the base of the middle-Miocene Kirkwood Formation, near Woodstown, New Jersey. These platelets appear to have resulted from breakdown of pre-existing illite and montmorillonite coupled with concurrent epitaxial growth and diagenetic growth of primary kaolinite. The alteration of the clay minerals is thought to be a product of upward leaching (dialysis) by groundwaters in the underlying Vincentown Sand. This is further evidenced by abnormally high percentages of clay-size kaolinite in the clays that lie above the Vincentown Sand but beneath the macro-kaolinite horizon. Growth of the macro-kaolinite was facilitated by face-to-edge sédimentation and the resultant high permeability of the stratum.