The effect of dry grinding of kaolin minerals by a mechanical mortar was examined by x-ray, thermal, and electron microscopic methods. Base exchange capacity and apparent density were also measured. In the early stage of grinding, kaolin crystals cleave and fracture and then split into fine crystals which are considered unit crystallites. Such crystallites gradually change to a disordered kaolin and partially decompose into a noncrystalline substance as grinding progresses. The crystalline and the noncrystalline substances promptly reaggregate and these reaggregated particles have a radial shape. After further grinding, the structure of the reaggregated particle seems to be zeolitic. Finally, the kaolin mineral thoroughly changes to a completely amorphous substance similar to a silica-alumina mixed gel. Consequently, the effect of dry grinding of kaolin is related to the degree of crystallinity of the original kaolin mineral.

Various physical properties of an acid-treated Texas bentonite were studied as a function of treatment time. The observed kinetics were pseudo first-order (large excess of acid) and on this basis and the x-ray results a model and mechanism are proposed that are consistent with observed surface area, exchange capacity, and surface acidity changes. A crude estimation of activation energy gave a value lower than that of Osthaus (1956) but of the right magnitude.

Further x-ray and chemical work on suspended sediment samples and cored samples from the James River and its estuary support earlier proposals by the author. A chlorite-like clay is forming from weathered illite through a mixed-layer illite—vermiculite-chlorite stage, and some illite is seemingly regenerated to a better illite by potassium fixation.

Chemical analyses of interstitial water, hydrochloric acid-leachate, and fused samples offer explanations regarding the chemical changes occurring in clays as composition of the environment changes.

Magnesium is adsorbed by clays to a far greater degree than potassium in the marine and brackish environment.

The variance between clays found in Recent and ancient sediments is related to and explained by the concept of the equivalence level. It is suggested that K+ is adsorbed preferentially to Mg2+ by clays when they have been buried to a depth that is greater than that of the Mg2+ — K+ — equivalence level; above this level Mg2+ is preferentially adsorbed by the clays.

The trifold nature of clay minerals in terms of their origin and distribution is briefly discussed.

An extensive survey of clay mineral relationships in the subsurface Wilcox formation (Eocene) has shown progressive diagenetic conversions with depth. Montmorillonite, a common constituent of Wilcox outcrop material, becomes loss evident below 3000 ft and is not normally found in an unmixed state below the 9000—10,000 ft overburden level. At depths between 3000 and 14,000 ft, montmorillonite lattices are commonly interpersed with illite components, the frequency of which increases with depth to a virtual elimination of montmorillonite swelling characteristics below 14,000 ft.

Chlorite is present at all stratigraphic levels including surface exposures. It appears to be a more dominant constituent at depth; however, observed increases in basal intensities in samples that had been more deeply buried may result from more perfect crystal development rather than quantitative differences.

The diagenetic conversion of montmorillonite to illite and possibly to chlorite has resulted in a distribution of the last two minerals that is related to estimated depositional environments as reconstructed from micro-paleontological criteria in at least one well in southern Louisiana. It is inferred, therefore, that different chemical characteristics in the ancient Wilcox seas are responsible for the distribution coincidence even though the mineral groups defining the distribution were not necessarily indigenous to the ancient Wilcox sea.

Correlations between clay minerals and environment are not particularly noted in the investigation of analogous environmental situations in Recent sediments, and it is thus concluded that the environment indicators which appear as minerals in lithified sediments such as the Wilcox are present in freshly deposited sediments in the form of submineralic chemical constituents such as absorbed ions, molecular groups or subcrystalline lattice configurations which are not detected by the ordinary clay mineral identification procedures. The change to mineralic form is aided by the natural “bombing” resulting from normal increases in temperature and pressure due to burial. The process is apparently a continuum through the entire subsurface residence of the sediment and the results are seemingly in accordance with phase equilibria.

Volcanic and intrusive rocks (mainly andesite and monzonite) of Tertiary age are associated with gold- and silver-bearing quartz veins. Hydrothermal alteration took place in two stages. The first and less intense alteration stage is associated with economic mineralization; metallization during the second and more intense stage is limited to pyrite.

Alteration zones from the vein outward include: (1) dickite, (2) illite—kaolinite, (3) vermiculite—halloysite, and (4) chlorite—montmorillonite. Mixed-layer associations of 2:1 clay minerals are abundant and represent transitions between discrete clay minerals. Apparent occurrence of allophane in the illite-kaolinite zone indicates that amorphous aluminum silicates are intermediate phases in transitions from 2: 1 to 1: 1 clay minerals.

Alteration of ferromagnesian minerals and plagioclase resulted largely in the formation of 2: 1 clay mineral types. Orthoclase is altered chiefly to illite and kaolinite. Dickite and quartz are end products of the most intense alteration in the area.

Chemical analyses of the altered rocks in general show a decrease in basic ions toward the vein. Less significant changes are shown by acidic ions. Analyses of illites indicate that substitution of K by Na, Fe, Ca and Mg has taken place near the vein as shown by an overall decrease in $\frac{K}{Na},\frac{K}{Fe},\frac{K}{Ca}$$\frac{K}{Na}, \frac{K}{Fe}, \frac{K}{Ca}$ and $\frac{K}{Mg}$$\frac{K}{Mg}$ ratios. Both sodium and potassium seem to be instrumental in the formation of well-developed illite crystals.

Alteration stages operative in the Cochiti district were developed by the action of solutions probably ranging in pH from 4 to 10. Alteration zones were formed penecontemporaneously with gradual outward migration of the least intense zones.

A mineralogical study by the Bureau of Mines and the University of Utah of the shale beds on the 1600 ft level of the Ophir Hill mine, Utah, disclosed that the micaceous minerals in the shale were altered as a function of proximity to the sulfide ore zones. The principal mineral in the shale distant from ore is 2M serieite. Within 100 ft along the bedding plane and 15 ft normal to the bedding plane, the serieite was altered successively to a modified form of serieite, two polymorphs of chlorite, and finally phlogopite.

A general sequence of the mineral changes from barren to ore-bearing ground was: (I) serieite; (2) modified serieite; (3) modified serieite and 7Å chlorite; (4) modified serieite, 14Å chlorite, and phlogopite; (5) 14Å chlorite and phlogopite; and (6) phlogopite. This sequence of alteration of serieite in the shale was not noted in the mine areas barren of ore.

An unusual halloysite deposit occurs on the west side of the Lake Mountains in Utah County, Utah. A field and laboratory study was made of this Fox deposit to determine the paragenesis of the clay minerals, especially halloysite. Successive lenses of unaltered tuff, partially altered tuff, clay and travertine beds, indicate that the clays have resulted from alteration of siliceous volcanic tuffs in a Tertiary (?) calcareous hot spring environment. Clay minerals identified were montmorillonite, kaolinite, halloysite·4H2O (endellite) and halloysite·2H20. Montmorillonite, which developed in a zone of less intensive silica leaching farthest from the hot spring vents, is the predominant clay mineral. Irregularly distributed pockets of halloysite and kaolinite developed nearest the hot spring vents in a high-calcium environment. In the clay, or associated with it, are calcite as travertine, quartz as rounded carbonate-corroded grains, feldspar, tridy- mite, biotite and glass.

Experiments that approximate the chemical environment prevalent in the halloysite alteration zone were conducted on phase relations in part of the system lime—alumina- silica—water. Results suggest the formation of halloysite·4H2O in this kinetic system from intermediate calcium aluminate or calcium silicate hydrates with halloysite-type structures, or both.

Examination of samples from shale outcrops on the periphery of the Utah Valley graben has revealed an unusual association of disseminated pyrophyllite with various clay minerals. Numerous samples were obtained from exposures in brick clay pits located in the Manning Canyon formation of Mississippian-Pennsylvanian age and from the Long Trail member of the Great Blue formation of Mississippian age. Associated clay minerals in these beds include illite, illite-montmorillonite mixed-layer clays, 7 Å chlorite, 14Å chlorite, kaolinite and sericite. Associated nonclay minerals include quartz, calcite, and small amounts of dolomite. Associated secondary minerals are calcite, aragonite, jarosite, gibbsite and gypsum.

Three possible explanations of genesis have been considered in the present study: deposition of detrital pyrophyllite, surface weathering under special conditions, and hydrothermal or pneumatolytie activity. The third alternative explanation, hydrothermal or pneumatolytie activity, is believed to be the most acceptable. It is hypothesized that magnesium, iron, and interlayer cations have been removed from some of the original 2 : 1 layer clays in the shale by solutions localized along fault zones.

The Hector, California, bentonite has been found unusual in many respects, and its thermal stability under hydrothermal conditions is no exception. It is important that the stability of this bentonite be known, as it is an end member of the saponite series with all cation positions, excluding interlayer, in the structure filled. The information is applicable to studies of wall rock alteration in which saponites occur as alteration products associated with ore deposition.

This saponite has an upper thermal stability limit of 750°C, at 15,000 psi water pressure, at which temperature it decomposes. The only products observed were talc and vapor; above 780°C the decomposition products observed were anthophyllite and vapor. Syntheses on the hectorite composition corroborate these decomposition temperatures and products.

Hydrothermal treatment at temperatures below about 450°C produce no change in the x-ray diffraction pattern of hectorite: on drying, the lattice collapses to a basal spacing (10–12Å) approaching that of mica, and with ethylene glycol expands to 17Å. However, after hydrothermal treatment above this temperature and to decomposition at 750°C, the hectorite, after subsequent cooling, collapses on drying at 110°C or less to 9.4Å, but expands as before to 17 Å with glycol saturation. Without the glycol treatment the resulting material could be mistaken for talc and considered the decomposition product of hectorite at the higher temperatures. This behavior also occurs using the synthetic hectorite but does not occur when using synthetic or natural saponites containing aluminum.

For comparison, the relative stabilities of some dioctahedral and trioctahedral layered minerals are given to show the consistently higher stabilities for the minerals whose octahedral positions are filled.

A Vermiculite from Beni-Uxera, North Africa, was refluxed with HC1 solutions of various concentrations to determine the effect of such a treatment on its surface area. The treated vermiculite was analyzed for “free” SiO2 and by x-ray diffraction, and the refluxing solutions were analyzed for the cations removed from the vermiculite. Surface areas were determined by means of adsorption isotherms of n-butane at 0°C.

The surface area of the acid-treated vermiculite increases with increase in concentration of the acid. Heating of the acid-treated samples or the natural samples decreases the surface area.

The increase in surface area by acid treatment is due to destruction of the vermiculite and its conversion to a “free” form of SiO2 which possesses a large surface area. x-Ray analysis indicated that this “free” SiO2 was not present in the interlayer positions of the crystal lattice of vermiculite.