Four chlorite polytypes of Bailey and Brown (1962) have been identified by X-ray diffraction in clay-size chlorites of soils, sediments, and sedimentary rocks: (1) IIb, the polytype of common metamorphic and igneous chlorites; (2) Ib(ß = 90°); (3) Ib(ß = 97°); (4) Ia. An additional stacking arrangement. Iba, is defined herein as disordered chlorite which lacks an h0l diffraction band in the 2.4–2.5 Å region.

Most type-I chlorites are authigenic, as demonstrated by thin-section petrography. Type-I chlorites form during diagenesis, or less commonly during halmyrolysis, at temperatures and pressures less than those of low-grade metamorphism. A type-1 crystallization sequence is proposed, from least to most stable: Iba → Ib(ß = 97°) → Ib(ß = 90°). Conditions of low-grade metamorphism usually are necessary to cause conversion of Iba(ß = 90°) to IIb, the most stable and common polytype. Chemical composition has little or no influence upon polytype relative stabilities; temperature is much more important.

Sediment source areas with high relief, abundant rainfall, cold climate, and which contain IIb-chlorite-bearing metamorphic rocks, may yield essentially unweathered IIb chlorite to sites of deposition. Thus, clay-size IIb chlorite in unmetamorphosed sedimentary rocks can be interpreted as detrital. Caution is required, however, because IIb may be able to form authigenically at submetamor-phic temperatures, because it is the most stable polytype. Petrographic evidence is useful in such cases.

Chlorite polytypism as a geothermometer can be applied to several geologic problems: (1) the authigenic versus detrital origin of clay minerals in sedimentary rocks, particularly in graywacke matrix; (2) the recognition of diagenetic facies or gradients, areally and stratigraphically, within given geologic provinces; (3) the detection of hydrothermal and incipient metamorphic effects. Chlorite polytypism merits general application as an interpretive tool.

When a mineral of the montmorillonite group, saturated with Na cations, is placed in a dilute solution of hydrochloric acid, and maintained at 200°C, it should alter and produce the corresponding mineral of the kaolinite group, according to the following reaction: triphormic clay + H+ ⇌ diphormic clay + SiO2 + Na+. The formation of the diphormic clay should depend only on the value of the [Na+]/[H+] ratio, for the various temperatures used in the process. In a number of experiments, a few minerals of various types were subjected to this alteration process, carried out under a wide variety of conditions; namely, duration of the alteration treatment, clay and acid concentrations and value of the [Na+]/[H+] ratio. Formation of kaolinite, was not found but instead either an amorphous gel, or a well-crystallized boehmite, or else no dissolution at all of the initial mineral. We have already shown that a feldspar, when subjected to an alteration under similar conditions, never produces kaolinite, but forms a poorly crystallized boehmite instead. This intermediate product is the only one able to fix silica in its numerous active sites, and thus produce kaolinite. In this investigation we sought to induce a formation of poorly crystallized boehmite, from montmorillonites, with a view to a subsequent growth of kaolinite. This was achieved by inserting layers of aluminum hydroxy-polymer between the layers of a montmorillonite, followed by an alteration conducted under the same conditions as previously. At the end of a 17 hr treatment, a number of fibers of poorly crystallized boehmite (pseudo-boehmite b) appeared. At the end of a 15 days period, large amounts of kaolinite were formed, and kaolinite alone remained at the end of one month. These experiments substantiate the need of some ‘storage’ of the aluminum in an intermediate poorly crystallized mineral. This is an essential preliminary condition to any formation of clay. This clay can be formed only when the silica-monomer can be fixed on active sites of the intermediate product.

An account is given of the interactions between clay minerals and organic polymers with particular reference to their applications in agriculture, foundation engineering and industry.

Active kaolinization of middle Tertiary obsidian and rhyolitic breccia in the San Andres range is effected by sulfurous hot waters and hydrous vapors emitted as springs, fumaroles, and geysers. The alteration preserves original rock textures and forms conspicuous aureoles around the hydrothermal vents, which shift about with slumping of the saturated clay. Systematic exploration by 87 test pits with maximum depth of 45 ft permitted inspection of the kaolin in place, and furnished bulk samples for laboratory work. Besides kaolinite, the clay contains unevenly distributed cristobalite and alunite, and traces of mercury. The natural brightness of the pure kaolinite approaches 90 if unstained by weathering, but little of it is pure. The yield of paper-coating fraction is small, and the composition and behavior in slurries are nonuniform. Most of the kaolinized material in the hydrothermal deposits is soft. Some, however, is hard and rock-like. Similar material occurs in clastic sediments of late Tertiary age, suggesting that the hydrothermal activity began at least as early as the Pliocene.