Halloysite clay minerals are ubiquitous in soils and weathered rocks where they occur in a variety of particle shapes and hydration states. Diversity also characterizes their chemical composition, cation exchange capacity and potassium selectivity. This review summarizes the extensive but scattered literature on halloysite, from its natural occurrence, through its crystal structure, chemical and morphological diversity, to its reactivity toward organic compounds, ions and salts, involving the various methods of differentiating halloysite from kaolinite. No unique test seems to be ideal to distinguish these 1:1 clay minerals, especially in soils. The occurrence of 2:1 phyllosilicate contaminants appears, so far, to provide the best explanation for the high charge and potassium selectivity of halloysite. Yet, hydration properties of the mineral probably play a major role in ion sorption. Clear trends seem to relate particle morphology and structural Fe. However, future work is required to understand the possible mechanisms linking chemical, morphological, hydration and charge properties of halloysite.

Halloysite clay minerals — a review

Homogeneous and heterogeneous nucleations in the polyol process for the preparation of micron and submicron size metal particles

Phyllosilicates, and among them clay minerals, are of great interest not only for the scientific community but also for their potential applications in many novel and advanced areas. However, the correct application of these minerals requires a thorough knowledge of their crystal chemical properties. This chapter provides crystal chemical and structural details related to phyllosilicates and describes the fundamental features leading to their different behaviour in different natural or technical processes, as also detailed in other chapters of this book. Phyllosilicates, described in this chapter, are minerals of the (i) kaolin-serpentine group (e.g. kaolinite, dickite, nacrite, halloysite, hisingerite, lizardite, antigorite, chrysotile, amesite, carlosturanite, greenalite); (ii) talc and pyrophyllite group (e.g. pyrophyllite, ferripyrophyllite); (iii) mica group, with particular focus to illite; (iv) smectite group (e.g. montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite); (v) vermiculite group; (vi) chlorite group; (vii) some 2:1 layer silicates involving a discontinuous octahedral sheet and a modulated tetrahedral sheet such as kalifersite, palygorskite and sepiolite; (viii) allophane and imogolite and (ix) mixed layer structures with particular focus on illite-smectite.

Structure and Mineralogy of Clay Minerals

The interaction between soil particles and the soil solution depends on the types
 and amounts of soil clays. Montmorillonite is the most reactive clay. Ca-clay forms
 tactoids (quasi crystals) and has limited swelling and dispersion. Na-clay forms single
 platelets and disperses freely. In a mixed Na/Ca system, “demixing” of the cations
 occurs as the Na ions concentrate on the external and the Ca ions on the internal
 surfaces of the tactoids. The demixing explains why a small percentage of exchangeable
 Na sharply increases the zeta potential and dispersion of montmorillonite clay.
 The hydraulic conductivity of a soil depends on both Na and the total salt concentration
 of the percolating solution. High hydraulic conductivity may be maintained, even at
 high exchangeable sodium percentage (ESP) values, if the solution concentration is
 above a critical (threshold) level. When waters of very low salt content are used,
 decreases in hydraulic conductivity and clay dispersion occur even in soils with low
 ESP values (<10).
 The main mechanism for hydraulic conductivity reduction in waters of medium to high
 salinity (replacecodegt0.5 dS/m) is clay swelling. High content of expansible clays
 increases the susceptibility of soils to intermediate (10<ESP<20) sodicity. The effect
 of low sodicity (ESP < 10) on clay swelling is minimal and the hydraulic conductivity
 of these soils is maintained at high values.
 When waters having low salt concentrations (salt concentration below the flocculation
 value of the clay) are used, clay dispersion, movement, and lodgement in the conducting
 pores comprise the main mechanism responsible for hydraulic conductivity reduction.
 These results occur even at low ESP. The most important soil property which determines
 its susceptibility to sodic conditions, when the soil is leached with rain or snow
 water, is the potential of the soil to release electrolytes from primary minerals
 and/or CaCO3 dissolution. Soils which release sufficient electrolytes to maintain the concentration
 of the soil solution above the flocculation value of the clay will not disperse, and
 thus will not be sensitive to low sodicity.

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Response of soils to sodic and saline conditions

The writer has established reference intervals in the subsurface on the basis of apparent systematic interlayer water loss of swelling clay minerals. The intervals are used in much the same manner as the familiar indicators for metamorphism, but are present at sufficiently shallow depths to be evident within oil-bearing strata of the Gulf Coast. The resulting conclusion is that clay-mineral diagenesis indicators may prove to be important petroleum-evaluation markers as well as fundamental properties of sedimentary basins. Sedimentary basins are viewed as combinations of gases, liquids, and semisolids distributed through a solid matrix. During geologic development the interstitial components segregate by migration and produce various commercially exploitable concentrations. Water, the principal fluid component of the sedimentary section, is thought to migrate in three separate stages. Initially, pore water and excessive (more than two) clay-water interlayers are removed by the action of overburden pressure. This initial water flow (which is essentially completed after the first few thousand feet of burial) reduces the water content of the sediment to about 30 percent, most of which is in the semi-solid interlayer form. A second stage of dehydration is thought to occur when the heat absorbed by the burie sediment becomes sufficiently great to mobilize the next-to-last water interlayer in an M(H2O)x + ^DgrHr = 1 + XH2O fashion. The final stage of sediment dehydration which extracts the last remaining water monolayer from clay lattices is apparently very slow, even by geologic standards, requiring tens or possibly hundreds of millions of years depending upon the geothermal and burial history of the sediment. Petroleum hydrocarbons which are distributed throughout the matrices of potential source beds in normal frequencies of 300-3,000 ppm are thought to be too sparse to initiate continuous fluid flow. In normal marine sediments, however, the water associated with clay minerals is present to a considerable depth in the order of 200,000 ppm, and therefore it is reasoned that this phase forms the connection between petroleum source and reservoir beds. The first and last dehydration stages are probably unimportant in Gulf Coast oil migration, inasmuch as they occur, respectively, at levels too shallow and too deep to intersect the interval of maximum liquid petroleum availability. The amount of water in movement during the second stage, at a level which does intersect this interval, is 10-15 percent of the compacted bulk volume and represents a significant fluid displacement capable of redistributing any mobile subsurface component. A measure of the degree to which the second-stage interlayer water has been discharged into the system can be noted on X-ray diffractograms. The movement appears to occur in a relatively restricted, depth-dependent temperature zone in which the average dehydration temperature of the points measured is 22 °F. With the use of an empirically derived P/T curve and a geothermal-gradient map, a set of regional subsurface dehydration contours can be constructed. A plot of 5,368 liquid petroleum production depths referenced to this dehydration "surface" shows an almost perfect Gaussian distribution. It seems significant that, although the dehydration depths range from 4,000 to 16,000 ft, hydrocarbon production depths are distributed in a statistically consistent relation to the calculated clay-dehydration contour surface.

George Brown

Papers

The x-ray identification and crystal structures of clay minerals