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

This review examines the detailed chemical insights that have been generated through 150 years of work worldwide on magnesium-based inorganic cements, with a focus on both scientific and patent literature. Magnesium carbonate, phosphate, silicate-hydrate, and oxysalt (both chloride and sulfate) cements are all assessed. Many such cements are ideally suited to specialist applications in precast construction, road repair, and other fields including nuclear waste immobilization. The majority of MgO-based cements are more costly to produce than Portland cement because of the relatively high cost of reactive sources of MgO and do not have a sufficiently high internal pH to passivate mild steel reinforcing bars. This precludes MgO-based cements from providing a large-scale replacement for Portland cement in the production of steel-reinforced concretes for civil engineering applications, despite the potential for CO2 emissions reductions offered by some such systems. Nonetheless, in uses that do not require steel reinforcement, and in locations where the MgO can be sourced at a competitive price, a detailed understanding of these systems enables their specification, design, and selection as advanced engineering materials with a strongly defined chemical basis.

Magnesia-Based Cements: A Journey of 150 Years, and Cements for the Future?

Publisher Summary The chapter provides details of the chemistry of the minerals frequently present in ultramafic rocks, and discusses the ecology of serpentine soil. Serpentine is used by biologists to describe a group of ultramafic rocks and the soils derived from them. Three principal polymorphic forms of serpentine are recognized: chrysotile, antigorite, and lizardite. The processes by which serpentine rocks develop into soils depend on climate, time, relief, and biotic factors along with the chemical composition of the parent material. As a result many types of soil occur on serpentines. The characteristics of serpentine are briefly discussed. Serpentine vegetation has two major characteristics: Physiognomic differences from the vegetation of surrounding rocks, and rare species and combinations of species. The vegetation of serpentines presents a wide range of appearances. Biologists have concentrated on the situations where serpentine vegetation is in sharp contrast with that of the surroundings. Such contrasts do not always occur and when this happens the serpentine vegetation is often not documented. The more closely studied serpentines have many vegetation features in common but the causes of these similarities can be very different. There is much scope for further work in many fields on serpentines.

George T. Faust

Papers

A restudy of stevensite and allied minerals

Thermal analysis studies on carbonates I. aragonite and calcite

The serpentine-group minerals

The hydrous nickel-magnesium silicates—the garnierite group,

Stevensite, redefined as a member of the montmorillonite group