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

▪ Abstract Like South Africa's Greater Cape Floristic Region, the Southwest Australian Floristic Region (SWAFR) is species rich, with a Mediterranean climate and old, weathered, nutrient-deficient landscapes. This region has 7380 native vascular plants (species/subspecies): one third described since 1970, 49% endemic, and 2500 of conservation concern. Origins are complex. Molecular phylogenies suggest multiple dispersal events into, out of, and within the SWAFR throughout the Cretaceous and Cenozoic; in many phylogenetically unrelated clades; and from many directions. Either explosive speciation or steady cladogenesis occurred among some woody sclerophyll and herbaceous families from the mid-Tertiary in response to progressive aridity. Genomic coalescence was sometimes involved. Rainforest taxa went extinct by the Pleistocene. Old lineages nevertheless persist as one endemic order (Dasypogonales) and 6–11 endemic families. Such a rich flora on old landscapes that have been exposed to European land-use pra...

The Southwest Australian Floristic Region: Evolution and Conservation of a Global Hot Spot of Biodiversity

A new kinetic approach to modeling water-rock interaction: The role of nucleation, precursors, and Ostwald ripening

Palaeoclimatic interprétation of clay minerals in marine deposits : an outlook from the continental origin.

Fire effects on soil aggregation: A review

This paper presents and reviews the processes responsible for the distribution and formation of regolith and associated landscapes of the Yilgarn Craton and highlights their implications for mineral exploration. It provides an analysis of regolith geology investigations that were conducted in many districts of contrasting contemporary geomorphic and climatic conditions.The Yilgarn Craton is composed of Archaean rocks, predominantly granitoids, that are crossed by north‐northwest‐trending belts of greenstones. It has an arid to humid climate at present. The gently undulating landsurface forms a partial etchplain and the topography is largely related to bedrock lithologies and a complex history of valley development and aggradation. Deep weathering has affected most lithologies and geological provinces across the craton. The depth of the weathered mantle may be as much as 150 m, but it varies considerably and rock outcrop may occur in any part of the landscape. The main factors influencing extent of weather...

Regolith geology of the Yilgarn Craton, Western Australia: Implications for exploration

Evolution of regoliths and landscapes in deeply weathered terrain — implications for geochemical exploration

Eucalyptus trees may translocate Au from mineral deposits and support the use of vegetation (biogeochemical) sampling in mineral exploration, particularly where thick sediments dominate. However, biogeochemistry has not been routinely adopted partly because biotic mechanisms of Au migration are poorly understood. For example, although Au has been previously measured in plant samples, there has been doubt as to whether it was truly absorbed rather than merely adsorbed on the plant surface as aeolian contamination. Here we show the first evidence of particulate Au within natural specimens of living biological tissue (not from laboratory experimentation). This observation conclusively demonstrates active biogeochemical adsorption of Au and provides insight into its behaviour in natural samples. The confirmation of biogeochemical adsorption of Au, and of a link with abiotic processes, promotes confidence in an emerging technique that may lead to future exploration success and maintain continuity of supply.

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Natural gold particles in Eucalyptus leaves and their relevance to exploration for buried gold deposits

During the weathering of gold deposits, exceptionally pure, <200 nm diameter, nanoparticulate gold plates (6 nm thick) are formed. The particles display controlled growth of both size and shape and signs of assembly to form belts and sheets. The gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event. The size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids. This represents the fi rst direct observation of colloidal nanoparticulate gold in nature, confi rming this as an active mechanism of gold transport during the weathering of gold deposits.

Naturally occurring gold nanoparticles and nanoplates

The Yilgarn Craton is one of the world's principal mineral provinces, with considerable resources of gold, nickel, bauxite and iron ore, as well as lesser amounts of a wide range of other commodities. As such, it is a major target for exploration. However, as most of the Craton has been exposed to sub-aerial conditions since at least the Late Proterozoic, it has had a long history of weathering and erosion under a wide variety of climatic conditions, resulting in a widespread and complex regolith cover. This diverse and extensive regolith poses considerable difficulties for exploration—as well as opportunities in terms of different sample media and the formation of resources within the regolith itself. The development of appropriate geochemical exploration procedures has been the subject of considerable research by government, university and industry for nearly 50 years. This guide is intended to summarise the results of this research in a single, stand-alone document to assist mineral explorers. It provi...

R. R. Anand

Papers

Regolith geology of the Yilgarn Craton, Western Australia: Implications for exploration

Evolution of regoliths and landscapes in deeply weathered terrain — implications for geochemical exploration

Natural gold particles in Eucalyptus leaves and their relevance to exploration for buried gold deposits

Naturally occurring gold nanoparticles and nanoplates

A guide for mineral exploration through the regolith in the Yilgarn Craton, Western Australia