Aluminium monohydroxide, also known as aluminium oxyhydroxide (boehmite - AlO[OH]), is water insoluble but crystallises into microcrystals of various shapes. When, by X-ray diffraction, the microcrystals present a basal reflexion (d[020]) of 0.611 nm, the crystalline structure is referred to as "well-crystallised" boehmite. Natural and synthetic crystals of well-crystallised boehmite can have a plate-like shape with either a rhombic or hexagonal profile. Synthetic crystals can also be lath-like or ellipsoid in shape. The purpose of this paper is to present a method of hydrothermal synthesis using a single temperature (200 ºC) for preparing plate-like crystals of well-crystallised boehmite with ellipsoid, rhombic, hexagonal, and lath-like profiles by using different precursors. Our observations suggest that all of these shapes are stages of growth of the microcrystals of well-crystallised boehmite along the c-axis direction of the rhombic crystals.

The simultaneous use of the specific values of some structural and chemical properties of clay minerals, such as kaolinite, montmorillonite and talc, allows the development of new properties for these materials, especially in relation to the external and internal microcrystal surfaces. These developments are very diversified for montmorillonite, due to the high specific surface area, expansible basal spacings, easy intercalation inside the 2:1 structural layers and a reversible and high cation exchance capacity. The review presents examples of chemical modifications on kaolins, montmorillonites (bentonites) and talcs.

Special clays are a group of clays different from the large volume of clay mineral products named "Industrial Clays": kaolins, ball clays, refractory clays, bentonites, fuller's earths, common clays. Two groups of special clays exist: rare, as in the case of hectorite and sepiolite and restricted areas, as in the case of white bentonite, halloysite and palygorskite (attapulgite). A review is given of the most important producers of the special clays and their properties in the Western World, as well as a discussion of the occurrence of these types of clays in Brazil.

Fuller's earth and acid activated smectitic clays are largely used as bleaching earth for the industrial processing of vegetable, animal and mineral oils and waxes. The paper comments about the nomenclature used for these materials, the nature of the acid activation of smectitic clays (bentonites), activation laboratory procedures and presents a review of the acid activation of bentonites from 20 deposits from several regions of Brazil. The activated clays were tested and show good decolorizing power for soybean, castor, cottonseed, corn and sunflower oils.

Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (ALCOA 123) and aqueous solution of acetic acid at 96ºC ±1ºC. The white powder of Al(OH)(CH3COO)2 is constituted by agglomerates of crystalline plates, having size about 10mum. The crystals were fired from 200ºC to 1550ºC, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300ºC); delta (750ºC); alpha (1050ºC). The aluminas maintain the original morphology of the Al(OH)Ac2 crystal agglomerates, up to 1050ºC, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550ºC. High surface area aluminas are formed in the temperature range of 700ºC to 1100ºC; the maximum value of 198m²/g is obtained at 900ºC, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al2O3 which acts as precursor for the sequence gamma- to alpha-Al2O3.

A dimensão do Brasil e a diversidade da geologia se reflete em vários meios nos quais montagens de caulinita foram formadas. Os meios geológicos dos caulins brasileiros podem ser divididos nos seguintes grupos - caulins sedimentares, caulins oriundos de pegmatitas, de rochas graníticas, de rochas vulcânicas, e caulins derivados de anortosito. As argilas sedimentares são encontradas principalmente na bacia amazônica e aquelas adjacentes ao rio Jari estão sendo exploradas comercialmente para exportação como argilas para recobrimento de papel. Os caulins amazônicos são caracterizados por alto teor de ferro e titânia (estruturais) com baixos níveis de álcalis e exibindo cristais de caulinita euédricos. As pegmatitas do sudeste, quando não recobertas com óxido de ferro, tem extremamente baixos teores de ferro e titânia e uma mistura de caulinita 7Å / haloisita 10Å ocorre em todos depósitos. As pegmatitas do nordeste produzem caulins constituídos somente por caulinitas euédricas com ausência de haloisita. Os caulins de granito tem geralmente maiores teores de ferro quando comparados com pegmatitas e são raros os depósitos constituídos somente por caulinita, sendo comum uma mistura de caulinita 7Å / haloisita. Os caulins obtidos da pegmatita e do granito são utilizados como cobertura de papel e em cerâmica em geral. Argilas de origem vulcânica são utilizadas em cerâmicas na região. Os caulins obtidos de anortosito são semelhantes em níveis de ferro e titânia àqueles obtidos de caulins graníticos. Montagens de caulinita e pequenas quantidades de haloisita 7Å são encontrados. Essas argilas são usadas tanto em cerâmicas de mesa quanto em preenchimentos para papel.

The size of Brazil and the diversity of geology is reflected in varying environments in which kaolinite assemblages have been formed. The geological environments of the Brazilian kaolin may be divided into the following groups - sedimentary kaolin, kaolin derived from pegmatite, from granitic rocks, from volcanic rocks and kaolin derived from anorthosite. The sedimentary clays are mainly found in the Amazon basin and those adjacent to the Jari River are being exploited commercially for export as a paper coating clay. Amazon kaolin is characterised by high iron and titania (lattice-held) with low levels of alkali and exhibiting euhedral kaolinite crystals. The South-eastern pegmatite, when not iron stained, are extremely low in iron and titania and a mixture of kaolinite 7Å/10Å-halloysite occurs in all deposits. The North-eastern pegmatite produces kaolin constituted only by euhedral kaolinites with an absence of halloysite. Kaolin from granites generally has higher iron levels when compared with pegmatite and deposits constituted only by kaolinite are rare, a mixture of kaolinite/7Å-halloysite being common. Both the pegmatite and granite derived kaolin are utilised as a paper filler and in general ceramics. Volcanic rocks on alteration produce a fine siliceous clay with titania levels higher than other types and are generally mixtures of kaolinite-7Å. Volcanic derived clays are utilized locally in ceramics. Kaolins derived from anorthosite are similar in iron and titania levels to those from granitic kaolins. Assemblages of kaolinite and small quantities of 7Å-halloysite are found. These clays are used in both whiteware ceramics and paper filler.