Recebido em 5/4/06; aceito em 17/11/06; publicado na web em 24/7/07 SPECIAL CLAYS: CHEMICALLY MODIFIED CLAYS -A REVIEW. 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.Keywords: special clays; chemically modified clay; surface modified clays. INTRODUÇÃOUm dos grupos das chamadas Argilas Especiais segundo a revista inglesa "Industrial Minerals" 1 compreende esmectitas quimicamente modificadas: as esmectitas (principalmente montmorilonitas) ativadas por ácidos e as "organoclays". O conceito de "ativação de montmorilonita", inicialmente restrito à ativação com o uso de áci-dos, foi ampliado por O'Driscoll 2 : tem-se a ativação ácida, a ativação alcalina, ativação "orgânica" ou "neutra" por compostos orgâni-cos e montmorilonitas naturalmente ativas (terras fuler). Barrer 3 chamou de ativação a troca do cátion sódio na montmorilonita-Na pelo cátion quaternário de amônio tetra-metil-amônio: essa conceituação foi utilizada neste trabalho, com compostos inorgânicos e orgânicos modificando as propriedades de argilas cauliníticas e esmectíticas, bem como de talcos. Há a proposta australiana 4 de, em lugar de usar o termo "ativação", ampliá-lo para a expressão mais abrangente "surface treated clays", a qual é aplicável a todas as argilas e argilominerais, abrangendo também as mudanças na superfície dos cristais causadas por transformações térmicas. ESTRUTURAS CRISTALINAS E MORFOLOGIAS DOS MICROCRISTAIS DE CAULINITA, DE MONTMORILONITA E TALCOAs estruturas cristalinas e características químicas da caulinita, da montmorilonita e do talco foram descritas por Brindley 5 e Newman 6 . A composição química da caulinita, expressa pela fórmu-la de óxidos, é Al 2 O 3 .2SiO 2 .2H 2 O. A fórmula da cela unitária da caulinita, eletricamente neutra, é Al 4 Si 4 O 10 (OH) 4 . A Figura 1 mostra esquematicamente, em vista lateral, duas camadas 1:1 da estrutura da caulinita 7 : a distância entre dois planos de oxigênios equivalentes em camadas sucessivas é de 7,16 Å ou 0,716 nm. Como a espessura da camada 1:1 é de 4,37 Å, o espaço entre as camadas tem 2,79 Å de espessura, valor que quase permite a acomodação de moléculas de água intercaladas entre as camadas, moléculas essas com cerca de 2,90 Å de diâmetro. Como todos os íons da caulinita são incolores, o cristal da caulinita é incolor; portanto, o pó constituído por cristais incolores de caulinita com dimensões da ordem de 2 m é branco. Essa cor branca do caulim é um dos principais fatores responsáveis por seu grande uso ind...

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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

1. The structure and distribution of the components of striated muscle cells vary with the species and with the specialization of muscle fiber function. 2. There appear to be two, easily distinguishable, general categories of striated muscle structure. A. High frequency muscle (represented by flight muscle of higher insects and hummingbird, and cicada tympanal muscle) is characterized by widely spaced, non-branching fibrils of large diameter and short period, little endoplasmic reticulum, and large quantities of large mitochondria (low fibril-sarcoplasm ratio). This structure is correlated with heavy tracheolization or vascularization, high oxidative activity, and dark color as compared with other muscles of the same species. B. Low frequency muscle is characterized, in general, by high fibril-sarcoplasm ratio, relatively long period, few mitochondria increasing with activity and decreasing with absolute power of the fiber. Oxidative capacity and color are proportional to the quantity of mitochondria. These fibers are further differentiated into (a) fibrillar arrangement of contractile material which permits a regular pattern of interfibrillar and segmental reticulum, and (b) afibrillar arrangement of contractile material leading to an unsystematic distribution of reticulum. 3. The endoplasmic reticulum appears as a complex coordination system in the muscle fiber. Peripherally, it links the Z and M lines of the fibrils to the sarcolemma and between the fibrils it links the cross-bands, forming the Grundmembran of earlier authors. By longitudinal linkage, it connects with the sarcolemma at the muscle extremity to form a digital arrangement into which the tendon fibrils are spliced. The extent of its development and its position have a definite relationship to the degree and site of fiber shortening. At present the reticulum is the only structure that one can consider to be an internal conducting system. It may distribute the excitation transversely from fibril to fibril, and lengthwise saltatorially to the symmetry centers of the sarcomeres. 4. The nucleus is the mediating element between the cytoplasmic phases within and without the tubular system of the endoplasmic reticulum. A possible mechanism which correlates nucleus, adenylic acid system, ion exchange, and reticulum with the initiation of contraction is postulated.

Brazil is a significant producer of kaolin with almost 2.5 Mt in 2005 representing 10% of the world's total of 25.0 Mt. Brazil is now the second largest producer in the world, after the USA, having overtaken the United Kingdom in 2005. The kaolin resources are widespread throughout the country and are varied in their origin, physical and chemical properties and morphology. The kaolin industry in Brazil has shown a dramatic rise over the last 15 years with production of beneficiated kaolin increasing from 0.66 Mt in 1990 to 2.5 Mt in 2005. The reason for the growth is solely the development of large kaolin deposits in the Amazon Basin that account for 90% of Brazil's production. In 2005 there are just two companies involved in the production and sales of kaolin from the Amazon Basin, namely Imerys (RCC — Rio Capim Caulim) and Caemi (a subsidiary of CVRD — Companhia Vale do Rio Doce) with kaolin operations of CADAM (Jari River) and PPSA (Capim River operations).

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 °C) for preparing plate-like crystals of wellcrystallised 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.

Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (AL-COA 123) and aqueous solution of acetic acid at 96 • C±1 • C. The white powder of Al(OH)(CH 3 COO) 2 is constituted by agglomerates of crystalline plates, having size about 10µm. The crystals were fired from 200 • C to 1550 • C, 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 (300 • C); delta (750 • C); alpha (1050 • C). The aluminas maintain the original morphology of the Al(OH)Ac 2 crystal agglomerates, up to 1050 • C, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550 • C. High surface area aluminas are formed in the temperature range of 700 • C to 1100 • C; the maximum value of 198m 2 /g is obtained at 900 • C, 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) Ac 2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al 2 O 3 which acts as precursor for the sequence gamma-to alpha-Al 2 O 3 .

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