The aim of this paper is to present the results of characterization of the particle shapes of six standard transition aluminas samples using transmission and scanning electron microscopies; selected area electron diffraction, in parallel with X-ray powder diffraction were used for confirmation of the different transition aluminas types. The transition aluminas -chi; kappa; gamma; theta; delta; and eta were supplied by ALCOA Central Laboratory. The chi-; kappa-;gamma-and delta-Al203 microcrystals are pseudomorphs from their respective precursors gibbsite and boehmite. However, theta-Al203 microcrystals are not pseudomorphs after the standard delta-Al203 sample. Also, eta-Al203 are not pseudomorphs after bayerite somatoids.

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 .

Duas argilas altamente aluminosas gibbsíticas, utilizadas em produtos refratários, foram estudadas por métodos óptico-eletrônicos, com a finalidade de caracterizar a seqüência de fases formadas durante transformações térmica. As argilas na forma de pós foram queimadas sobre placas de platina entre 200 ºC e 1500 ºC e com resfriamento programado. Após o aquecimento a 300 ºC, foi possível distinguir no microscópio eletrônico de transmissão entre cristais hexagonais de gibbsita e de caulinita, tendo o mesmo tamanho e forma. As transformações de fase da gibbsita e os cristais de caulinita pouco defeituosos seguem series independentes até 1100 ºC / 1200 ºC. Foi fácil distinguir morfologicamente os pseudomorfos das aluminas-chi e -kapa dos pseudomorfos dos cristais da caulinita e da metacaulinita no intervalo 400 ºC / 800 ºC. Cristais de espinélio alongados podem ser caracterizados dentro dos pseudomorfos da metacaulinita a 900 ºC. A mulita, em ambas as argilas, aumenta em teor e nas dimensões dos cristais de 900 ºC / 1550 ºC, enquanto o teor de alumina-alfa cresce até 1300 ºC e decresce em seguida até 1550 ºC, indicando haver interações entre as diversas fases, especialmente entre sílica e alumina-alfa.

Some species of anuran amphibians possess a calcified dermal layer (the Eberth-Kastschenko layer) located between the "stratum spongiosum" and the "stratum compactum." This layer consists of calcium phosphate deposits, proteoglycans, and glycosaminoglycans. Although regarded as a protective layer against desiccation, a calcium reservoir, or possibly a remnant of a dermal skeleton present in anuran ancestors, very little is known about its origin, structure, and function. Thus, we studied the structure and composition of the mineralized dermal layer of Corythomantis greeningi, a peculiar hylid from the Brazilian semiarid region (caatinga), using conventional and cryosubstitution methods combined with transmission, scanning, and analytical electron microscopy. Results show that the dermal layer consists of dense, closely juxtaposed, globular structures. Although the electron opacity of the globules was variable, depending on the type of preparation, crystal-like inclusions were present in all of them, as confirmed by dark field microscopy. Electron probe X-ray microanalysis showed calcium, phosphorus, and oxygen, and electron diffraction revealed a crystalline structure comparable to that of a hydroxyapatite.

By heating powders of the aluminum monohydroxide fibrillar pseudoboehmite from 200 o C to 1400 o C several high surface area aluminas are prepared and characterized by X-ray diffraction and electron optical methods. Aqueous sols with pseudoboehmite fibrils of different lengths were dried by two methods: at room temperature and spray-dried. The following aluminas were obtained after treatment of the powders at increasing temperatures and having a range of specific surface areas: ). Spray-dried powders, fired at the same temperature than the ground powders, showed higher specific surface areas. The higher surface area alumina have values of the same order of magnitude of the commercial "ad-cat" aluminas.