Sudoite from diagenetic to very low-grade metaclastites of the Betic Cordillera was studied by X-ray diffraction and transmission/analytical electron microscopy. Sudoite formed directly from dickite, the assemblage dickite + sudoite + illite being replaced at increasing metamorphic grade by the assemblage pyrophyllite + sudoite + illite. Sudoite ranges in composition from Mg-rich to Fe-rich chemistries. In addition, a wide variety of mixed-layered structures (illite-sudoite, pyrophyllite-sudoite, and dickite-sudoite) was also identified. Mg-rich sudoite shows a mean chemical composition of (Al2.91Fe0.252+Mg1.80)(Si3.10Al0.90)O10(OH)8, and a IIb ordered structure with b = 9.055 Å. Intermediate Fe-Mg sudoite exhibits a very variable composition, the Fe-rich phases having a mean composition of (Al2.09Fe0.613+Fe0.872+Mg1.44)(Si3.31Al0.69O10(OH)8. These are disordered polytypes with b values ranging from 9.070 to 9.101 Å. Fe occurs in both octahedral sheets, according to two types of substitutions: Fe3+ for Al in the dioctahedral sheet and Fe2+ for Mg in the trioctahedral sheet. Sudoite with such a composition has not been described previously.

Allophanes are poorly crystalline and quasi-stable aluminosilicate minerals, the structures of which are sensitive to chemical treatment. In the present study, solid-state 27Al and 29Si nuclear magnetic resonance (NMR) spectra of allophane samples were monitored as they went through several purification procedures. It was confirmed that no significant structural changes were caused by boiling with 6% H2O2 to remove organic matter, by size fractionation (sonification), by sedimentation, by precipitation at pH 4.0, or by dithionite-citrate-bicarbonate treatment for the removal of Fe (hydr)oxides. Hot 5% Na2CO3 treatment for the removal of reactive silica-alumina gels and adsorbed citrate from allophane samples, however, decreased signal intensity corresponding to imogolite-like Si (Q33VIAl, −78 ppm in 29Si NMR) and increased signal intensities corresponding to IVAl (55 ppm in 27Al NMR) and possibly X-ray amorphous aluminosilicates (centered at −85 ppm in 29Si NMR). Cold (room temperature) 5% Na2CO3 treatment for 16 h proved to be effective in avoiding these structural changes.

The Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

The Mevaiela kaolin deposits are located in the northern part of the anorthositic-gabbro massif within the Cunene complex (southern Angola) and were formed by the alteration of basic anorthosites and gabbros. The Mevaiela area is part of an elevated region which is located between two extensive NNW-SSE fracture systems. Several kaolinite samples were collected from a quarry (main excavation) and from drill-holes as well as from surficial occurrences in the Cunene complex. Chemical analyses, X-ray diffraction, differential thermal analysis, scanning electron microscopy and isotope analyses were performed in order to model the kaolinite occurrences. The alteration of the anorthosite to kaolin approaching the main excavation is characterized by significant decrease in alkaline-earth and transition metals (Ca, Mg, Fe, Co, Ni and Mn) between the average anorthosite and the kaolin. The crystallinity indices suggest that the kaolin contains kaolinite with a reasonably well ordered structure and near the transition between T (triclinic) and pM (pseudo monoclinic).

Mineral exploration tools have been evaluated during this study to assist in future kaolin exploration in the Cunene anorthosite complex.

Isotopic analysis of O and D indicates that Ca-feldspar alteration is essentially due to meteoric fluids, over a different range of temperatures. Furthermore, the presence of quartz-feldspar veinlets in the kaolinite bodies could be the result of hydrothermal activity linked to post-anorthosite granite intrusions of the so-called ‘red granite’. Kaolinite from Cunene plots on or close to the kaolinite line into the ‘warm temperature in tropical region’ area (surficial samples). Samples from drill-holes plot on the left and show the largest displacement from the KS line; these samples also have a relatively reduced δD range of values (−65 to −98%). However, if supergene processes take place in the presence of waters of meteoric origin at temperatures similar to typical surface temperatures, the clays thus formed should plot either in the vicinity of the KS line or be displaced towards lower δO18 and higher δD, depending on both the temperature and relative proportion of clay to water.