Characterization of halloysite (North East Rif, Morocco): evaluation of its suitability for the ceramics industry

ABSTRACTA halloysite clay from Nador (NE Morocco) was studied to evaluate its suitability in the ceramics industry. A cross-section involving all the Messinian facies was performed in the Melilla Neogene basin, at the foot of the Gourougou volcano, to establish the origin of the halloysite and estimate its reserves. White layers of halloysite and red clays rich in smectite occurring in contact with basal-reef limestone were characterized by mineralogical (XRD, IR), textural (SEM) and physico-chemical analyses (grain-size, Atterberg limits, DTA/TG, XRF and specific surface area). Ceramic properties were evaluated for halloysite fired from 500 to 1100°C to evaluate technical processing for ceramic production.The halloysite clay consists of fine particles with a high plasticity and a large specific surface area. The XRD investigation revealed the presence of 7 Å non-hydrated halloysite along with gibbsite, alunite, K-feldspar and traces of smectite and illite.The presence of halloysite was confirmed from the characteristic IR bands at 3695 and 3618 cm−1and the predominance of tubular crystals observed in the SEM. The chemical analysis revealed high contents linked to the presence of Al-rich phases (gibbsite and alunite). DTA/TG and XRD results of fired clay samples proved the dehydroxylation of halloysite and a rearrangement of metakaolinite to form mullite and spinel at 975°C.The Moroccan halloysite might be suitable for refractory ceramic applications. However, addition of quartz sand might be necessary to avoid crack development during firing and to reduce the plasticity of raw halloysite and minimize shrinkage during sintering.

Preparation of single-layered enzyme 2D crystals for electron crystallography

Electron crystallography allows for a wide range of membrane proteins to be studied once conditions for two-dimensional (2D) crystallization have been identified. Two-dimensional crystallization is most frequently achieved via the dialysis approach, where the detergent-solubilized membrane protein is reconstituted into a lipid bilayer [1]. Vesicles, planar-tubular crystals, and sheets are the three most common 2D crystal morphologies. Vesicle and planar-tubular morphologies are observed for the largest percentage of 2D crystals of membrane proteins. Upon negative stain as well as electron cryo-microscopy (cryo-EM) grid preparation, each planar-tubular and vesicle 2D crystal will result in two ordered bilayers that can be analyzed separately by image processing. If any of these morphologies, however, contains a larger number of stacked crystals, data of tilted crystal stacks in particular can currently not be analyzed. Sheets constitute the most desirable morphology, allowing for the preparation of very flat samples for cryo-EM [2]. This is at present the only type of morphology that may be amenable to collection and analysis of electron diffraction data of highly ordered samples [3]. We could reproducibly induce single-layered sheet formation in the large majority of 2D crystals of two different enzyme samples and are working towards a general protocol applicable to other membrane protein 2D crystals.