Characterization of halloysite (north east Rif, Morocco); evaluation of its suitability for the ceramics industry

A 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-1 and 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.