Unravelling and characterizing halloysite deposits in pegmatite provinces from southeastern brazil

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Ernesto Adler Licursi, Luiz Carlos Bertolino1*, Victor Matheus Joaquim Salgado-Campos, Francisco José da Silva

Kaolin is a fine-grained rock formed by hydrated aluminum phyllosilicates, which may contain impurities such as quartz, mica, feldspar, iron oxides, among others. Mineralogically, it consists mainly of kaolinite and halloysite. More rarely, dickite and nacrite may also occur. Halloysite is a kaolinite polytype that has two water molecule in its structure, differing mainly by its tubular shape and reduced size. Because of these properties, there has been a growing interest in the study of halloysite due to applications in different sectors of nanotechnology with an emphasis on health and agriculture areas. The clay mineral occurs in two types: 10Å-halloysite (Al2Si2O5(OH)4.2H2O) when water molecules are present in the interlamellar region and 7Å-halloysite (Al2Si2O5(OH)4) when such molecules are missing. Halloysite deposits can be classified as primary, when formed in situ due to alteration of silicates by weathering or hydrothermalism, or as sedimentary/secondary when originating from sedimentary processes. Primary deposits are more common being halloysite generally associated with intrusive acidic rocks, granites, anorthosites and pegmatites. The objective of this study was to identify and characterize different occurrences of halloysite from four samples of weathered pegmatites from southern Brazil. Under a hand magnifying glass, these pegmatites are whitish in color, friable and the main mineralogy is composed by quartz grains, feldspar, mica and kaolin. The samples were also analyzed through X-ray diffractometry (XRD) and scanning electron microscopy (SEM). XRD indicates the presence of kaolinite/halloysite at 7Å, quartz at 3,34Å and 4,25Å, mica at 10Å, K-feldspar at 3,24Å and plagioclase in one sample at 3,18Å. Images generated by SEM showed kaolinite in booklet format and tubular halloysite. Based on these results, the presence of halloysite is marked in several pegmatites in the states of Rio de Janeiro and Minas Gerais (southern Brazil), nonetheless open regarding its true economic potential in the region.

Keywords: Halloysite; Pegmatite; Nanotubes; Clay mineral

Proscience vol. 9

Pp 31-36

DOI: 10.14644/EC2023.006