Progressive Formation of Halloysite From the Hydrothermal Alteration of Biotite and the Formation Mechanisms of Anatase in Altered Volcanic Rocks From Limnos Island, Northeast Aegean Sea, Greece

Abstract

Occurrences of halloysite-rich material in altered volcanic rocks, principally trachyandesites, dacites, and tuffs, extend over an area of~1 km 2 in the southwestern part of Limnos, Island, northeast Aegean Sea, Greece. The present study was designed to investigate the alteration processes which acted on the biotite in these volcanic rocks, to describe in detail the mechanism of formation of the halloysite, and to specify the mechanisms of formation of anatase during the alteration processes. Samples were examined using polarized-light microscopy, X-ray powder diffraction, scanning electron microscopy, scanning electron microscopy-energy dispersive spectroscopy, and Fourier-transform-Raman techniques. The extensive alteration of the parent rocks, triggered by the circulation of hydrothermal fluids through faults and fractures, resulted in the alteration of biotite to halloysite. Six stages of alteration were recognized. Nanoparticles of halloysite were initially formed on the mica layers, which progressively grew through short-tubular to well formed tubular halloysite, with increasing alteration. In the most altered samples, laths and interconnected laths with the composition (Al 3.96 Fe 0.04 )Si 4 O 10 (OH) 8 , were the dominant halloysite morphologies. Anatase was encountered as an alteration product of both ilmenite and biotite. Ilmenite was altered to anatase and Fe oxides. The altered ilmenite crystals constrained most of the newly formed anatase within the space occupied previously by ilmenite, leading to the formation of skeletal anatase. The layered structure of the micas was the main factor governing the morphology of newly formed anatase developed outside ilmenite margins in the form of layers parallel to those of mica. An unusual ringlike structure of anatase was thought to be the result of the uncommon alteration of inner parts of mica folia to tubular halloysite oriented perpendicular to the mica layers. The detachment of the halloysite tubes by circulating hydrothermal fluids was considered to be the reason for the creation of holes which were subsequently surrounded by the anatase ring forms.

Study of a terrestrial Martian analogue: Geochemical characterization of the Meñakoz outcrops (Biscay, Spain)

“…As rutile, it may be formed by hydrothermal conditions but also during the cooling of the lava . Anatase could arise as an alteration product of titanium‐rich minerals, and its formation suggests, again, a hydrothermal origin . On Mars, they were identified in Gale Crater as trace minerals …”

Properties and applications of halloysite nanotubes: recent research advances and future prospects

“…In addition, the crystallization conditions and geological occurrence of Hal are closely related to its morphology. For example, the Hal exhibits spheroidal morphology when recrystallized from supersaturated solutions of volcanic glass and pumice (Joussein et al, 2005); however, the Hal exhibits a tubular morphology when formed by the hydrothermal alteration of biotite (Papoulis et al, 2009), by the weathering of feldspar in granitic rocks (Adamo et al, 2001), and by the topological alteration of platy Kaol (Singh and Gilkes, 1992). More details concerning the geological genesis of Hal can be found in previous reports (Joussein et al, 2005;Pasbakhsh et al, 2013).…”

Hydrothermal Alteration Zones Detection in Limnos Island, Through the Application of Remote Sensing

“…Remote sensing can assist and provide valuable information in bedrock mapping, detection, identification and estimating affluence of specific minerals at a specific scale (Ahmed and Beiranvand Pour, 2014;Sabins, 1999;Hunt, 1977). In Limnos Island, field geological studies have referred the detection of hydrothermal alteration zones in several sites like Sardes, Roussopouli and Fakos peninsula (Papoulis et al, 2014;Fornadel, 2010;Papoulis et al, 2009;Skarpelis and Voudouris, 1998). Positive results can be obtained using band rationing and false colour using these ratios.…”