Dehydration-hydration reactivity of laumontite: analyses and tests for easy detection ARTURO BRAVO 1,2 , OSCAR JEREZ 1 , URSULA KELM 1,* AND MAURO POBLETE 3 1 Instituto de geología Económica Aplicada (GEA), Universidad de Concepción, Concepción, Chile 2 Current address: Institut für Mineralogie, Brennhausgasse, TU Bergakademie Freiberg, Freiberg, Germany 3 Facultad de Ingeniería, Universidad Católica de la Santísima Concepción, Concepción, Chile (Received 20 October 2016; revised 14 September 2017; Associate Editor: David L. Bish) ABSTRACT: Hydration reactions are known to affect rock or aggregate stability in construction; laumontite is not usually considered to be a ‘problem-mineral’ though drill cores from the very low-grade metamorphic altered andesites and volcanoclastic rocks from Central Chile showed detachments of shotcrete in a tunnel exposed to periodic water flow, with expandable clay phases presumed to be responsible for the observed failure. Abundant laumontite detected in the cores may also be responsible for the detachment, however, resulting from the structural expansion and contraction in response to hydration and drying. Clay reactivity in construction projects is often tested on site by 30 days of ethylene glycol exposure, but adequate monitoring options for laumontite are not deployed. Options for laumontite characterization involving a combination of water immersion and slaking and modified oedometer-based expansibility tests were used here to observe the response to laumontite expansion pressure. All tests were formulated considering minimal implementation efforts for building sites or the easy availability of analytical and testing facilities. Laumontite was identified by optical microscopy, semiquantitative X-ray diffraction, and automated mineralogical analysis. A combination of the latter two methods provided reliable information about the presence of sub-microscopic laumontite and a visual impression of the textural arrangement of the zeolite in the rock. A slaking test based on four cycles of immersion followed by drying and final application of weight (simulated overburden) is best suited to indirect detection and for demonstrating rock reactivity due to the presence of laumontite. Rocks with laumontite show expansion when crushed, recompacted and fitted into an oedometer, but mineralogical information is required for adequate interpretation of the results. KEYWORDS: laumontite, hydration, expansion pressure, andesite, Chile. The hydration/dehydration of laumontite at ambient or near-ambient temperatures and the associated changes in crystal structure have been studied thoroughly since its first description by Coombs (1952). Later studies provided further crystal-structure refinements, with emphasis on water-site occupancies as a function of humidity and temperature, and also focused on low- temperature petrological modelling of zeolite-contain- ing systems (e.g. Armbruster & Köhler, 1992; Artioli & Stahl, 1993; Gabuda & Kzlova, 1995; Kiseleva et al., 1996; Stahl et al., 1996; Fridriksson et al., 2003; Kol’tsova, 2009). Neuhoff & Bird (2001) reported laumontite in equilibrium with leonhardite at 70–80% * E-mail: ukelm@udec.cl https://doi.org/10.1180/claymin.2017.052.3.03 © 2017 The Mineralogical Society Clay Minerals, (2017) 52, 315–327 Brought to you by | Imperial College London Authenticated Download Date | 4/9/18 7:38 PM