Brucite—Industrial Mineral with a future

Geoscience Canada, June, 2007 by George J. Simandl, Suzanne Paradis, Melanie Irvine

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SUMMARY

Brucite, Mg[(OH).sub.2], is an uncommon mineral primarily known to mineral collectors, and to specialists studying contact metamorphic and ultramafic rocks. It is an environmentally friendly flame-retardant and is in commercial demand; it also represents a potential ore source for the metal, magnesium, which is itself in great demand. The present brucite market for flame-retardants is less than 50 000 tonnes annually, but it is increasing exponentially. Brucite has the advantage of not containing C[O.sub.2]; hence none is released during calcination, a positive feature in today's society concerned with climate change. This review paper summarises the topic for scientists studying the thermodynamic properties of brucite, geologists studying its contact metamorphic characteristics, exploration geologists and potential end-users. Given the demand for the mineral and metal, high-grade brucite deposits may become hot exploration targets within the next few years.

SOMMAIRE

La brucite, Mg[(OH).sub.2], est un mineral plutot rare connu surtout des collectionneurs de mineraux et des specialistes du metamorphisme de contact et des roches ultramafiques. La brucite est un materiau ignifuge ecologique qui est en demande commercialement; il represente aussi une source potentielle de magnesium metallique, pour lequel existe une forte demande. La demande actuelle de brucite comme materiau ignifuge est de moins de 50 000 tonnes annuellement, mais elle croit exponentiellement. La brucite a l'avantage de ne pas contenir de C[O.sub.2]; et donc, aucun C[O.sub.2] n'est produit lors de sa calcination, caracteristique tres appreciee en ces temps d'inquietudes en regard des changements climatiques. Notre article de synthese presente un resume de la question a l'intention des scientifiques interesses par les proprietes thermodynamique de la brucite, aux geologues interesses par ses caracteristiques de mineral de metamorphisme de contact, ainsi qu'aux geologues en general et aux utilisateurs. Etant donne la demande de brucite comme mineral et comme source de metal, les bons gisements de brucite pourraient constituer des cibles d'exploration dans un avenir rapproche.

INTRODUCTION

Brucite is a magnesium hydroxide Mg[(OH).sub.2]. It has a higher magnesium content than any other raw material, commonly used or considered as ore (Table 1). Brucite forms soft, waxy to glassy, white, pale-green, grey or blue crystals, plate aggregates, rosettes, fibrous masses and fracture fillings. It is relatively soft (2.5 on the Mohs scale) and has a low density (2.38-2.40 g/[cm.sup.3]). It is soluble in hydrochloric acid but has no effervescence. Weathering transforms waxy, fresh brucite into a chalk-like material.

Brucite is widely distributed in ultramafic rocks (Khan et al. 1971; Hora 1998). It is also found in a variety of exotic settings such as kimberlites (Malkov 1974) and carbonatites (Lee et al. 2000). Most of the economic brucite deposits appear to be hosted by marbles affected by high-temperature, low-pressure metamorphism (mainly in pluton-associated, contact metamorphic aureoles). The fibrous variety of brucite, nemalite, is common in ultramafic rocks, where it coexists with chrysotile (Ross and Nolan 2003; Khan et al. 1971). Ultramafic-hosted deposits were considered as potential sources of brucite in the past (Khan et al. 1971) and are being considered again because brucite fibres from the Shaan Nan Asbestos Mine in Shaan Xi Province, China, were tested as a reinforcing material for concrete (Liu et al. 2004). The unfortunate association of brucite with asbestos in ultramafic settings, is the main reason why carbonate-hosted brucite deposits are the recommended and preferred exploration targets. The early negative studies on the inhalation effects of brucite dust are invalid because the supposedly pure brucite samples used in the experiments, contained up to 10% chrysotile (Davies et al. 1985). Liu et al. (2004) list a number of recent studies indicating that pure brucite is virtually harmless and this should allay some of the health concerns raised in earlier studies.

Examples of carbonate-hosted brucite deposits of economic significance are Cross Quarry near Wakefield, Quebec, Canada (Jacob et al. 1991; Hebert and Pare 1990; Perreault 2003); Kuldur, eastern Russia (Anonymous 2005); Granasen, Norway (Overeng 2000); Gabbs magnesite--brucite deposit, Nye County, Nevada, USA (Schilling 1968) and Marble Canyon, Culberson County, Texas, USA (Newman and Hoffman 1996).

Other undeveloped or exhausted brucite deposits occur in China, Arizona, United Kingdom, Ireland, North Korea and Canada; however, fundamental, publicly available technical data are missing or not available.

World brucite reserves and resources are impossible to estimate with any reasonable accuracy because there are many inconsistencies in the reported numbers. Kramer (2001) estimates brucite reserves for Nevada, USA, at 3 million tonnes and for North Korea at 2 million tonnes; however, there are no indications of their grades. In Canada, under the 43-101CP Standards of Disclosure for mineral projects, such tonnage estimates cannot be referred to as reserves. Because the potential economic importance of brucite escaped the attention of most economic geologists, many carbonate-hosted occurrences worthy of geologic follow-up, such as those in British Columbia (Simandl et al. 2006), were never investigated in detail.