Dissolution Process of Phlogopite in Acid Solutions

-The alteration experiments of phlogopite with 0.01 N HC1 solution containing 0.1 M NaC1 at 50*, 80* and 120"C have been carried out to aid in the understanding of the dissolution process of mica and the formation of secondary phases such as vermiculite and interstratified mica/vermiculite. Twenty milligrams of phlogopite samples were suspended in 20 ml or 100 ml of leaching solution. In these experiments, the dissolution ofphlogopite occurred incongruently, where the preferential release of K occurred in almost all stages of the alteration reaction. In the 100 ml experiments, the priority in dissolution in the initial stage was in the order; K > Fe > Mg, Al > Si. This supports that phlogopite leaching is controlled by the mineral structure. At 80* and 50"C in the 20 ml experiments, the release of all elements except for K was nearly congruent. At 120"(2 in the 20 ml experiments, the dissolution was outwardly incongruent, which Fe decreased remarkably after six days and A1 was released most slowly compared with all other elements in phlogopite. This is probably due to the precipitation of secondary phases such as aluminum and iron oxides and/or hydroxides. Vermiculite and Rl-type interstratified mica/vermiculite, containing 70 50% mica, were formed in the alteration process of phlogopite. The following two processes were confirmed for the formation of interstratified structure: Interstratified structure was formed (1) directly from phlogopite or (2) from vermiculite which was produced earlier from phlogopite by regaining of K from the ambient solution. It may depend on the release rate of K from phlogopite whether mica-vermiculite layer sequences develop or vermiculite-vermiculite sequences do. Key Words--Alteration, Biotite, Dissolution, Interstratification, Leached layer, Mica/vermiculite, Phlogopite, Transmission electron microscopy, Vermiculite. I N T R O D U C T I O N Alteration o f micas has been experimentally carried out by several investigators to simulate natural weathering and the hydrothermal alteration processes. Barshad (1948) found that K in biotite was replaced with hydrated Mg when in contact with a MgC12 solution. Since then, the exchange reaction experiments of interlayer K in mica with salt solutions have been actively studied (Norrish 1973), and the diffusion control model has been proposed to explain the K exchange mechanism (Mortland 1958, Reed and Scott 1962, Leonard and Weed 1970). Schnitzer and Kodama (1976) pursued compositional changes in a fulvic acid solution on dissolution of three micas (phlogopite, biotite, and muscovite) and showed congruent dissolution. However, phlogopite dissolved incongruently when in contact with Ca-bearing hydrochloric acid solution produces vermiculite and interstratified mica/vermiculi te (Inoue et al 1981). Biotite and phlogopite can transform to vermiculite, directly (Walker 1949, Wilson 1966) or through an interstratified structure (Boettcher 1966, Wilson 1970, Brindley et al 1983) during natural weathering or hydrothermal alteration. Transmission electron microscopic (TEM) study for the different stages of mica alteration have been carried out by Banfield and Eggleton (1988) and Reichenbach et al. (1988) to demonstrate the structural modification. Rhcades and Coleman (1967) and Sawhney (1969) represented that Copyright 9 1995, The Clay Minerals Society the reverse transformation of vermiculite to mica also takes place through an interstratified structure. Tsuzuki (1985) has commented that alteration products such as vermiculite and interstratified mica/vermiculite may be a leached layer which is formed throughout a mica crystal. The investigation on the formation of the leached layer constitutes an important contribution to the understanding of alteration of micas, but more information is necessary. Phlogopite was reacted with Na-bearing hydrochloric acid solutions to better understand the dissolution process of micas and the formation of secondary phases. Both solution and solid data were monitored as a function of time at 50", 80 ~ and 120"C. E X P E R I M E N T A L