MINERALOGICAL MAGAZINE JOURNAL OF THE MINERALOGICAL SOCIETY Vol. 35 September 1965 No. 271 The thermal decomposition of amosite By A. A. HODGSON Cape Asbestos Fibres Limited, Barking, Essex, A. G. FREEMAN, 1 and H. F. W. TAYLOR Department of Chemistry, University of Aberdeen [Read 17 September 1964] Summary. When amosite (fibrous grunerite, F%.sMgl.sSisO~(OH)~), is heated in argon or nitrogen, physically combined water is lost up to 500-700 ~ C. Above 500~ (static) or 700~ (dynamic), dehydroxylation occurs endothermieally, giving a pyroxene as the main product. Under dynamic heating conditions, part of the hydroxyl water is lost as hydrogen, with concurrent oxidation of the iron. At about 1000 ~ C the pyroxene is decomposed to olivine and cristobalite ; at about 1100 ~ C melting begins. In oxygen or air, physically combined water is again lost below 500-700 ~ C. At 350-1200 ~ C a sequence of overlapping dehydrogenation, oxygen absorption, and dehydroxylation reactions occurs, which gives rise to a broad exotherm on the d.t.a, curve. The main products (for static heating condi- tions) are an oxyamphibole at 350-800 ~ C, and a spinel, hematite, a pyroxene, and X-ray amorphous material at 800-1100 ~ C. Silica crystallizes as cristobalite at 1100-1350 ~ C, and as tridymite at 1450 ~ C. Most of the products in either neutral or oxidizing atmospheres are formed topotactically. The mechanisms and rates of the reactions are discussed, and the problem of determining the chemically com- bined water in amosite and other minerals of similar composition is considered. MOSITE is the fibrous variety of the monoclinic amphibole, grunerite. It is known to occur only in the eastern Transvaal, South Africa, in metamorphosed banded ironstones of the Transvaal System. It is mined on a large scale, current annual production being around 70 000 tons. Its composition approximates to Fe~.+Mgl.sSisO2~(OH)2. Its thermal decomposition in air was first studied by Vermaas (1952), who used d.t.a, and X-ray powder patterns. He reported a broad exothermic effect at 58~830 ~ C, which he attributed to oxidation and decomposition, and showed the products of decomposition at 1200 ~ C to be magnetite and cristobalite. Heystek and Sehmidt (1953) also reported d.t.a, data, which agree broadly with those of Vermaas. 1 Present address: Victoria University, Wellington, New Zealand. Ff 446 A.A. HODGSON, A. G. FREEMAN~ AND H. F. W. TAYLOR ON No detailed X-ray study of amosite has been reported, but Ghose and Hellner (1959) determined the crystal structure of a non-fibrous grunerite similar in composition to amosite. They found that iron was almost exclusively preferred in the M 4 sites, that is, at the edges of the octa- hedral bands, and that iron and magnesium were distributed at random in the other cation sites. Vermaas showed that non-fibrous grunerite, when heated in air, behaved like amosite but that decomposition and oxidation occurred more slowly and at higher temperatures. In contrast, a finely fibrous variety of amosite, called montasite, reacted more rapidly than typical amosites and at lower temperatures. The aim of the present work was to study the thermal decomposition of amosite in greater detail, in both oxidizing and neutral atmospheres. Material and experimental methods Several specimens from the asbestos mines at or near Penge, in the Steelpoort-Lydenburg area of eastern Transvaal, were used (specimens PRS 3, 4, 5, and 6, H 7, 9, 10, 12, 16, and 18, WRS 2 and 3, and KRS 1 from the collection at the Cape Asbestos Fibre Laboratory). Despite the fact that extensive mining operations have been carried out for many years, the geology of the area is not known in detail; studies are now in progress and will be reported elsewhere. The specimens consisted of cross-fibre seams, often about 10 cm thick, in a coarse-grained grey rock. The fibres, which were white, were often contaminated with magnetite grains, and sometimes also with other minerals. These included a carbonate, which appeared to be a magnesian siderite of approximate composition Feo.sMg0.~COa. X-ray photographs showed that the fibres were composed of very thin crystallites rotated at random around the fibre axis, so that only powder and fibre rotation photo- graphs could usefully be obtained. However, occasional amphibole crystals were found among the fibres that were large enough to give true single-crystal patterns. All the fibre specimens had closely similar optical properties: parallel extinction, positive elongation, refractive indices 1.689 and 1.668 for light vibrating respectively parallel and perpendicular to the fibre direction. They also had virtually identical cell parameters, which agreed substantially with those reported by Ghose and Hellner (1959) for a grunerite of roughly similar composition, viz., a 9.56~, b 18"302, c 5.34 s A, fl 101 ~ 50' (a sin fl 9-36 ~), space group C2/m. X-ray powder patterns agreed closely with that reported by AMOSITE 447 Flood (1957) for a grunerite from Collobri6res, and less closely with that reported by Vermaas (1952) for amosite from Penge. Chemical analyses showed some variation between specimens, but all corresponded approximately to the formula Fe~+Mgl.sSisO2z(OH)2 (table I). The thermal behaviour was studied in argon and in air or oxygen mainly by differential thermal analysis (d.t.a.), thermogravimetric TABLE I. Chemical analysis and atomic ratios for amosite from the Penge area. 1 2 3 Si02 49-76 Si 7-92 7.92 AlcOa 0.24 A1 0.05 0.05 1 8-00 Z Fe20 a 0.49 FeIII 0.06 ~ 0"03 FeO 40.09 ~0.03 MgO 6.15 Mg 1.45 1.45 }5.00 (il§247 MnO 0.53 Feii 5.30 t 3-52 ) CaO 0.91 ( 1.78 Na~O 0.10 Mn 0.07 0.07 ~ 2.00 M4 K20 0.13 Ca 0.15 0.15 ) CO2 0-18 Na 0.03 0.03 / 0"06 A H20 +6~176 1-87 K 0.03 0"03 J H20 -8~176 0'27 H 1'99 1"99 1"99 H 100.72 1. Chemical analysis ; mean for 10 specimens (see text). H20 +6~176and H20 -~0~from dynamic dehydration curves. Analyst, W. Benns. 2. Atomic ratios calculated from col. I, referred to (O§ = 24. CO2 in col. 1 was assumed to occur as a magnesian siderite of composition Fe0.sMgo.2COa. 3. Suggested allocation of atoms to sites. analysis (t.g.a.), dynamic dehydration, static weight change curves, and X-ray fibre rotation photographs and Fe e+ analyses of heated specimens. The techniques were the same as those used with crocidolite (Hodgson, Freeman, and Taylor, 1965). The heating rates were 10 ~ C/rain for d.t.a. and 2-5 ~ C/min for t.g.a, and dynamic dehydration. With each of these three methods, 5 to 13 specimens were studied and gave results in broad agreement with each other. Samples were prepared for examination by breaking up selected portions of the fibre seams with a hammer followed by cutting into short lengths with scissors and sieving or shaking to remove grains of mag- netite and other impurities. Samples used for d.t.a., dynamic dehydra- tion, and t.g.a, determinations were dried at 100 ~ C in air before use; those used for static weight-change curves, Fe ~+ determinations, and X-ray studies were not thus dried. 448 A.A. HODGSON, A. G. FREEMAN, AND H. F. W. TAYLOR ON Thermal decomposition in inert atmospheres Visual examination. When amosite is heated in argon or nitrogen, the tensile strength falls markedly at 200-400 ~ C. The colour darkens progressively above about 700 ~ C, especially when dynamic heating conditions are used. Samples heated dynamically to 1000 ~ C contain nearly black fibres that have largely retained their original form, together with fluffy, white material. Melting begins at about 1100 ~ C. 0 ~ 200 ~ 400 ~ 600 u 800 ~ I000 ~ 1200 ~ J I I I I I i I ' I ! 0 m A 0 q/I OLI. --0--- CRI. --@--_ 0j- PYR. B I~. AMO. L I , I i l , I , I , 0 o 200 ~ 400 ~ 600 ~ SO0 ~ I0000 1200~ Temperature ~ FIG. l. Static heating in nitrogen (specimen PRS 5). (a) Weight-loss curve. (b) Phases detected by X-rays (amo. ~ amosite, pyr. ~ pyroxene, cri. = cristo- bMite, oli. olivine; full and open circles denote major and minor constituents respectively). Static heating. Fig. la shows the static weight-change curve for a typical specimen, and fig. lb shows the phases detected from X-ray fibre rotation photographs on heated samples. There is a gradual loss in weight up to about 550 ~ C. No change is detectable with X-rays, and it was concluded that, as with crocidolite (Hodgson, Freeman, and Taylor, 1965), the loss was of physically combined water. At 550-950 ~ C there is a sharp loss of 1.85%, and the X-ray results show that decomposition occurs concurrently. The only product detectable with X-rays up to 950~ is a pyroxene. It showed strong preferred orientation; the orientation relationship was determined fully by employing a single crystal of grunerite picked out from, one of the fibre speeimens and was AMOSITE 449 found to be the same as with the pyroxenes formed from tremolite (Freeman and Taylor, 1960) or riebeekite (Hodgson, Freeman, and Taylor, 1965). The pyroxene formed from amosite had parameters near to those given by Brown (1960) for hypothetical FeSiO a (asinfi 9-3, b 9.1 _~). The decomposition can be represented, at least approximately, by the equation: Fe gi. SisO H = Fe Mgl. SiTO I+SiO2+H O amosite pyroxene though, as Taylor (1962) has pointed out for the analogous case of tremolite, the amorphous product is not necessarily pure silica.