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The Intergrowth of Fibrous Brucite and Fibrous

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The Intergrowth of Fibrous Brucite and Fibrous Canadian Mineralogist Vol. 17, pp.699-7V2 (1979) THE INTERGROWTHOF FIBROUSBRUCITE AND FIBROUSMAGNESITE WITH CHRVSOTILE E. J. W. WHITTAKER AND A. P. MIDDLETON Depanment of Geology and Mineralogy, University of Oxford, Parks Road, Oxford lXI 3PR, England ABsrRAcr de Jeffrey rappelle Ia soi-disantn6malite de Cassiar, C.B., qui consisteen mas6site fibreuse avec chry' Nemalite (fibrous brucite) from Jeffrey mine, sotile accessoire,et dans laquelle l'orientation va- Qu6bec, has been investigated by electron micro- riable des fibrilles de magn6site s'expliquerait par ssopy and diffraction. The material consists of an des consid6rationsst6riques plut6t que structurales. association of fibrous brucite with chrysotile-2itf.r and paraohrysotile. The brucite is present in laths (Traduit par la Rddaction) elongated along the .r axis and not invariably lying on (0001). The laths are associated with fibrils of chrysotile had been a common initial constituent of hqtnopucttoN orientation of tle nemalite would correspond to the orientation of the outer brucite-like layer of the fibrils, and therefore be dependent on whether Nemalite, a fibrous form of brucite, was those fibrils are normal or parachrysotile. Elonga- first described by Nuttall (1821). It commonly tion of the laths is frequently opposite to that occurs in slip-fibre veins in serpentine; the predicted. This may be due to the fact that para- fibres can be very longo some specimens ap- chrysotile had been a corrmon initial constituent of proaching a length of 3 m, although the distance the material during the formation of the brucite, but that it subsequently recrystallued to more over which fibres are continuous is difficult to stable normal chrysotile. The genetic relationship establish. between the brusite orietrtation in the Jeffrey ne- Textures of different specimens have been malite and its chrysotile component is paralleled investigated a number of times by X-ray dif- by supposed nemalite from Cassiar, B.C,. which fraction (Berman 1932, Garrido 1936' de Jong is composed of fibrous masesite with minor chry- has been shown sotile; the varied orientation of the magnesite fibrils 1938, Eckhardt 1956), and it is explained by steric rather than structural con- by these authors that the ultimate fibrils are siderations. sufficiently fine that a handleable specimen gives a rotation-type X-ray diffraction pattern. These authors found that the z axes of the component Sorvrrvrernp brucite crystallites are perpendicular to the lengfh of the fibres. It follows that they have Nous avons 6tudi6 la n6malite (brucite fibreuse) grown to disproportionate lengths in different (Qu6bec) par de la mine de Jeffrey microscopie et symmetry-related directions in the basal plane, 6lectroniques. ks dchantillons montrent diffraction with the ratio of length to width being in the brucite fibreuse, chrysotile-2M"1 et lassociation than 1 pm' parachrysodle. Ia brucite se pr6sente en languettes order of 1tr if the fibril width is less (1936) by qui sont allong6es suivant tZ110l, ne reposant pas as has been shown by Garrido and touiours sur (0@1). Ces languettes sont associ6es Donnay (l9M\. The morphology of the brucite i des fibrilles de chrysotile; on s'attendrait i une fibres was investigated by electron microscopy orientation de la n6malite correspondant i celle de by Eckhardt (1956), who found the fibres to la couche ext6rieure, de type brucite, des fibrilles' consist of laths lying on their (0001) faces. qui d6pendrait donc de 16 16fir1g min6- orientation This work was extended further by Liebling & ralogique - chrysotile normal ou parachrysotile - (1972). de ces dernidres. Ia pr6diction de fallongement Langer est souvent contraire aux faits. On peut supposer This remarkable morphology seems to be que, pendant la formation de la brucite. le para- connected with intergrown chrysotile fibres, chrysotile ait 6t6 un constituatrt initial recristallis6 which are usually, if not invariably, present' par la forme plus stable du chrysodle la suite sous intergrowths ale mentioned by most of La relation entre I'orientation Such normal. e€n6tique quoted above and are present in de la brucite et celle du chrvsotile dans la n6malite the authors 699 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/17/4/699/4004849/699_17_4_cm.pdf by guest on 02 October 2021 700 THE cANADTANMINERALOGIST several specimens we have examined. Berman PnrsENr OnsrnvatroNs (1932) suggestedthat a structural relationship between the brucite and chrysotile had led to A specimen of nemalite from a slip-fibre an oriented replacement of chrysotile by brucite, vein at Jeffrey mine, Asbestos, Qu6bec, was but this was suggestedbefore the structure of examined by electron microscopy and diffraction chrysotile was known. Now that the external on an A.E.I. EM6G electron microscope oper- surface of chrysotile is known to be virtually ating at 100 kV. The specimenswere dispened identical to that of brucite. it is clear that such in water and supported on carbon films or a relationship would require a correspondence sometimesdirectly on copper grids. The brucite between the fibre repeat of the two minerals. component consisted of laths lz Io | ;am wide, The fibre repeat of chrysotile (a = 5,34 ]i elongate along the r axis [2110] with a fibre-axis corresponds structurally to the orthohexagonal repeat of 3.t A. As the laths generally did not D dimensiql along the six related zone-axes of give a diffraction pattern of hexagonal sym- brucite n0101, [1100] and [0110] and their nega- metry, they were evidently not lying on (00O1) tives (in terms of Weber symbols).However, Ber- faces. Associatedwith the brucite were fibres of man found the length of brucite fibres to be along chrysotile-2M"r, chrysotile-D" and parachryso- the .r axis with a fibre repeat of 3.16 A,-3rnd tile, the last showing evidence of polygonal y1t! the corre_spondingWeber symbols [2110], morphology (Middleton & Whittaker 1979). [121O]and [1120] and their negatives.De Jong Another specimen of supposednemalite from (1938) also found this as the major orientation Cassiar mine, B.C., was studied by X-ray dif- for the brucite fibres in a specimenfrom Lojana, fraction. It consisted of an intergrowth of a Yugoslavia, but in addition he reported a second minor quantity of chrysotile with magnesite,not component with the e axis perpendicular to the brucite, and its texture also differed from that fibre axis but otherwiserandom. Ganido (1936) of nemalite in several ways. Although the part- also found that the main layer lines indicate a ing of the material into fibres appearedto be on 3.15 A repeat, but a very few weak reflections quite a fine scale, X-ray specimens (of the indicate a 2.74 A repeat, (i.e,, Yz X 5.48 A, order of 0.1 mm thick or more) had a co- in agreement with the orthohexagonal 6 axis). herent single crystallographic orientation and lNote that there is some confusion in nomen- gave oscillation photographs that could contain c-lature in Garrido's paper in that the 3.15 weak reflections of a chrysotile (rotation-type) A repeat is said to be along [10T0] and the photograph. Furthermore, in spite of a very 2.74 A repeat along [120]. These two zone-axis perfect morphological fibrous direction, such symbols should clearly be interchanged. There oscillation photographsshowed that the magnesite are also misprints in the indices of some of was slightly offset from a rational crystallographic the reflections, leading to impossible indices axis. Divergences of 0.5 and 4o between the such as 1l0o.l Strong Debye arcs with specific fibre axis and the nearest crystallographic axis cutoff points (as in de Jong's specimen) showed were noted in different specimens. When the the presence of fibres with their lengfhs in all crystal was adjusted to that the crystallographic possible crystallographic directions perpendi- axis coincides with the oscillation (or rotation) cular to their e axis. Bckhardt (1956), on tle axis, indexable photographs were obtained. One other hand, foun{a 5.5 A fibre-repeat which showedthat the crystallographic axis was close to he described as [110] (i,e., in terms of the rr, (though not coincident with) the fibre axis, xz otrd e axes only, equivalent to the Weber which had a fibre repeat of 7 34 A and was symbol [110OI); he noted the relationship of identified as lllfi (referred to rhombohedral this to the a dimension of_chrysotile. He also axes). The other specimen had two sets of found, however, that this [110O] axis was offset layer lines of comparable strengths giving re- from the fibre axis by amounts varying from peats of 7.9 and 11.8 A. These were identified 2 to 7o by rotation of the crystallites about as [112] and [113], respectively. their z axes. ft seems that the crystallographic orientation DrscussroN of nemalite presents an anomaly. On occasion, the orientation is what one would expest if the It is surprising that the brucite laths in the growth. of the brucite had been controlled nemalite from Jeffrey mine do not lie on epitaxially by the replaced chrysotile, but in (0001), since this implies that their smallest general the observed orientation is perpendi- dimension is not generally perpendicular to the cular to the one expected. basal plane. No complete explanation of this Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/17/4/699/4004849/699_17_4_cm.pdf by guest on 02 October 2021 BRUCITE- AND MAGNESITE_CIIRYSOTILE INTERGROWIHS 701 orientation can be offered, but it may be due suggest that the magnesite has grown between, to interfibrillar constraints during growth. and replaced, fibres of chrysotile and that this The discovery of parachrysotile in the ne- processhas exerciseda geometrical control over malite suggests a solution to the anomalous the morphology, but that the crystallographic crystallographic orientation of the brucite fibres orientation of the magnesite is a result of the that has been found to predominate in all the accidents of nucleation, modified by the effects specimens studied, exsept that of Eckhardt.
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