THE STRUCTURES OF THE MINERALS OF THE DESCLOIZITE AND AD ELITE GROUPS: III- BRACKEBUSCHITE D. M. DONALDSON! AND W. H. BARNES, Division of Physics, National Research Council, Ottawa, Canada. ABSTRACT The structure of brackebuschite, Pb2(Mn, Fe) (VO.).' H20, has been determined, al- though there is still some uncertainty about the location of the water molecule (if it is in- deed present). Atoms of Pb, Mn and Fe, and V are in special positions. The coordination of oxygen with vanadium is tetrahedral; it is fourfold or sixfold with manganese depending on the H20-sites, and it is eightfold with one lead atom and tenfold with the other. Metal-to- oxygen distances are given and the accuracy of the results is discussed. By comparison with the structure of pyrobelonite it is shown that brackebuschite should not be classified with the minerals of the descloizite and adelite groups. INTRODUCTION Several attempts have been made to classify brackebuschite according to the chemical composition deduced from the analyses of Doering (1883) which are the only ones available in the literature. Thus Dana (1892) suggested that the formula is "perhaps A3 V208+ H20* with A = Pb chiefly, also Fe, Mn", but observed that Damour regarded Mn, Fe, (and Cu) as impurities; these data are repeated by Mellor (1947). Dana also credits Groth with the suggestion that brackebuschite might be the monoclinic equivalent of descloizi te. Strunz (1939) referred brackebuschite to a type A2(X04)(OH),* and, on the basis of x-ray diffraction powder photographs, concluded that it must be completely different structurally from the minerals of the des- cloizite group. On the other hand, Richmond (1940), without examina- tion of brackebuschite by x-ray methods, assigned it the formula (Pb, Mn)2V04(OH) with Pb:Mn=2:1 and suggested that it forms a series with pyrobelonite, which he formulated (Mn, Pb)2V04(OH) (see, Don- aldson & Barnes, 1955). He, therefore, placed brackebuschite "tenta- tively" in the orthorhombic section of family ABX04(Z) with pyrobelon- ite, descloizite, higginsite (i.e., conichalcite; Berry, 1951), etc. (Rich- mond, 1940, pp. 444, 476, 477). After a careful examination of Doering's analytical results in the light 1 National Research Laboratories Postdoctorate Fellow, now I.C.I. Fellow, The Uni- versity, Edinburgh, Scotland. * The letter R is used by Dana (1892) for the cation whereas it is employed by Strunz (1939) for the metalloid of the anion; in this paper it has been changed to A for the former and X for the latter in conformity with later formulae (Richmond, 1940; Berry & Graham, 1948; Dana, 1951). 597 598 D. M. DONALDSON AND W. H. BARNES of original specific gravity, unit cell, space group (P21/ m), and indexed powder data, Berry & Graham (1948) came to the conclusions that the ideal structural formula of brackebuschite is 2[Pb2(Mn, Fe) (V04)2' H20], that the mineral belongs to the chemical type A3(X04)2' nH20 with n = 1, and that their investigation had failed to show any relationship with pyrobelonite. These results have been incorporated in the Seventh Edi- tion of Dana's System of Mineralogy (1951). The formula of Berry & Graham (1948) is in complete agreement with that suggested by Doering in 1883 and Dana in 1892. During a recent survey of unit cell and space group data for a number of vanadium minerals in this laboratory (Barnes & Qurashi, 1952), sev- eral interesting similarities and differences were observed between pre- cession photographs of brackebuschite on the one han.d and those of members of the descloizite and adelite groups on the other (see, Qurashi & Barnes, 1954, Fig. 1). With the information now available on the structures of descloizite and conichalcite (Qurashi & Barnes, 1954) and pyrobelonite (Donaldson & Barnes, 1955) it is of particular interest to compare the atomic arrangement in brackebuschite with that charac- teristic of the minerals of the descloizite and adelite groups. EXPERIMENTAL, AND CRYSTAL DATA The experimental work was carried out with two crystals from the same (Sierra de Cordoba) specimen (Harvard Museum, 96255) as those examined by Berry & Graham (1948); the crystals had the dimensions 45X30X180j.l and 50X35X80j.l. Intensities were estimated visually from precession and Weissenberg photographs (MoK" radiation) using multiple exposure technique for the former and multiple film technique for the latter, and were corrected for the Lorentz and polarization fac- tors. The absorption coefficient for MoK" (X=0.7107 A) is 535 per em. and approximate absorption corrections were applied, assuming cy- lindrical crystals. Brackebuschite is monoclinic, space group P21/m(C2h2) or P21(C22), with a=7.681, b=6.15s, c=8.880 A, and ,8=111°50' (Barnes & Qurashi, 1952). There are 2[Pb2(Mn, Fe)(V04h.H20] per eel!. The calculated density is 6.11 gm. per m!.; observed specific gravity (Berry & Graham, 1948), 6.05 (Doering, 1883, found 5.85 which he acknowledged must be low because the specimen employed contained up to 4 per cent of quartz and ferruginous matter). In order to facilitate direct comparison with descloizite and pyro- belonite it is convenient to refer brackebuschite to a B-centered cell (see, Barnes & Qurashi, 1952; Qurashi & Barnes, 1954) given by the trans- formation matrix 100/010/102. The dimensions of the new cell are a' = MINERALS OF THE DESCLOIZITE AND ADELlTE GROUPS 599 7.681, b'=6.155, c'=2X8.262 A, {3=93°45', and the space group is E2J/m or E21; coordinates, diagrams, and discussion throughout this paper are in terms of this E-centered cell. ANALYSIS OF THE STRUCTURE Direct comparison of corresponding zero-level precession photographs of brackebuschite, pyrobelonite, and degcloizite (see, Qurashi & Barnes, 1954, Fig. 1) shows that corresponding hkO reflections from the three minerals are almost identical in relative intensities and Bragg angles, cor- responding Okt reflections are very similar, but there are important dif- ferences between the hot reflections from brackebuschite and correspond- I --------- -... ( ~- , ...) - - - - - J <::: ... '... --- --------.... ~ -- "' ( ------ ----- ..." .......----------- FIG. 1. Patterson map for the {Okl} zone of brackebuschite. Contours are regular, but arbitrary, intervals; zero contour broken. ing ones from pyrobelonite and descloizite. Although it would thus have been feasible to assume that the x and y coordinates of the lead atoms in brackebuschite probably are virtually the same as those in descloizite (Qurashi & Barnes, 1954) and in pyrobelonite (Donaldson & Barnes, 1955) it was decided to start the structure analysis of brackebuschite by computing the (100) Patterson map. The result is shown in Fig. 1 from which it is apparent that the majority of the peaks lie along y=O and t. These must represent most of the metal-metal vectors. It follows, there- fore, that the lead together with some of the lighter metal atoms must lie on, or close to, the same planes which are separated by b/2. The positions of the other peaks in Fig. 1 suggest that the remainder of the lighter 600 D. M. DONALDSON AND W. H. BARNES metal atoms must be situated on, or near, planes midway between these. If all the metal-atom sites are exactly in the planes specified, they are consistent with space group B2dm for the metal atoms, and hence this space group was adopted provisionally as the basis for the structure an- alysis. The equivalent positions of B2dm are (a) O,O,O;O,~,O;!,O,~;~,!,! (b) !,O,O;i,~,O;O,O,!;O,!,! (c) x, ~,z;x, 1,z; !+x,~, ~+z;! -x, -I,~-z (d) x, y, z; x, !.-y, z; !+x, y, !+z; !+x, !-y, !+z; x, y, z; x, !+y, z; !-x, y, !-z; !-x,!+y,!-z. From a detailed interpretation of the (100) Patterson map y, Z co- ordinates were obtained for lead, manganese*, and vanadium, with lead and vanadium at y=::!::t (special positions (c» and with manganese in special positions (a) or (b). In view of the small difference between the atomic numbers of manganese and vanadium, the choice of which should be placed in the available positions was governed by the sites occupied by zinc and vanadium in descloizite. The structure was not prejudged by this selection because an "average" atom X of f=!(fMn+fv) could equally well have been placed in each of the positions at this stage. A (100) Fourier synthesis with metal-atom coordinates obtained from the analysis of the Patterson map was then carried out (see Fig. 2). The positions of Pb, Mn, and V were refined and termination-of-series errors were corrected by successive difference syntheses (Cochran, 1951) using coordinates for these atoms alone until those for the oxygen atoms had been determined. After inclusion of the latter only minor shifts in the positions of the metal atoms were required in succeeding difference syn- theses. Examination of the hkO intensities showed that, in addition to the absences characteristic of the B-centering, all reflections for which h = 4n+ 2, k = 2n and for which h = 4n, k = 2n+ 1 were either unobservable or very weak. On the assumption that this effect must be caused by the positions of the lead atoms, it follows that the x, y coordinates of Pb are x=t, y=t, etc. (special positions (c». Because the lead atoms determine the phases of most reflections, it was possible to carry out a Fourier syn- thesis for this zone immediately (see Fig. 2) and then to proceed with re- finement by successive difference syntheses as in the case of the {Okl} zone. Finally the {hOl} zone was refined by the same procedure, commencing * Since the atoms of Fe in brackebuschite must be occupying some of the Mn-sites, "manganese" refers to both Fe and Mn unless otherwise noted in the text.