MINERAL CHEMISTRY AND MINERALOGY OF THE LEAD BEARING MEMBERS OF THE BEUDANTITE AND RELATED MINERAL GROUPS. By W.E. BAKER B.Sc. Hon (Tas.), M.Aus.I.M.M. A thesis presented for the Degree of Master of Science, The University of New South Wales. September, 1962. PLATE 1 Dundas Pyromorphite showing pseudomorphous Hinsdalite, a Beudantite Group mineral, at low centre (x2). CONTENTS Page Summary 1 • Introduction 4. Part I - Mineral Synthesis 11. Part II - Studies of Mineral Equilibria 23* Part III - Mineralogical Studies 49. Part IV - Concluding Remarks 96. Appendix - Analytical Procedures and Detailed Results 104. References 124. (iii) SUMMARY. Following the recognition of a pseudomorph after pyro- morphite from Dundas, Tasmania, as being either hinsdalite PbAl^CPO^) (SO^) (OH)^ or plumbogummite PbAl^(P0jL+)2(0H) H^O, a general study of these and related minerals was under­ taken. The minerals, beudantite, PbFe^(AsO^)(SO^)(OH)^, cork- ite, PbFe^(PO^)(SO^)(OH)^, hinsdalite and hidalgoite PbAl^ (AsO^)(SO^)(OH)^, all members of the Beudantite Group, were synthesized as were also the compounds PbAl^CVO^)(SO^)(OH)^ and PbFe^CVO^)(SO^)(OH)^. Synthesis of plumbogummite and its iron analogue PbFe^(POi+)p(OH)^.H^O were also successful but attempts to produce the chromium analogues of these min­ erals and compounds related to plumbojarosite PbFe^CSG^)^ (0H)12 failed. Specimens of hinsdalite, plumbojarosite and the syn­ thetics were examined by means of X-ray diffraction and the rhombohedral cell constants found to be: Hinsdalite = 61°8 1 arh = 6,88 a Hidalgoite 6*97 60°43 Corkite 7.00 63°00 Beudantite 7.08 62°36 PbAl3(V04)(S04)(0H)6 7.00 60°20 PbFe3(V04)(S04)(0H)6 7-12 62°24 Plumbogummite 6.90 61 °8' 2. PbFe3(P04)2(0H)^.H20 7.04 63°36 Plumbojarosite 11.94 35°00 It was not found possible to distinguish between hinsdalite and plumbogummite on the basis of X-ray results. The pres­ ence of water in only the latter mineral indicated that differential thermal analysis might detect the difference and this was found to be the case. The formation of hinsdalite, corkite and pyromorphite was subjected to physico-chemical studies which enabled the calculation of the free energies of formation of these min­ erals as -1034.4, -669*8 and -842.3 k.cal. respectively. These values were used to test the feasibility of a volume constant reaction proposed to explain the alteration of pyromorphite to hinsdalite. -| # 9Pb?(P04) Cl + 72A13+ + 24S0^_ + 1440H“ (+ 24C1_) ^ 24PbAl (P04)(S01+)(0H)6 + 21Pb2+ + 3P0^_ + 9CI (+24C1-). The reaction was found to result in a decrease in free energy and would thus be a spontaneous process. Hence it is likely that the equation demonstrates the manner in which the alteration of pyromorphite took place in the oxidized zone. The genesis of either hinsdalite or plumbogummite re­ quires the presence of abundant aluminium which is not a 1. Bracketed chloride ions included only to preserve elec­ trical neutrality. • 3- common metal in either the hydrothermal or oxidation envir­ onment. The abundance of the eLement in the Dundas area is indicated by the widespread occurrence of gibbsite. The origin in this case is possibly to be found in the hydro- thermal alteration of the eugeosynclinal sediments with which the Dundas ore-bodies are associated. Since hinsdal- ite and plumbogummite both appear to be formed quite readily the mineral which is produced may depend largely upon the time of entry of the aluminium into the environment. If this occurs during the oxidation of the ore-body then the abundance of sulphate will favour the formation of hinsdal- ite. Once the sulphides have been oxidised down to the water table, the concentration of sulphate in the ground water will fall to a far lower level and, provided that sufficient phos­ phate is present, the formation of plumbogummite will be favoured. 4. INTRODUCTION. The data presented in this thesis have resulted from the.study of a number of the less common secondary lead min­ erals. The investigations began in 1957 whilst the writer was a teaching fellow at the University of New South Wales and were completed at Broken Hill in 1961. The programme was initiated by the study of a specimen from the Comet 0 Mine, Dundas, Tasmania *. This consisted largely of a pale green mass of hexagonal prisms of pyromorphite distributed through a limonitic gossan. In parts of the specimen there were white pseudomorphs after pyromorphite, some of them occurring merely as thin shells of hexagonal outline. (Plate 1, frontispiece.) A qualitative spectrographic examination of the pseudo- morph showed that the dominant metals were aluminium and lead whilst silver was present as a minor constituent. X-ray powder diffraction data were obtained by means of a Phillips Model 1050 X-Ray Diffractometer. Comparison of these data with those available in the A.S.T.M. Card Index showed that plumbogummite, PbAl^(PO^)^(OH) H^O (Card 2-O683), gives a similar diffraction pattern and has a composition compatible with the spectrographic results for the pseudomorph. 2. Examination of the specimen was requested by Associate Professor L.J. Lawrence, then Senior Lecturer in charge of the Geological Section of the School of Mining Eng­ ineering and Applied Geology. .?• Plumbogummite is a member of one of three isostruct- ural groups of minerals, namely the Alunite, Plumbogummite and Beudantite Groups. As is the case with the Plumbo­ gummite Group the other two take their names from member minerals alunite, KAl^(SO^)^(OH)^, and beudantite, PbFe^ (AsO^) (SOj^) (OH)^. One member of the latter group, hinsdal- ite, PbAl^CPO^)(SO^)(OH)^, has a composition near to that of plumbogummite. No X-ray data were available for hinsdalite and a specimen was sought so that X-ray diffraction studies could be made. The Australian Museum in Sydney kindly don­ ated a specimen from the original locality of the Golden Fleece Mine, Hinsdale County, Colorado, U.S.A. The diffrac­ tion patterns of the pseudomorph, plumbogummite and hinsdal­ ite are given in Table 1 (page 6). From these data it can be seen that the identity of the pseudomorph cannot be re­ solved by X-ray diffraction methods alone. Solution of the problem was not attempted at this stage and the line of study was changed to investigate the genesis of the pseudomorph. By refluxing pyromorphite with a solution of aluminium sul­ phate, a material yielding a diffraction pattern similar to that of hinsdalite was produced. As a result of this success the writer decided to extend the studies to include the lead bearing members of the Beudantite, Plumbogummite and Alunite Groups. KENSI NGTON £ TABLE 1. Comparison of X-Rav Diffraction Patterns Hlnsdalite. Plumbogummite Dundas A.S.T.M. Card Hinsdalite Pseudomorph 2-0683 (Colorado) d d d 1/11 i/q 1/1 1 5-71 95 5.70 80 5.70 92 5-5 8 15 - 5-57 30 4.92 15 4.8V 4o 4.90 3 3-51 60 3-79 40 3-51 44 3.45 20 3.45 60 3-43 18 2.97 100 2.97 100 2.97 100 2.94 10 - 2.93 4 2.86 15 2.82 20 2.85 11 2.79 15 - 2.79 15 2.46 15 2.44 4o 2.45 10 + In Parts I and II of this thesis only a part of the diffraction patterns is given. Complete data are given in Part III - Mineralogical Studies. The minerals concerned are listed in Table 2 (page 8). In­ vestigations were planned in three fields, namely mineral synthesis, mineral equilibria and mineralogy. Mineral syntheses were undertaken to produce the 7 required minerals and other lead bearing compounds of sim­ ilar structure. It was also planned to examine the possibil­ ity that other secondary lead minerals, like pyromorphite, were altered by the action of aluminium sulphate. Studies of mineral equilibria were designed to enable the determination of free energy of formation data which it was thought would allow a more accurate consideration of the alteration of pyromorphite in the presence of aluminium and sulphate ions. Mineralogical studies were undertaken mainly to obtain X-ray diffraction data for the minerals concerned since such data have been unavailable up to the present time. Because of the similarity between the diffraction patterns of hins- dalite and plumbogummite, other means of distinguishing these minerals were sought. Chemical spot-testing and differential thermal analysis were thought to be applicable to this prob­ lem. .8. TABLE 2. Lead Bearing Members of the Alunite. Plumbogummite and Beudantite Groups. ALUNITE GROUP Plumbojarosite PbFe6(S04)lf(0H)12 PLUMBOGUMMITE GROUP Plumbo gummit e PbAl3(P04)2(0H)?.H20 BEUDANTITE GROUP Beudantite PbFe3(As04)(S04)(0H)6 Corkite PbFe3(F04)(S04)(0H)6 Hinsdalite PbAl (P04)(S04)(0H)6 Hidalgoite PbAl3(As04)(S04)(0H)6 The minerals concerned have been studied by a number of workers. A chemical classification of the members of the three groups was proposed by Schaller (1911)* Morphological and optical studies of beudantite have been made by Lacroix (1915)5 of corkite by Miers (1900), of hinsdalite by Larsen and Schaller (1911) and of hidalgoite by Smith et al (1953)* Plumbogummite has been examined by Prior (1900) and plumbo- jarosite by Hillebrand and Penfield (1902) and Hillebrand and Wright (1910)• X-ray data have been published for hid­ algoite by Smith et al (Op.Cit.). Interplanar spacings and intensities for plumbogummite are listed in the A.S.T.M. Card Index (Op.Cit.), apparently derived from unpublished 9 records of the British Museum. The cell dimensions and structure of plumbojarosite have been elucidated by Henricks (1935) and interplanar spacings and intensities have been published by Kauffman et al (1950)* These data have been summarised by Palache et al (1951) and are presented in Table 3 (page 10).