SHORT COMMUNICATIONS MINERALOGICAL MAGAZINE, DECEMBER 1982, VOL. 46, PP. 499-501 Chemistry of formation of lanarkite, Pb2OSO 4 W E have recently reported (Humphreys et al., 1980; sion which is at odds with the widespread occur- Abdul-Samad et al., 1982) the free energies of rence of the simple sulphate and the extreme rarity formation of a variety of chloride-bearing minerals of the basic salt, and with aqueous synthetic of Pb(II) and Cu(II) together with carbonate procedures for the preparation of the compound and sulphate species of the same metals includ- (Bode and Voss, 1959), which involve reaction of ing leadhillite, Pb,SO4(COa)2(OH)2, caledonite, angtesite in basic solution. PbsCu2CO3(SO4)3(OH)6, and linarite, (Pb,Cu)2 Kellog and Basu (1960) also determined AG~ for SO4(OH)2. By using suitable phase diagrams it has Pb2OSOa(s) at 298.16 K using the method of proved possible to reconstruct, in part, the chemical univariant equilibria in the system Pb-S-O. They history of the development of some complex obtained a value of -1016.4 kJ mol-1 based on secondary mineral assemblages such as those at literature values for PbO(s), PbS(s), PbSO4(s), and the Mammoth-St. Anthony mine, Tiger, Arizona, SO2(g) and another of - 1019.8 kJ mol- 1 based on and the halide and carbonate suite of the Mendip adjusted values for the above compounds. These Hills, Somerset. two results, for which the error was estimated to A celebrated locality for the three sulphate- be about 4.5 kJ mol-1, seem to be considerably bearing minerals above is the Leadhills-Wanlock- more compatible with observed associations than head district of Scotland (Wilson, 1921; Heddle, the earlier values. I923, 1924) from which the minerals leadhillite and We have redetermined the free energy of forma- caledonite were first described, and in which several tion of lanarkite at 298.2 K using previously other rare species have been noted including published solution techniques (Alwan and Wil- susannite, the hexagonal dimorph of leadhillite, liams, 1979). The lanarkite, (BM 44522) from the hydrocerussite, Pba(CO3)2(OH)2 and lanarkite, Susanna mine, Leadhills, Scotland was donated by Pb2OSO ,. Preliminary attempts to incorporate the British Museum (Natural History). The value this last species into our thermodynamic model for obtained is -1014.6+0.8 kJ mo1-1, which is in secondary mineral formation were frustrated excellent agreement with those by Kellog and Basu because of the wide variation in the reported values (1960). The solubility of lanarkite in water at 298.2 for its free energy of formation. We have therefore K is 7.57(36) x 10 -5 mol dm -3 at pH 7.67 and the redetermined this quantity, and use it to outline computed concentrations (Perrin and Sayce, 1967) possible conditions for the crystallization oflanark- of Pb(aq)2+ and SO4(aq)2- in equilibrium with solid ite from aqueous solution. lanarkite are 7.24(31) and 7.27(36) • 10 -5 mol dm -3 An early estimate of AGf(29s.0 2 K) (--1083.2 kJ respectively. The value of the equilibrium constant tool -1) was made by Latimer (1952) and more for (3) is: recently a value of -- 1071.9 kJ mol- 1 was reported PbzOSO4(s) + 2H(+~q)~- by Wagman et al. (1968). However, calculation of 2Pbtaq)2+ + SO,(aq2- ) + H20(1 ) (3) the free energy change between angiesite and lanarkite as given in equation (1) and using AG~ corrected to zero ionic strength and AG~pb~oso,) values by Wagrnan et al. (1968) for lanarkite and calculated in the usual way. Thus the pH value those listed in Barner and Scheuerman (1978) for shown in (4) has been calculated as the boundary the other species in the reaction, leads to a AG~ condition for the reaction given in (1). value of 46.6 kJ mol-1. This value can be used to pH = 9.10+0.5 logaso,~- (4) derive equation (2) which defines the boundary This result may be used to explain the known between the two minerals at equilibrium and at occurrences of lanarkite. Our conclusions are T = 298 K. reinforced by calculations involving other minerals 2PbSO4(s) + H20(1) formed from carbonate-bearing solutions. Pb2OSO4(s) + 2H(,q)+ + SO4(aq)2-- (1) The boundaries between anglesite and lead- pH = 4.08 + 0.5 log aso,~- (2) hillite (5), 4PbSO,,(s) + 2H2CO~ + 2H200) ~.~ Such a result suggests that lanarkite would be Pb4SO4(CO3)2(OH)2(s) + 3SO4(aq2 - ) + 6H(aq)+ thermodynamically stable, even at the expense of anglesite in many chemical environments, a conclu- pH = 6.24+0.5 logasol --0.33 1Ogaa~co o (5) 500 SHORT COMMUNICATIONS anglesite and hydrocerussite (6), conclusion that an~co~ rose during the later development of the Susanna oxidized zone, after 3PbSO4ts ) + 2H200) + 2H2CO~ ,~- the formation oflanarkite, leadhillite, and anglesite. Pba(COa)2(OH)2ts) + 3502(~q) + 6H(+aq) Two more recent reports have appeared con- pH = 6.4940.5 logaso,~---0.3~ log a n,c%o (6) cerning the mineralogy of the Leadhills-Wanlock- head mining field (Temple, 1956; Gillanders, 1981). leadhillite and lanarkite (7), Some paragenetic sequences of mineral formation Pb4SO4(CO3)2(OH)2(s) + 2Htaq)+ + SO4(aq)2 pertinent to our work are listed, including the 2Pb2OSO,(~) + 2H2CO~ noteworthy observation that cerussite was the first mineral in many sequences. In the 'exclusively lead pH = 0.51 +0.5 logaso~ -logan~c%o (7) ores' Temple (1956) notes leadhillite after cerussite, leadhillite and hydrocerussite (8), followed by lanarkite, but less commonly than the 3Pb4SO4(CO3)2(OH)2(s ) + 2H2CO~ + 2H200) .~- classic order galena, PbS ~ anglesite ~ cerussite. A further observation of hydrocerussite after cerus- 4Pb3(CO3)2(OH)2(s) + 3SO4(aq)2-- + 6Htaq)+ site firmly establishes that during the formation of pH = 7.26+0.5 logaso,~---0.3] logan~cog (8) the secondary minerals, the an,co o fell and then and lanarkite and hydrocerussite (9), rose again, parallelling observations concerning the emplacement of the complex oxide zone at Tiger, 3Pb2OSO4(s) + 4H2CO~ + H20(I) Arizona (Abdul-Samad et al., 1982). All these 2Pb3(CO3)2(OH)2(s ) + 3SO2~q)+ 6H~q~ observations neatly fit the experimental results on the thermodynamic stability of lanarkite, and thus pH = 3.88 +0.5 log aso~ -- 0.66 log an,co 0 (9) its rarity. are all dependent upon aH2coo and thus the Finally, a recent report (Crowley, 1980) outlines stability fields of these minerals, and of cerussite, the first documented occurrence of lanarkite in the PbCO3, must be limited by this quantity. Inspec- USA at the C. and B. mine, Gila county, Arizona. tion of equations (4)-(9) indicates that lanarkite is All the specimens were altered, being partially or thermodynamically stable only at very low activi- completely replaced by cerussite, with fewer than ties of H2COa(aq).o It is clear from other studies a dozen out of some one hundred cerussite speci- involving secondary copper and lead minerals mens, some of which were completely replaced (Abdul-Samad et al., 1982) that such low activities pseudomorphs, still containing any of the basic are achieved only rarely in nature and hence sulphate. Neither at this deposit nor at the Susanna lanarkite should be an uncommon mineral. mine was the expected alteration sequence lanark- In appropriate conditions with au2co~ c. 10-8.6, ite ~ hydrocerussite --* cerussite observed, and the thermodynamic model predicts that the asso- further examination of the specimens especially at ciated minerals that might be expected, not taking grain boundaries, may reveal the basic carbonate. into account those species containing Cu(II), are anglesite and leadhillite. Field observations on the Acknowledgement. The Royal Society is gratefully ack- occurrence of lanarkite bear this out. nowledged for a travel grant (to P.A.W.) to facilitate a Even in the Leadhills district, lanarkite was study of mineral occurrences and collections in Arizona. identified with certainty from only one mine, the Susanna, although Brown (1919) reported its REFERENCES occurrence on dumps from other veins in the Abdul-Samad, F. A., Thomas, J. H., Williams, P. A., district. It was adjudged to be 'the rarest mineral Bideaux, R. A., and Symes, R. F. (1982) Transition Met. occurring in the district' (Wilson, 1921) and 'much Chem. 7, 32-7. the rarest of the minerals occurring at Leadhills' Alwan, A. K., and Williams, P. A. (1979) Ibid. 4, 128-32. (Heddle, 1924). The latter author lists lanarkite as Barner, H. E., and Scheuerman, R. V. (1978) Handbook of being associated with susannite and caledonite, Thermochemical Data for Compounds and Aqueous rarely with cerussite, but also notes its association Species. John Wiley and Sons. with anglesite, linarite, leadhillite, and pyromorph- Bode, H., and Voss, E. (1959) Electrochim. Acta l, 318-25. ite. Anglesite was abundant in the Susanna mine, Brown, R. (1919) Trans. Dumfriesshire and Galloway Nat. associated with linarite, cerussite, and lanarkite; Hist. and Antiquarian Soc. 6, 124-37. leadhillite was also found in large quantities in Crowley, J. A. (1980) Mineral. Rec. 11, 213-18. Gillanders, R. J. (1981) Ibid. 12, 235-50. combination with cerussite or with cerussite and Heddle, M. F. (1923) The Mineralogy of Scotland, 2nd edn. lanarkite. A series of pseudomorphs of particular Vol. 1. W. C. Henderson and Son. interest here is listed by Heddle (1923) and Wilson --(1924) 1bid. Vol. 2. (1921), specifically cerussite after lanarkite, angle- Humphreys, D. A., Thomas, J. H., Williams, P. A., and site, and leadhillite. These replacements support the Symes, R. F. (1980) Mineral. Ma9. 43, 901-4. SHORT COMMUNICATIONS 501 Kellog, H.