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The Chemical Stability of Mendipite, Diaboleite, Chloroxiphite, and Cumeng6ite, and Their Relationships to Other Secondary Lead(II) Minerals

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The Chemical Stability of Mendipite, Diaboleite, Chloroxiphite, and Cumeng6ite, and Their Relationships to Other Secondary Lead(II) Minerals MINERALOGICAL MAGAZINE, SEPTEMBER I980, VOL. 43, PP. 901-4 The chemical stability of mendipite, diaboleite, chloroxiphite, and cumeng6ite, and their relationships to other secondary lead(II) minerals D. ALUN HUMPHREYS, JOHN H. THOMAS, AND PETER A. WILLIAMS Department of Chemistry, University College, Cardiff CFI IXL AND ROBERT F. SYMES Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD SUMMARY. The chemical stabilities of mendipite, xiphite assemblage. Chloroxiphite, mendipite, and Pb302C12, diabole'ite, Pb2CuC12(OH),, chloroxiphite, diaboleite were all originally described from Pb3CuC1202(OH)2, and cumeng6ite, Pb19Cuz,C142 deposits in the Mendip Hills, Somerset, England (OH)g,, have been determined in aqueous solution at (Spencer and Mountain, I923), and from which 298.2 K. Values of standard Gibbs free energy of forma- area paralaurionite has also recently been recorded tion, AG~,for the four minerals are - 74o, - I I60, - 1129, and -I5163+20 kJ mol-1 respectively. These values (Symes and Embrey, I977). Chloroxiphite is known have been used to construct the stability diagram shown only from this area. in fig. I which illustrates their relationships to each other The occurrence of mendipite, paralaurionite, and to the minerals cotunnite, PbCl2, paralaurionite, diaboleite, and the bol6ite group in settings which PbOHC1, and litharge, PbO. This diagram shows that could only derive from aqueous solutions (Winchell mendipite occupies a large stability field and should and Rouse, I974; Palache et al., 1951) suggests that readily form from cold, aqueous, mineralizing solutions all of these minerals can have a secondary origin. containing variable amounts of lead and chloride ions, These observations have prompted us to determine and over a broad pH range, The formation of para- the stability fields of several members of the group laurionite and of cotunnite requires a considerable at 298 K in equilibrium with aqueous solutions. increase in chloride ion concentration, although para- laurionite can crystallize under much less extreme condi- Here we report the chemical stabilities of mendip- tions than cotunnite. The encroachment of the copper ite, Pb302C12, diaboleite, PbaCuC12(OH)4, minerals on to the stability fields of those mineral phases chloroxiphite, PbaCuC1202(OH)2, and cumeng6- containing lead(II) only is significant even at very low ite, Pb19Cu24CI#2(OH)44, and their relationships relative activities of cupric ion. Chloroxiphite has a large to cotunnite, PbC12, paralaurionite, PbOHC1, and stability field, and at given concentrations of cupric ion, litharge, PbO. diabole'ite is stable at relatively high ac~-. Cumeng~ite Method. Mendipite and chloroxiphite from the will only form at high concentrations of chloride ion. Merehead Quarry, Somerset, England (Symes and Embrey, i977), diaboleite from the Mammoth HALIDE salts are generally soluble in water, mine, Tiger, Arizona (Anthony et al., I977), and which explains why halide minerals of the transi- cumeng6ite from Santa Rosalia, Baja California, tion elements are usually absent from the vicinity of Mexico (Wilson and Rocha, 1955), were used in oxidizing orebodies. In such an environment low this study. Individual crystal fragments were hand- pH values also tend to facilitate the solution of picked under the microscope and freed from any most species. Nevertheless, many halide minerals contaminating material. The compounds were are known and occur in considerable quantities at checked by their X-ray powder patterns. For the some localities. Two groups of halide minerals determination of thermodynamic stabilities, the containing Cu(II) and Pb(II) are outstanding in compounds were ground finely in an agate mortar their complexity and rarity. These are the bol6ite and added in excess to boiled-out, CO2-free water group (Winchell and Rouse, I974) and in flasks from which all gas was excluded. The the mendipite-paralaurionite-diaboleite-chloro- aqueous systems were allowed to come to equili- )Copyright the Mineralogical Society 902 D. A. HUMPHREYS ET AL. brium (constant pH) in a thermostatted water bath Vuceta and Morgan (i977) , and Zirino and (298.2_ o.2 K). The solutions were then treated as Yamamoto (/972). An example of the calculations previously described (Haacke and Williams, 1979) , used is (for mendipite): the pH measured (Orion 47o-A Ionaliser), and the total concentrations of Pb(aq)2 +' Cu~aq),2 + and C~q) in PbaOzC12(s) + 4FI(a+q)~ 3 Pbi2a~+ 2C~aq) + equilibrium with each solid phase determined by 2H20(1), and KH+ = apb2. a~l-/aH+ , where at atomic absorption spectroscopy (Pb and Cu) using is the activity of the species i. a Varian AA6 instrument fitted with carbon rod a~i is available directly from the pH measurement, attachment, and colorimetry (C1) using standard apb2. and acl- are obtained from the free concentra- techniques (Huang and Johns, 1967). For deter- tions of the p~2~ and C~aq) ions and their calcu- mination of solubility products, the activity of the lated activity coefficients. From KH+,AG ~ for the free ions involved must be determined and for this reaction is obtained from the relation AGO= purpose ionic species distributions in the aqueous -RTlnKH. and using AGO values for P~aq),2+ solutions were calculated using a previously C~0, and H20~1~ (Barner and Scheuerman, t978), described method (Alwan and Williams, i979) AGf for the mineral is calculated. Full lists of ionic involving the well-tested computer program species distributions for all of the minerals are COMICS (Perrin and Sayce, I967). The extended available from the authors on request. A series of Debye-Hfickel approximation (Truesdell and repeat experiments established the limits of errors Jones, I974; Kielland, 1937) was used to obtain in the AGO values below as + 20 kJ mol-x. The individual ionic activity coefficients (7i). In the AGO values calculated in this study are given in species distribution calculation, the ions considered Table I, together with those of cotunnite and were CuCl(+q), CuCl~ CuCI~aq) ' CuCl~(aq), litharge (Barner and Scheuerman, i978 ) and para- PbC~aq),oPbC12(aq),+ 0 PbC13(aq),- PbC14taq),2- CuOF!~aq),+ laurionite. No distinction is drawn here between Cu(OH)2(~q), Cu(OH)~aq), Cu(OH)~aq), paralaurionite and its dimorph, laurionite (Palache Cu2(OH)~(a*q), PbOHta+q), Pb(OH)~ Pb(OH)3~aq), et al., I95i ) which has not been observed in the Pb2OH(aq),3+ Pb3(OH)4(ac0,2+ Pbr 4-+ and Mendip Hills associations. The value of AG~ for Pb6(On)ggq). PbOHClts ) of -4o8. 3 kJ mol given in Wagman et Equilibrium constants for the formation of these al. (1968 and 1969) is clearly too high as evidenced ions were obtained from Baes and Mesmer (I976), by comparison with values for PbFC~s) and Pb(OH)E(s), and by calculation of the stability field of PbOHC~s), relative to that of cotunnite. For- tunately a value for PbOHC~s) can be derived from -0.5- II \ c....... the work of Yadava et al. (I968) who studied the ionization of PbOHCI~s) in aqueous solution at 308.2 K. The ion product [PbOH § l-C1-] at this temperature was found to be 7.6 x to -7 and from -2.5-- this value AG~ ~: was calculated for the solid phase (see above). The value obtained, -383. 7 kJ mol- 1, agrees well with calculations of other phases (see below) and although there is a temperature discrepancy between the value chosen for para- -4.5-- laurionite and all other solid phases, this will have a minimal effect on the nature of the stability field log ac1--- of the former mineral (Garrels and Christ, I965). -6.5~ Of course, a difference in AGO between para- laurionite and laurionite is to be expected. Yadava et al. do not report, in their study of PbOHC1, to / which dimorph their work refers. However, any -8.5 difference in AGO for the two minerals will be so I I I I I I I 5 7 9 small as to make no visible changes to the stability pH field diagrams shown in figs. 1 and 2. FIG. I. Stability fields for the copper and lead halide Discussion. The results obtained above have been minerals in terms of ac~ and pH at 298 K. The bold used to construct the stability field diagram shown line delineating the Cu-containing species refers to an in fig. I. Calculations were made to eliminate acu2. of Io -8 mol dm -3. The limits for acu2+ = apb2* as a variable in the system. For the Cu- to -6 mol din-3 are shown with a dashed line. containing phases, the bold boundaries refer to an CHEMICAL STABILITY OF MENDIPITE 903 TABLE I. AG~ * derived and used in this study Mineral Formula AG~ mol- 1 mendipite Pb302CI 2 -- 74o___ 20 ] diaboleite Pb2CuCI2(OH)4 - 116o + 2o this work chloroxiphite Pb3CuCI202(OH)2 - I 129 _ 20 / cumengeite Pbl 9Cu24CI42(OH),,4 - 15163 ___20 l cotunnite PbCI2 - 314-2 ] litharge PbO - I89.1 see text paralaurionite PbOHC1 - 383.7 J * Calculated at 298.2 K except for PbOHC1 (see text). equilibrium acu2§ of IO -s mol dm -a. The although it can crystallize under much less extreme boundaries with acu2+ = Io -6 mol dm -3 are also conditions than cotunnite. Activities of Pb(Zao~ and shown in faint lines. A diagram showing Cu-free C~q) of the order of IO -2 tool dm -3 and a pH of minerals is shown in fig. z, on which is also about 5.5 are sufficient for paralaurionite to be contoured apb2+ in equilibrium with the mineral stable. It is also worth'noting, in view of the possible phases. involvement of mineralizing solutions at greater Conditions under which the Cu-free minerals temperature than those used in the experiments, can form vary considerably. Cotunnite can only that PbC12ts) and PbOHCI~s) are appreciably more form from acid solutions where both act and soluble in hot water than in cold.
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