The aqueous chemistry of uranium minerals. Part 2. Minerals of the ...

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The aqueous chemistry of uranium minerals. Part 2. Minerals of the liebigite group. ALWAN K. ALWAN AND PETER A. WILLIAMS. Department of Chemistry ...





VOL. 4 3 , PP- 6 6 5 - 7

The aqueous chemistry of uranium minerals. Part 2. Minerals of the liebigite group ALWAN K. ALWAN AND PETER A. WILLIAMS Department of Chemistry, University College, Cardiff, CFI IXL

SYNOPSIS FREE energies of formation of liebigite, Ca2UO 2 (CO3) 3" I o H 2 0 , swartzite, CaMgUO2(CO3) 3. I 2 H 2 0 , bayleyite, Mg2UO2(COa) 3. I 8 H 2 0 , and andersonite, N a 2 C a U O 2 ( C O 3 ) 3 - 6 H 2 0 have been



9 /






determined from solution studies at various temperatures, AGf(298.2K) o values for the above minerals are - 6 2 2 6 _ 12, - 6 6 o 7 _ 8, - 7924 + 8 and - 5651 _ 24 kJ m o l - 1 respectively. AHg298.2K) values respectively are - 7037 +--24, - 7535 - 20, - 9 1 9 2 + 2 o , and -5916+_36 kJ mo1-1. These results have been used to construct the stability diagram for the four minerals shown in fig. I. Andersonite can only form when the activity of the sodium ion, aNa§ is relatively high and aca2§ and a~ag2+ are small. The interconversions of the sodium-free species are defined. When aMg,+/ aca,§ 7.94, bayleyite forms preferentially. One might therefore expect the bayleyite-andersonite association to be more c o m m o n than liebigiteandersonite, but this is entirely dependent upon the relative concentrations of Mg2+(acO and Ca2+(acO in the solutions from which the minerals form. REFERENCE











log aca2+

FIG. I. Stability fields for the minerals of the liebigite group at 298.2K in terms of aMg2+ and aca2+. The firm line defining the andersonite field is calculated for aNa+ = IO-3 mol dm -3. The faint lines indicate the same boundary for aNa+ = 10 -2 and IO-~ mol dm -3.

Haacke (D. F.) and Williams (P. A.), I979. Mineral. Mag. 43, 539-4 I.

[Manuscript received 8 July 1979] [Note added in proof. FIG. I. For log aNa+ read aNa+.]

@~Copyright the Mineralogical Society





P. A. W I L L I A M S


and P.A. Willia~ts

Department of Chemistry, University College, Cardiff, CF1 1 ~ . The important role of carho~ato-c~plexes of the uranyl ton ~022" (aq)) in the transpo~ os uranium in ground waters has loag been recognized. In an ~idizing. Cnvlr~ment, _the s t a b i l i t y field os the ccmplexes [DZO~3(aq), ID2(OO3)22(eq). and ID2(COs)34faq). impinge significantly on that of LD2(S) coordination. Fo#ever, whe~ such anions as V04~-~' , ~O 3" (aq) or AsO~" _ (aq) are present, several suites of more insoluble minerals can form preferentinlly ~ i r , 1978 a,b}. Although the t~2(CO3)3~q ) . _ ion is rmmrkably stable ~ d soluble in aqueous solution, should it beccme concentrated enough, several secondary minerals based upon i t s structure can for~ depending upon the a w i l a b i l i t y of suitable counterio~s. These a t i ~ r a l s are often, but not a l ~ y s , found as effloroscences on the ~a/ls of old mine ~ o r k i ~ s where the rate of evaporation is high. In a sense the minerals are intemediate between oxidized uranium ore deposits and uranium species dissolved in ground ~aters and as such are of particular interest to the e ~ l o r a t l o n g~ochomist. Ptn'baps the most w t d e s p r ~ of the mLnerals based on the trism(carbonato)-dioxouraniumC/l) ion are those containing the alkali earth cations. Liebigite, Ca2L~z(COs)3.10~20) the ~ost ommon species ~ms recognlsed as early as 1848 (Smith 1848, 1651). Other e a r l i e r occurrences are described by Frondel (1958). NDre recent occurrences have b ~ m noted in Gera~ny, Japan, Canada and Sadden (Metsobara, 1976; Rimsaite, 1977; welenta, 1977; Natan~be, 1976; welin, 195~). The other c ~ m ~ l y found members of the l i e b i g i t e g r o ~ are bayleyite, Mg2UO2(CO3)yl~b~O , m ~ r t z i t e , ~ 2 ( C O 3 ) y 1 ~ { 2 0 , and andersonite, Na2CaUO/(CO3)y6H2O, none of which is iscmoepho~ with liobigite (Frondel, 19~8), and which were a l l i n i t i a l l y found at the Hillside m.h*e, Yavapei County, Arizona (~xelrod et a_[1., 1951). All of these minerals have since been noted at other localities (Frondel, 1958; Ri~,aite, 1977; Wolin, 1958; Matsobara, 1976; Wilkins. 1971; Jedlicka, 1959; Barczat, 1966; Davidson and Kerr, 1968; Wilkins, 1971). A rather more enigmatic but possible member of this g r o ~ is r a b b i t t i t e , Ca~493(UD2)z(CO3)6(OH)4.18HlO, but this mneral Is only reported s ~r~ locality (Th~pson, Weeks and ~nerwood, 1955) and rm~y one aMlysis has been made.

Full l i s t s of a l l cancentrations of m~xaxxts species at equilibrium at each temperature are awailable from the authors on request. For each aineral, at each tmaperature, the solubility product, g ~ , was calculated using the usual procedure 0taacke and Wilii~as, 1979). An Arrhenius plot of lag gSP against 1/T for eacb mineral (Figure 2) yielded a good straight l i n e frca ~ i c h the pertinent tNo~o~Tna~ic quantities f o r the c~mpo~ds could be obtained in the usual ~ y . ,~f and ~ for the ions coacernod and for ~ t e r were taken fz~m Banter and Sche~enmn (1978). For example for the dlssoclati~n of liebigite, CazUO2(CO3)2.1OH20(s)~ 2Ca2(eq)+ + UO 2. 2(eq) + ~ 0 2" 3(aq)

+ IOH20~b"~

AH~ for the reaction as written is 26.8(3.3)kJ mole-1. ValUes of the correspo.dlng quantity for s~mrtzite, bayleyite and ~ndersonite are 19.70.3), -I ~.6(0.8) and 109.6(9.2) kJ mole respectively. Derived values of AG~ and N~f for the four minerals are given in Table I I , together ~ t h their estimated errors. Using these values, we have ccnstructeq a stability field diagram, -29

l ~ gSP






Figure 2. A ~ i t l s plots of KS~ values for a, bayleyite; b, l l e b i g i t e ; c, s ~ r t z i t e ; d, and~rscnlte. for the four m n e r a l s , ~ i n h is shown in Fig. 1.

We have chosem to

represent the field in terms of acal* and ~492§ The full contour for the audersonite s t a b i l i t y field is chosen at aNa§ - 10-3~1 dm-3. Fainter

The chemical and thermudyn~c stabilities of the ~'arie~s members of the liebigite group are r~t tm~rstoud, we have therefore undertaken a study Of the stablllties of the group cc~prislng l i e b i g i t e , s~arZzite, heyleyite and andersoHite in order to place the formation and inlerconversion os the compounds from and in aqueo~ solution ~ a l i r a f ~ t i n g , we present the *%~Jults of our studies below. The compo~nde were synthesised by published metNocL~ (M~rowitz and Ltnd~rg, 1960; [email protected]~x)witz and Rgss , 1961; Weyrowitz, 1962; Meyrowltz, ROSs and Necks, 1963) and checked by the the1~ogravlmetric analysis (Stanton F.edcroft 1~7~0 balance) and published poeder X-ray data (Pr~del, 1958). For detellination of solubility, an excess os solid phase ~ s admitted to a flask containin~ a m ~ e t i c f o l l o ~ r (teflon covered) and f i l l e d with boiled= Out CO2=free ~mter. No gaseous i~ase ~es l e f t in the flasks (to avoid C02(g) equilibration) ~ich ~ r e then iamersed in a chemostatted ~ter bath m~d i~intained at constant temperature (-" O.Z~ Preliminary axperiments SNO*'~ that eq~librium ~ s achieved after about 48 hours (constant I~l and ~UDZZ* ]- readings). After o~e week, samples of solution were removed frma the flasks, filtered through Wnat~n G~/F flbreglass f i l t e r paper (,0.7 pm retention, pre~ashed), the !~ of the f i l t r a t e recorded and the f i l t r a t e retained for analysis. For analysis, an aliquot of the s~pernatant ~as acidified with HNO 3 and [ U 0 2 2 ~ O r ~ s u r e d spectrophotometrically at 412 rm using a Bac~man DK2A ratio recording spectroghotmaeter and uranium standards p r e ~ r e d in the smae c ~ t ~ t i ~ of HNOy Total concentration of carbonate and alkali earth and alkali metal cations ~ere then calcalated from the stoichiometry of the appropriate mir~ral phase. Species distrtbutiolts in solutin~ a t each temperature were then calculated using the well=tested progrm~e C~MICS (Perrin and Sa),ce, 1967) a~d reflmment carried ~ t as de~rihod p r e ~ ; i ~ l y (Alwan and Willinms, 1979). The complex species i n c h ~ d in the programme are tabulated in Table I. S t a b i l i t y constants TA~E i.

Species included for the calo~latic~s





~Oc~3?j )


C~O2)~(OH)~(~q ) M~4COU).C~q)



were taken from Baes and Me~aer (1976), L a ~ i r (1978), Truesdell and Jo~s (1974), Reardon and (1974), Babko and Kodenskaya (19~0), Serge/eva et a l . , (1972), Cirm~ide ~ a l . , (1975) and Scanion (1977).


~inor.1 lieblgite s~4rtzite

bayleyite audersccdte

T h ~ i C quantities at 298.2K for the Rtnerals of the l i e p i g i t e gr~x~.


CazLD2(OD3)3.1OH20 C~4gUD2(C03)3.12~20 Mg2002[CO3)3.18H20 Na2CaUO2(CO3)Y&q20


-6226 -6~7 -7924 -5651

.oi -i ~er,~ ..1-1

-" 12 -+ 8 -~ 8 -+ 24

-7037 -7538 -9192 -5916

=~ 24 -+ 20 -+ 20 f 36

contours are also given for aNa+ - I0-2 and I0"4 tool dm -3. It should first be noted that ~ r ~ o ~ i t e is o~ly Stable Whe~ the ar of sodi~ is very high and the ~ctivlties of Calcium and m4~.esium ions wery low. concentrations of sodium ion in g S t e r s are most easily achieved via evaporation and correspond with obsercations of these carbonates on the surfaces of mine workings, welln (1988) has suggested that almost c(mplete evaporation of the uranitm- bearing solutions is necessary for deposition. This of course is a conseqiam~e of the stability of the LD2(COs)~'(eq) lore, and its SOlubility, T~O relationships between the thre~ Na-free minerals are sho~m clearly in the same diagram. ~ring the synthesis of the minerals fzxl nitrate-containing SOlutions, Meyrowitz (1962) noted the apparently small field of stability of ~rtzite relative to liebiglte and hayleyite, an observation we ccnfim. Under his empcrinental condition5 he reported that if Ca:Ng is 7:8 or greater, liebigite formed from solutions containing Ca 2+, Mg z+, NO3 and U02(CO3)~-. If the ratio w~s 5:8 or less bayleyite formed. The studies reported here allow the interco~versinns to he riguro~sly defined. If ~ate2*/aca2+ ,0.32, liebigite is the stable phase. If a~2*/aca2- > 7.94, hoy1eyite forms preferentially. S~mrtzite is stable under the intermediate conditions. Finally it should be noted that increasing aNa* impinges far more oc the fields of liobigite and ~rtzlte relative to that of bayleyite. O.e might therefore expect the associstion of andersonite-bayieyite to be more c~mcn that andersonlte-linbigite. Koacver, this will of CCLLtse depend on the r e l a t i v e a v a i l a b i l i t i e s of calcium and m~nesitm ions to the SOlutions f1~m ~ i n h the minerals form. The authors wish to thank the Natural Rnvirmment Research Cotmcil for a grant under ~ c h this work ~as carried OUt,








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