American Mineralogist, Volume 59, pages 471474, 1974 Precipitationof Norsethiteat Room Temperature Wrurteu C. Hoon, Pnrnn F. SrEroL,nno De,vp G. Tscnopp Departmentof Geology,Southern Illinois Uniuersity, Carbondale.Illinois 6290I Absfract Norsethite, BaMg(COa)a can be formed from aqueous solution by mixing a solution containing 0.06 molar barium chloride and 0.1 molar magnesiumchloride with an equal amount of 0.5 molar sodium carbonate solution and allowing the precipitate to age in contact with the solution for three days or longer. Other barium and magnesium concentrations give rise to precipitates containing witherite alone, witherite plus norsethite, norsethite alone, norsethite plus hydrated magnesium carbonates,or hydrated magnesiumcarbonates alone. The wide range oforiginal barium to magne- sium ratios of solutions from which norsethite can be precipitated suggeststhat it should be a more common mineral than is presently recognized. fntroduction precipitates and solutions were removed from the and allowed to stand at room tem- The mineral norsethite, BaMg(CO')r, although stirrer, covered, perature(20'C) for a period of time rangingfrom three apparentlyrare in nature, has receiveda considerable months. At the end of the aging period, amount of attention sinceits discoveryby Mrose el a/ days to nine from the precipitatesand (1961) in the Green River Formation in Wyoming. the liquids were separated portion solids smearedonto a glassslide for It was first synthesizedby Chang (1964) at 500"C a of the by X-ray diffraction. and later by Lippmann (1967a, 1968b) at 2AoC. identification Norsethite is structurally similar to dolomite (Lipp- Results mann, 1967b,1968a) but, in contrast to dolomite, is easily formed at low temperatures.Hence, study of The resulis of experimentsthat were terminatedat the stability relations of norsethite may aid in the the end of threedays and thosethat lastednine months understandingof the origin of dolomite and other are essentiallyidentical. Both sets gave five mineral orderedrhombohedral carbonate minerals. This paper assemblages,(l) witherite (BaCO,), (2) witherite plus reports the synthesis of norsethite at 20"C from norsethite,(3) norsethite,(4) norsethiteplus hydrated aqueous solutions of widely varying barium and magnesiumcarbonates, and (5) hydrated magnesium magnesiumratios. carbonates (Fig. l). One of the most interesting discoveriesof this investigationis the very wide range Eryrerimental Procedure of Ba/Mg ratios in the starting solutionsfrom which Reagent grade BaCl"'zHrO, MgCL.6H,O, and norsethite can be precipitated.Although the field in NarCO, were used as starting materials. Stock which only norsethite forms is fairly limited, the solutionsof 0.1 molar BaClr, 0.1 molar MgClr, and assemblagesnorsethite plus witherite or norsethite 0.1 molar BaCl, plus 0.1 molar MgCl, were prepared plus hydratedmagnesium carbonates were precipitated and usedto make the various experimentalsolutions. from solutions with original Ba/Mg ratios ranging The desiredconcentrations of barium and magnesium from l0 down to 0.1. Norsethite by itself can easily were obtained by mixing various amounts of these be formed by mixing a solution containing0.06 molar three stock solutions plus enough distilled water to barium chloride and 0.1 molar magnesiumchloride bring the volume to 25 milliliters. Precipitation of with an equal amount of 0.5 molar sodium carbonate the carbonate minerals was achieved by rapidly and allowing the mixture to agea few days,It should adding 25 milliliters of 0.5 molar NarCOesolution to be emphasizedthat the mineral assemblagesshown on the cation-bearingliquid while the latter was being Figure I are those that precipitated from starting stirred with a magnetic stirrer. After approximately solutionsof variousconcentrations. The diagramdoes two minutes of stirring, the beakerscontaining the not show the stability fieldsof the minerals. 471 472 HOOD, STEIDL, TSCHOPP a mixture of the two single-cationsolids in the presence of excesscarbonate. Since the resultsof the solutions C' aged three days and those aged nine months are essentiallyidentical, time doesnot seemto be a major c^ o factor in determiningthe results.These observations b. .= E taken together indicate that the presenceof two 2 3 mineralscoexisting in a field in Figure I is a result of €o there being insufficient amounts aE of either barium or TE magnesium to convert all the solid to norsethite. Lippmann (1967a, 1968b) synthesized norsethite ag by rl the reaction of solid barium carbonate (witherite) with dilute solutions of magnesium chloride and sodium bicarbonateaccording to the reaction: dl BaCO3 Mg'* : 0.00 o.o2 o.o4 oJ6 l.os----- o:lo * * COr'- BaMg(CO3)r. ilg in startingsotution He obtainedcrystals of norsethiteas long as 0.1 mm after times as short as two days, and his work gives (molest titer) insight into the problem of formation of an ordered Flc. 1. Mineral assemblagesprecipitated as a function of two-cation mineral from a pre-existingsingle cation various original barium and magnesiumconcentrations. Fields carbonatemineral such as the formation of dolomite are (1) witherite, (2) witherite plus norsethite, (3) norsethite, (4) from calcite. However, his work doesnot resolvethe norsethite plus hydrated magnesium carbonates, and (5) question of whether an ordered double-cation car- hydrated magnesiumcarbonates. bonate mineral can be precipitated directly from solutions of proper composition. The hydrated magnesium carbonates found To examinewhether the experimentsdescribed in during this investigation consisted of mixtures of this paper precipitatednorsethite directly or followed nesquehonite(MgCOr.Mg(OH),.4 HrO), lansfordite somereaction similar to that describedby Lippmann, (MgCO..5 H,O), and hydromagnesite(4MgCO,. samples of precipitate in the norsethite field were Mg(OH)r'4 H,O). The relativeamounts of the three removedat time periodsof two minutes,four minutes, minerals appearedto changerandomly with time in twenty minutes, one hour, twenty-four hours, and precipitatesin which they occurred,and it is uncertain three days. The precipitatesextracted in one hour or which of them representsthe stable phaseunder the less were amorphous, whereas those extracted in conditions of the experiments.The solution composi- oneand threeday intervalsgave well-defined norsethite tions and temperatureare not far from the nesque- X-ray patterns. Well crystallizednorsethite does not honite-lansfordite-hydromagnesitetriple junction on form directly under the conditions of very rapid Langmuir's(1965) P"o.-T diagram,so it is not very precipitationused in theseexperiments. surprising that all three phases occurred in the Lippmann (1973)points out that the equation for precipitates.Langmuir's work suggeststhat lansfordite precipitation of norsethite can be written easily dehydratesto nesquehoniteat temperatures : higher than about lOoC and both lansfordite and Ba'* * Mg" * 2COs2- BaMg(CO3), (1) nesquehonite are metastable relative to hydro- for which the equilibrium constant would have the magnesiteat relativehumidities less than fifty percent. form: This implies dehydration after precipitation may accountfor part of the observedvariation in amounts K: * * - of the three phases. (aBa' )(aM g' 1laco 1' "' Discussion If carbonate activity remains constant the activities of barium and magnesium should be inversely Norsethite can be formed from either witherite or relatedto one another.On the other hand if norsethite hydrated magnesiumcarbonate by adding the other formed by the process:. cation in the presenceofexcess carbonate and allowing the precipitateto agea few days.It will alsoform from 2BaCOs* Mg'* : BaMe(COr),* Ba'* (2) PRECIPITATION OF NORSETHITE 473 the equilibrium constantwould be: Tlsr.r 1 Barium and Magnesium Remaining in Solution After Precipitation of Carbonate Phases y : 1aBa2')/(oMe'*) Original Solid ?hases Barium in Magnesium in which case the activities would change in direct Ba:Mg Identified** Solution in Solution Ba:Mg Ratio* (ppn) (ppm) Ratio proportion to one another. In experimentsin which the cation concentration 10:0 w 1.9 1 0.2 0.0 J 0.00 8t2 W'N 0.43 I 0.I0 1,0 r 0.05 0.43 in the starting solution is 0.1 molar and the anion 6t4 N,W 0.35 I 0.07 2.0 1 0.1 0.18 concentration0.2 molar, the barium and magnesium 4t6 N 0.25 I 0.06 62 ! 2 0.004 2:8*** N,H 0.05 I 0.06 26 ! | 0. 002 are almost entirely precipitated out and hence car- 0:10 B 0.08 1 0.06 85 t 2 0.00r bonate activity in the remaining solution can be *l4oLeculqr ratio consideredto be nearly constant. Some solutions of d*il4ithe"ite, Nwrs e thi. te, H=hAdtated rcgne s im earbonate s tlese cation and anion concentrationswere agednine ***s@nple e)aporated soneDhat dwin4 ecperirent. months, analyzed, and the ratios of barium to magnesium in the solutions calculated (Table l). Thus the possibilityexists that there may be an upper Analyseswere made by atomic absorptionspectro- limit of the cation to carbonateratio in which norse- photometry. The detection limit for barium was thite is stable. A similar observation was made in approximately0.05 ppm and little relianceshould be Lippmann's (1967a, 1968b, 1973) work, where no placed on the two measurementsnear this value. alteration of witherite to norsethite occurred with Even with these limitations, the concentrations of magnesiumchloride solutions unless sodium bicar- barium and magnesium(and presumablythe activities bonate was addedas well. becauseboth elementsare presentin low concentra- Judging from the wide