American Mineralogist, Volume 58, pages 347-342, 1973 Synthesisof Benstoniteat Room Temperature Wnrrena C. Hooo, aNo pErpn F. Srenr, Department ol Geology, Southerrc Illirwis (Jniaersity, Carbon:dale,Illinois 6290I Abstract Benstonite;MgCaoBae(COr)'", has been synthesizedat room temperatureby mixing a solu- tion containing 12.5ml of 0.2 molar MgClz, 12.5ml of 0.2 molar BaCl", and 5.0 ml of 0.1 molar CaCl: with 25 ml of 0.5 molar sodium carbonate solution and allowing the precipitate to age in contact with the remaining solution for two weeks. During the course of an investigationof mineral and a smear mount was made of the precipitate. relationshipsin the systemBa-Mg-Ca-CO3 in aque- The material was identified as benstoniteby X-ray ous solutionat room temperature,the mineral ben- diffraction. All of the more intensediffraction peaks, stonitewas synthesized.This mineralhas not previ- as well as many of the minor ones,reported by Lipp ously been synthesized, so we report here the mann (1962) and Sundius(1964) are presenton condition under which it was formed. diffraction traces made from the artificial material Benstonite was first described by Lippmann (Table 1). Intensitiesof the peaks are in general (1,961a,l96lb, 1962) from the barite mine near agreementwith thosereported by Lippman. No other Magret Cove,Arkansas. ft has sincebeen identified peaks are presenton the diffractometertrace, so the at Langban,Sweden (Sundius, 1964; Moore, I97O) benstonite seems to be present in relatively pure and at the Minerva Mine near Cave-in-Rock,Illinois forms. (White and Jarosewich,1970). The chemicalfor- The composition of the solution and the method mula is not yet known with certainty. Lippmann of precipitation of the mineral is very similar to (1962) suggestedeither (Ca,Mg,Mn)7(Ba,Sr)6that used in a study of Mg-Ba-CO3 in aqueous (COr)r* or possiblyMgCa6Ba6(COr)rr. The mate- solution (Hood and Steidl,in preparation),the only rial from Illinois contains2.1 wt percent magnesium, differencebeing the presenceof calcium and a longer which correspondsvery closely to one magnesium agingperiod. The magnesium,barium, and carbonate per formula, so White and Jarosewich (1970) concentrations are such that norsethite (BaMg stronglysuggest that magnesiumis an essentialcon- (COr)r) plus witherite would have formed in the stituent and does not substitutefor calcium. In view absenceof calcium. The presenceof calcium allowed of the composition of the solution from which we the formation of benstonite rather than norsethite synthesizedthe mineral, we are inclined to agreewith this interpretation. and perhapsexplains the apparentabsence of norse- Illinois fluorspar district. The Benstonitewas precipitatedfrom aqueoussolution thite in the Southern pres- by first mixing togetherI2.5 ml of 0.2 molar MgCl2, assemblagewitherite, benstonite,and calcite is (White 12.5ml of 0.2 molar BaCl2,and 5.0 ml of 0.1 molar ent in the Minerva Mine and Jarosewich, CaClz. To this cation-bearingsolution 25 ml of 0.5 1970) and the assemblagebenstonite, barite, and molar sodium carbonate solution was added while calcite is presentin the Magnet Cove locality (Lipp- stirring with a magnetic stirrer. A white precipitate mann, 1962). Calcite, benstonite,norsethite, and formed immediately. After approximately two min- barytocalcite(BaCa(COg)s) have all beenreported utes of stirring, the beaker containing the solution from Langban, Sweden(Moore, 1,970).Relations and suspendedprecipitate was covered and allowed amongthese various mineralsare not clear. Study of to sit for two weeksat room temperature.At the end the system Ba-Mg-Ca-COsat room temperatureis of this time, the aqueoussolution was decantedoff continuing in this laboratory. 341 342 MINERALOGICAL NOTES Tlsrn l. X-ray Powder Data for Natural and Artificial Benstonite MagnetCove, Ark. Lanaban,Srreden Arti fi cral d (A) d (i) d (A) 9.21 3 7.63 3 5.83 2 5.28 3 534 5 4.57 3 4.19 t6 421 50 4.21 l3 392 67 3.92 38'l 3.89 60 3.80 3.60 9 3 .60 6 3.45 4 335 10 3.37 40 3.37 3 309 95 307 100 3 l0 100 298 l 300 60 3.01 1 289 I 2.755 3 2.637 I ?.62 t0 2.557 2 2.55 3 2.536 28 2 -52 2.52 50 2 .495 I 2.46 1 2.445 9 245 2.38 2 2.327 2 2.28 I 2.233 5 2.217 l0 221 20 2.19 1? 2.19 4 2.15 1 215 2.123 23 2.1',| ?.12 45 2.il3 4 2 097 4 2.042 2 2.03 2.006 3 I 994 3 I 973 1.957 i0 t98 23 I 945 il I .948 40 1.96 20 1.905 17 1 902 50 1.92 25 1 .886 5 'I1.863 4'I I 869 40 .834 1.757 I 1.764 1.746 l 'I1 728 2'l .698 'I 1 692 1 691 ']1 .670 I 664 I 1.65'] 3 'I1.645 I .630 7 I .630 t8 1.616 ? I .620 t0 1.626 7 1.547 6 I .548 30 1.556 17 References from the barite deposit in Hot Spring County, Arkansas. Amer. Mineral. 47, 585-598. Moonr, P. B. ( 1970) Mineralogy and chemistry of LtrrlreN, F. (196la) Uber ein barium-calciumkarbonataus Langban-type deposits in Bergslagen, Sweden. Mineral. Hot Spring County, Arkansas (abstr.). Fort. Mineral. Record, l, 154-172. 39.81. SuNorus, N. (1964) Carbonatesin the manganeseores at (1961b) Benstonite,Ca"Bau(CO.)o, ein neues min- Langban. Ark. Mineral. GeoI. 4, 279285. eral. D ie Naturwissenschaften, 4E, 550-55 1. Wnrra, J. S., lNo E. Jlnosrwrcn (1970) Secondoccurrence (1962) Benstonite, CazBao(COr)o,a new mineral of benstonite.Mineral. Record, l, l4Ll4l..