Volume 27, pages 614-628, 1942

      New data on rhodochrosite are presented, including unpublished analyses. Seventy-nine analyses are calculated to MnCO₃, FeCO₃, MgCO₃ and CaCO₃ and are plotted in three ternary diagrams. The frequency of appearance of Ca, Fe, and Mg in significant quantities as isomorphous constituents of the 79 rhodochrosite specimens is found to be in the ratio 21:17:12. Complete isomorphous miscibility with calcite and siderite is demonstrated. The series to magnesite is incomplete.

      The refractive indices and specific gravities of rhodochrosite are shown to vary directly with the composition, and to be calculable from the proportions of the four end members of the system. The composition of a specimen, however, cannot be deduced from the specific gravity and optical properties alone. The nO of pure MnCO₃ is shown to be 1.816, and the specific gravity is 3.70. The theoretical specific gravity calculated from the molecular weight and the volume of a unit cell is considerably larger.

      Rhodochrosite from Butte, Montana, is described and analyses of rhodochrosite and ankerite from Butte are given.

      INTRODUCTION

      In 1917 W. E. Ford¹showed that the refractive indices and specific gravities of members of the calcite group of minerals could be calculated directly from their percentage composition with reasonable accuracy. Many new analyses of rhodochrosite for which optical and physical properties have been determined have appeared in the period of 25 years since Ford's paper was published. The interest in rhodochrosite as a source of manganese has been heightened by the present emergency, and it was in the course of an investigation of manganese reserves at Butte, Montana, with Charles F. Park, Jr., that the writer decided that further quantitative study of some of the properties of rhodochrosite might be worthwhile.

      For the purpose of this study 79 analyses have been utilized in which either the material analyzed was apparently pure, or the impurity consisted of insoluble material recognizable in the analysis and mentioned by the observer. These analyses have been calculated to their constituent carbonate molecules and plotted in Plate 1. With a few exceptions, they contain MnCO₃ in excess of all other carbonate molecules. Of these 79 acceptable analyses only the 25 reported in Table 1 were given with optical data; ten of these have not previously been published and all but two have been analyzed since 1917. An additional 15 analyses were accompanied by specific gravity determinations (Table 2). Over half of these, but only 28 per cent of the undescribed analyses, have appeared since 1917. Hence the trend toward correlation of chemical and physical data in mineralogical studies is noticeable in the rhodochrosite series, just as it is in other series which have been receiving attention in recent years.

      Nevertheless, the relative scarcity of high grade analyses of pure, clean material, accompanied by carefully determined physical data, is distinctly evident in the rhodochrosite series. Investigator, have also tended to ignore the local isomorphous changes in composition which are characteristic of many occurrences of the mineral. Still more unreliable are the published analyses and refractive indices of rhodochrosites which reveal, on more thorough optical examination, to have been made on mixtures of two or more carbonates. The writer has omitted a few "rhodochrosites" from consideration because of this fact.

      COMPOSITION

      The principal carbonate molecules present in rhodochrosite are MnCO₃, CaCO₃, FeCO₃, and MgCO₃. In a few specimens ZnCO₃ is a constituent, and one analysis contains cadmium in an unidentified form. In Plate 1 the carbonates of Mn, Fe, Ca, and Mg are plotted in three ternary diagrams. A mineral appears in two diagrams if its third and fourth most abundant metals are about equal in quantity, and it appears in all three fields if Ca, Mg, and Fe are equally abundant. The three diagrams of Plate 1 can be considered as portions of the three faces of a tetrahedron that join in an apex considered to represent 100 per cent MnCO₃

      Of the 79 analyses plotted in Plate 1, 56 are either three-component carbonates or they contain a fourth metal in such small quantity that it may be neglected. In plotting, this fourth metal is added to the others according to the following scheme: if Ca, it is added to Mg; if Fe, it is added to Mn; and if Mg, it is distributed ¾ to Ca and ¼ to Mn. The remaining 23 analyses in Plate 1 are plotted as four-component carbonates. Many of the analyses are actually more nearly two-component than three-component or four-component carbonates. These lie near the edges of the tetrahedron whereas the three and four-component analyses lie, respectively, on the triangular faces or well within the trihedral angle forming the apex of the tetrahedron.

             a Goldschmidt, V. M., Faraday Soc. Trans., 25, 253 (1929).
             b Paining, Linus, Jour. Am. Chem. Soc., 49, 763 (1927).
             c Zachariasen, W. H., Zeits. Krist., 80,146 (1931).

      The MnCO₃-FeCO₃ series appears to be completely miscible. Ford² had noted a considerable break in the series between 50 and 70 per cent MnCO₃, but analyses H and J from Ouray and Leadville, Colorado, fall in this zone.

      In the MnCO₃-CaCO₃ series Ford³ deduced from a study of molecular volumes, rhombohedral angles, and analyses, that only comparatively small amounts of other molecules would be found replacing CaCO₃. Of these he considered MnCO₃ the most likely to replace CaCO₃ isomorphously because of the closest relation between molecular volumes and plane angles of the rhombic faces. By means of an x-ray study and new analyses, Krieger⁴ later showed complete isomorphism in the series from CaCO₃ to analysis Z which is 42 per cent MnCO₃. The series is completed through specimens T from Batesville, Y from Japan, X from Tennessee and others here assembled, indicating that Ca and Mn may substitute for each other in any proportion. Specimens T and X were studied optically by the writer and were seen to consist essentially of single carbonate minerals, variable within limits but not composite. However, it is well to point out that many of these analyses which complete Krieger's series are not of the very best material. Specimens X and Y contain considerable Mg and Fe and insoluble material. Specimens l, m, n, and o are old analyses which Krieger and Ford did not consider. Ford mentions specimen l but classes it as exceptional. In summary, the MnCO₃-CaCO₃ series is well demonstrated at the manganocalcite end by Krieger, and somewhat substantiated in the range from 42 to 85 per cent MnCO₃ by the present study.

      The series MnCO₃-MgCO₃ is incomplete. The magnesite highest in manganese quoted by Ford⁵ contains 13.33 per cent of MnCO₃ and 16.99 per cent FeCO₃. Twelve other analyses of manganoan magnesite contain less than 3.5 per cent MnCO₃. At the rhodochrosite end of the MnCO₃-MgCO₃ series, with the exception of specimen p, the highest MgCO₃ content is 9.4 per cent in specimen V. In only one rhodochrosite in nine is MgCO₃ the second most abundant constituent. Specimen p (Table 2) with 27.5 per cent MgCO₃ is from Hambach (Nassau), Germany, and was analyzed in connection with a study of the thermal dissociation of rhodochrosite. The material is not described, but the analysis should probably be accepted because the oxides calculate directly to carbonates and the only impurity seems to be 0.74 per cent SiO₂.

      REFRACTIVE INDICES

      The observations of the writer indicate that many natural rhodochrosites vary in their composition and optical properties more than seems hitherto to have been recognized. This is especially true of rhodochrosite from Butte, Montana, but is equally true of many specimens from elsewhere that have come to the writer's attention. Most banded rhodochrosites have small changes in composition from band to band. Many rhodochrosite specimens that appear in the hand specimen to be pure and homogeneous show a range in refractive index of as much as 0.010 in the space of an inch or so. Single crystals may be zoned. Commonly it is not enough to make only one observation of refractive index on a single grain of an analyzed sample. Some published descriptions give indices to 0.0001 but are actually questionable at 0.01. In Table 1 those observations which were found by the writer to have been made on variable material are reported as a range rather than a single observation or an average.

Fig. 1. Observed and calculated refractive indices.

      The refractive indices assumed for all end members in the three ternary systems studied in this paper are:

      L. Yosimura, Jour. Fac. Sci., Hokkaido Imp. Univ., series 4, 4, 313-451 (1939); from the Kaso mine, Japan, dense turbid rhodochrosite with SiO₂, A1₂O₃, H₂O, and insoluble totalling 10.95 per cent.

      M. Yosimura, op. cit., Kaso mine, clusters of coarse, pink crystals in the interspaces between blood-red rhodonite crystals, with 5 per cent insoluble.

      N. Emma mine, Butte, Montana, deep pink, coarse, zoned cleavage rhomb, anal. Fairchild, optical data, Wayland.

      O. Ross, C. P., U. S. Geol. Survey Bull. in preparation; from Whitlatch dump, Austin, Nevada, 15.08 per cent insoluble, 1.04 per cent Fe₂O₃, anal. Steiger, optical data, Glass.

      P. Ross, op. cit., from Belle Wilder, Austin, Nevada, 24.45 per cent insoluble, anal. Steiger, optical data, Glass.

      Q. Ross, op. cit., from Cummings lease, Austin, Nevada, 32.03 per cent insoluble, anal. Steiger, optical data, Glass.

      R. Ross, op. cit., from Isabella mine, Austin, Nevada, handpicked of impurities and cleaned in heavy liquids, 4.58 per cent insoluble, anal. Wells, optical data, Glass.

      S. Ross, op, cit., from Ruby Incline, Austin, Nevada, 26.12 per cent insoluble, anal. Steiger, optical data, Glass.

      T. Miser and Hewett (U. S. Geol. Survey, Bull. 921-A, in preparation), U. S. Geol. Survey, Bull. 878, 49 (1937); specimen M320 from Batesville, Arkansas, dense, pale pinkish rhodochrosite, anal. Wells, optical data, Wayland. Indices of most grains of the sample lie between 1.79 and 1.80. Other analyses reported are composite.

      U. Goddard, U. S. Geol. Survey unpublished analysis, 1940; from Algonquin mine, Philipsburg, Montana, anal. Fahey, optical data, Wayland.

      V. Goddard, op. cit., from Algonquin mine, anal. Fahey, optical data, Wayland.

      W. Goddard, op. cit., from Scratch Awl mine, Philipsburg, Montana, anal. Fahey, optical data, Wayland.

      X. Keith, in U. S. Geol. Survey, Bull. 878, 95 (1937); from Sevier, Tennessee, impure, zoned, turbid rhodochrosite, 15.54 per cent SiO₂, 2.42 Al₂O₃ anal. Steiger, optical data, Wayland.

      Y. Yosimura, op. cit., from the Kaso mine, Japan, dense grayish rhodochrosite replacing rhodonite; with sericite and quartz. Turbid, pleochroic, impure, with SiO₂, A1₂O₃, H₂O and insoluble totalling 19.26 per cent.

      Z. Krieger, Am. Mineral., 15, 23-29 (1930); from Sparta, North Carolina, anal. Shannon.

      The nE indices calculated from Ford's value of 1.597 for pure MnCO₃  differ an arithmetic average of 0.0064 from observed indices, if specimen F is disregarded. The average deviation is 0.0032. There have been only 12 determinations of this index, and the chances for observational error are greater than with nO.

      SPECIFIC GRAVITY

      Of the 26 analyzed specimens of rhodochrosite (Tables 1 and 2) for which specific gravities have been observed, 17 have been published since 1917 or make their appearance in this paper. Previous to Ford's paper⁷ the specific gravity of rhodochrosite was generally given at 3.45 to 3.60. As Fig. 2 shows, some natural rhodochrosites do lie in this range. The value of 3.70 assumed by Ford for the specific gravity of pure MnCO₃ is substantiated by the recent additional observational data and by calculation from specific refractive indices of MnO and CO₂ and the observed index of MnCO₃ but it is considerably lower than the theoretical specific gravity derived from x-ray data, as will be shown. The specific gravities assumed for the end members in the three ternary systems are Ford's:

      FeCO₃ 3.89 MnCO₃ 3.70 MgCO₃ 2.96 CaCO₃ 2.715

      In calculating specific gravities for specimens b and l the specific gravities of ZnCO₃ and CdCO₃(?) were taken as 4.45 and 5.6, respectively.

      Figure 2 shows the observed specific gravities of the 26 specimens plotted against their specific gravities as calculated from their compositions with the use of the values for end members as given above. In a number of specimens the observed specific gravity falls short of the calculated specific gravity, and in many others they are equal, but in none are they substantially greater. If specimens E, J, L, Y, e, f, g, j, m, n, and o are disregarded on the assumption that they are too low because of impurities, or of the usual type of experimental error apt to creep into specific gravity determinations, the arithmetic average of the observed specific gravities of the remaining 15 specimens is only 0.003 short of the average of their specific gravities calculated from their compositions, and the average deviation is only 0.015.

      a. Kunz, Am. Jour. Sci., 3d series., 34, 477-478 (1887); from the John Reed mine, Alicante, Lake County, Colorado, rich red, transparent rhombohedrons, pleochroic, nO salmon pink, nE light yellow, anal. Mackintosh.

      b. Zsviny (Földtani Közlöny, Budapest, 1927), Min. Abs., 3, 508 (1928); from Rákos, Slovakia, small scalenohedrons on limonite.

      c. Baric and Tucan, Annales Geol. de la Peninsule Balkanique, Belgrad, 8, 129-130 (1925); from Ljubija, Bosnia, light rose-colored secondary rhodochrosite on limonite.

      d. Barn and Tutan, op. cit., from Ljubija, Bosnia, brown rhodochrosite on limonite.

      e. Vesely (Rozpravy, Ceské Akad., 1922), Min. Abs., 2, 142 (1923); from Litosice, Bohemia.

      f. Vesely, op. cit., from Chvaletice, Bohemia.

      g. Sandberger, Neues Jahrb. Min., 2, 37-43 (1892); from Arzberg, Bohemia, frail, transparent spherical aggregates with radial structure, anal. Hilger.

      h. Brush and Dana, Am. Jour. Sci., 3d series, 18, 50 (1879); from Branchville, Conn., pink rhodochrosite in lithiophilite, and white or slightly pink granular masses with apatite and quartz, anal. Penfield.

      i. Yosimura, Jour. Fac. Sci., Hokkaido Imp. Univ., series 4, 4, 313-451 (1939); from the Kaso mine, Japan, pink rhodochrosite ore with tephroite, 2.7 per cent SiO₂.

      j. Yosimura, op. cit., from the Kaso mine, Japan, pink rhombohedra in drusy quartz veins, 2 per cent insoluble.

      k. Kersten, (Jour. pr. Chem., 37, 163, 1887), Dana System, V ed., 691(1884); from Voightsberg, Saxony.

      1. Browning, Am. Jour. Sci., 3d series, 40, 375-376 (1890); from Franklin Furnace, N. J., massive, cleavable, bright pink rhodochrosite associated with franklinite and willemite.

      m. Bukovsky, Zeits. Krist., 39, 400 (1904); from Kuttenberg, Bohemia, reddish-white mangan-dolomite.

      n. Roepper, Am. Jour. Sci., 2nd series, 50, 37 (1870); from Stirling Hill, Franklin Furnace, N. J., delicate pink cleavage masses, with willemite.

      o. Weibull, Tsch. Min. Petr. Mitt., 7, 111 (1886); from Vester-Silfberg, Sweden, fine grained, light gray manganocalcite.

      p. Manchot and Lorenz, Zeits. anorg. Chem., 134, 297-316 (1924).

      q. Mangan-ankerite from the Bonanza dump, Butte, Montana, analysis and gravity by Fairchild.

      FIG. 2. Observed and calculated specific gravities. 

      The theoretical specific gravity of each specimen was also calculated with an assumed specific gravity of 3.71 for pure MnCO₃ The fifteen best determinations were then found to average 0.013 below their calculated specific gravities. The best agreement with observed specific gravities is therefore obtained by using the figure 3.70 for pure MnCO₃

      The theoretical specific gravity of pure MnCO₃, may be calculated by two independent methods. Application of the law of Gladstone and Dale,

 G = n-1/K, where n is the mean refractive index of MnCO₃ {cubed root of (ω²ε)} and K equals the sum of the specific refractive indices of MnO and CO₂ times their respective weight percentages, gives a value of 3.707 for pure MnCO₃,

                                                 (Mol. wt.) (No. of mols./unit cell)
      The second method, G =  --------------------------------------
                                               (Vol./unit cell)(No. of mols./gm. mol. wt.)
 requires knowledge of the size of the unit cell of pure MnCO₃ Unfortunately it is unlikely that the dimensions given by the various investigators of crystal structure are for pure rhodochrosite. In only one case has the unit cell of an analyzed specimen been studied.

      Brentano and Adamson⁸ measured a rhodochrosite on which they had observed the specific gravity to range between 3.47 and 3.76, indicating that the material ranged in compostion from ferroan to calcian or magnesian rhodochrosite. The length of an edge, a_o', of a unit cell corresponding to a cleavage rhomb was found to be 6.064Å, and the angle α'  of a rhombic face was 102°50'. Such a unit cell contains four molecules of MnCO₃. From this data Brentano and Adamson calculated a specific gravity of 3.747. They may have measured a ferroan portion of their sample.

      The true unit cell of rhodochrosite and of other members of the calcite group has been found by Wyckoff⁹ not to correspond with the cleavage rhombohedron but to be a steep rhombohedron which contains only two molecules. Wyckoff obtained some of the analyzed material of specimen A from Lake County, Colorado, and determined the length of the edge of its unit rhombohedron, a_o, to be 5.61Å and the rhombic face angle a to be 47°46'. Later Wyckoff changed the a_o value to 5.84Å.¹⁰ Using the values 5.84Å and 47°46' for the Lake County rhodochrosite and calculating the molecular weight from the analysis, the writer obtained a density of 3.785. This is higher by 0.075 than the observed specific gravity, 3.710.

      Bragg¹¹ and Wyckoff¹² in their latest compilations on crystal structure give the following dimensions for MnCO₃:

         ao                      α                    ao'                       α'         
        5.84Å              47º20'              6.01Å              102º50'

       Using the values 5.84Å and 47°20' the writer obtained a theoretical specific gravity of 3.851, and using the values 6.01Å and 102°50' the specific gravity is 3.837. These are much higher than observed values for rhodochrosite.

      If we now assume that the specific gravity of pure rhodochrosite is 3.70 and its nO is 1.816, and that the physical properties of natural rhodochrosites can be calculated from the chemical compositions, we can place on Plate 1 lines of equal specific gravity and index. On doing this we see that the reverse procedure is not possible, that is, knowing the specific gravity and the refractive indices we still cannot tell the composition, because the lines on the diagram are parallel or intersect at small angles. Simple qualitative chemical tests would serve to indicate the field in which some specimens would lie, but in most cases a quantitative analysis is necessary.

      In distinguishing rhodochrosites from minerals of the siderite-magnesite series the refractive index and specific gravity data may be useful. For a given nO the specific gravity of a magnesian siderite will be about 0.1 lower than for rhodochrosite.

      Having studied the relationships of composition, specific gravity, and refractive indices of a number of rhodochrosites from a wide variety of localities, it remains to be seen in what ways rhodochrosite from Butte is similar or different from other rhodochrosites. As suggested above, it was the erratic relationship between color banding and the optical data of Butte rhodochrosite that induced this study, and it is especially because of this that the following descriptions of occurrence, mineral association, texture and isomorphism of Butte rhodochrosite are considered of interest to this paper.

      RHODOCHROSITE AT BUTTE, MONTANA

      The Butte district has produced over 400,000 tons of high grade rhodochrosite ore since 1917. During the present emergency lower grade ores are being beneficiated by flotation. The rhodochrosite is calcined in large rotary kilns to produce an oxide which contains about 58 per cent manganese. Most of the ore has come from the Emma mine on the Black Chief lode near downtown Butte, but rhodochrosite is common in most of the veins of the southwestern and western portions of the district.¹³

      Two bulk analyses of Emma ore are available. Hewett and Pardee¹⁴ give the average composition of about 250,000 tons of ore from the Emma mine and indicate the molecular composition of the rhodochrosite free from impurities to be MnCO₃ 91 per cent, MgCO₃ 7 per cent and CaCO₃ 2 per cent.

      The average percentage composition of 13,961 tons of rhodochrosite ore delivered to the Southern Manganese Corporation in 1918 and 1919 and the variation in monthly average analyses are given below:

      If the FeO is assigned to pyrite rather than to FeCO₃ following Hewett and Pardee,¹⁵ the molecular composition of the rhodochrosite free from impurities is approximately as follows:

      In addition to the abundant rhodonite of the northern part of the district, minor quantities of mangan-ankerite, manganosiderite,¹⁶ huebnerite,¹⁷ helvite¹⁸ and alabandite¹⁹ are found. A specimen of pale pink, massive, crystalline mangan-ankerite from the dump of the Bonanza mine was analyzed by J. G. Fairchild: CaO 30.52, MgO 15.92, MnO 4.28, FeO 3.31, ZnO none, SO₃ none, insoluble none, and CO₂ 46.08 (calculated); total 100.11. The observed nO ranges between 1.690 and 1.698 and the calculated nO is 1.694 .The specific gravity and molecular proportions are given in Table 2 as specimen q.

      Greenish wolframite similar to that described as an alteration product of huebnerite by Winchell²⁰ was noted in a specimen of banded rhodochrosite from the Emma mine. The mineral occurs as extremely tiny, greenish prisms intimately intergrown with quartz. Spectrographic tests by K. J. Murata and chemical tests by Michael Fleischer showed the presence of Fe, Mn, and W. The identity of the wolframite was proved by x-ray powder pictures taken by W. E. Richmond.

      ACKNOWLEDGMENTS

      The writer is pleased to acknowledge the constructive criticism and helpful suggestions of Drs. Waldemar T. Schaller, D. F. Hewett, Charles F. Park, Jr., J. B. Mertie, Michael Fleisher, and Duncan McConnell. The Anaconda Copper Mining Company permitted the examination of the Butte carbonate ores. Persons responsible for the chemical and some of the optical data have been acknowledged throughout the text. The writer is indebted to Messrs. E. N. Goddard, W. S. Burbank, C. P. Ross, H. D. Miser, and D. F. Hewett of the U. S. Geological Survey for permission to include some of their analyses previously unpublished.

NOTES