Volume 12, pages 10-14, 1927

      The standard treatises on mineralogy all refer rhodonite, bustamite, fowlerite and babingtonite to the triclinic pyroxenes, but show no agreement at all regarding the other members of the group. Dana¹ includes hiortdahlite in the group, but it is excluded by others because it is not a metasilicate; Hintze² includes jadeite as a triclinic pyroxene, but other authors agree that it is monoclinic; Groth³ adds schizolite and margarosanite to the triclinic pyroxenes, but their crystallographic angles and constants differ decidedly from those of any pyroxenes. Washington and Merwin⁴ add sobralite, pyroxmangite and vogtite to this triclinic group of minerals, but suggest that the group should not be regarded as pyroxenes.

      Every mineralogist understands that mineral formulas are practically always simplified too much to represent accurately the real composition. Such simplification is highly desirable so long as no elements which are necessary to the mineral are excluded from the formula. For example, it is proper to consider that ZnS is the formula of sphalerite since the iron, usually present, is entirely unnecessary and merely proxies for part of the zinc. Similarly, it is correct to state that NaAlSiO₄ is the formula of nephelite since other constituents (including excess SiO₂) are quite unnecessary to the mineral.

      With these facts in mind what formulas should be assigned to rhodonite and babingtonite? According to all authorities the formula of rhodonite is MnSiO₃ according to Miers⁵ that of babingtonite is FeSiO₃. Analyses show that MnSiO₃ forms about 85 molecular percent of rhodonite and FeSiO₃ forms only about 15-20 percent of babingtonite. Since FeSiO₃ forms such a small portion of babingtonite it is not surprising that other writers do not agree with Miers in the view that the simplified formula of babingtonite is FeSiO₃; but the case is interesting as illustrating to what lengths this process of simplification of formulas is sometimes carried in our text books. It is the prevailing view that the formula of babingtonite must be more complicated than FeSiO₃, but there is no agreement concerning it. Rammelsberg⁶ regarded it as composed of (Ca,Fe,Mn)SiO₃ and Fe₂(SiO₃)₃. Doelter⁷ explained it as composed of Ca(Fe,Mn)Si₂O₆, CaFe₂Si₄O₁₂ and CaSiO₃. Silvia Hillebrand⁸ regarded it as a mixture of Ca₂Si₃O₈ and CaFe₂Si₂O₈. More recently Washington and Merwin⁹ have concluded that it is a mixture of Ca(Fe,Mn)Si₂O₆, Fe"Fe"'₂Si₄O₁₂, CaSiO₃, and H₂CaSi₂O₆. It seems possible that the correct explanation of the composition of babingtonite is still undiscovered.

      In regard to rhodonite, the simplification involved in considering that it is MnSiO₃ is no greater than that involved in deriving many mineral formulas. However, the fact that the simplification is not greater than in many other cases does not prove that it is permissible in this case. That depends upon whether the constituents omitted from the formula are necessary or unnecessary to the mineral. In other words, it depends upon whether the mineral rhodonite is essentially the same as, or essentially different from, pure crystallized MnSiO₃. Kallenberg¹⁰ has investigated this matter with the following results:

      1. Natural rhodonite is negative and biaxial of large optic angle.

      2. Natural rhodonite, when fused and recrystallized, is negative and biaxial of large optic angle.

      3. Artificial MnSiO₃ crystals are positive and biaxial of small optic angle.

      4. Crystallization of artificial MnSiO₃ at lower temperatures with fluxes does not change the optic sign.

      5. The addition to MnSiO₃ of small percentages of FeSiO₃ (similar to the tenor of FeSiO₃ in some natural rhodonites) does not change the optic sign to negative, but this result is obtained by adding 30-40 per cent of FeSiO₃.

      6. The addition to MnSiO₃ of MgSiO₃ does not change the sign.

      7. The addition to MnSiO₃ of 5 percent (or more) of CaSiO₃ changes the substance to negative and biaxial.

      Kallenberg supposed that MnSiO₃ and CaSiO₃ form an isomorphous or isodimorphous series, but his evidence on this point is not conclusive; in fact, his fusing point curve may well pertain to a pair of substances miscible as liquids, but showing little or no solubility as crystals; and one or more intermediate compounds are quite possible, especially if they are unstable at their melting points.

      The writer would therefore reinterpret the evidence supplied by Kallenberg to mean that:

      1. Natural rhodonite is essentially different from pure MnSiO₃. This difference is not due to inversion, nor is it due to admixed FeSiO₃ or MgSiO₃.

      2. Natural rhodonite is essentially the same as MnSiO₃ with some CaSiO₃. Therefore Ca can not correctly be omitted from the formula of rhodonite.

      A brief study of the analyses of rhodonite shows that calcium is always present and does not vary very radically in tenor. The best analyses may be represented fairly well by writing the formula as CaMn₅(SiO₃)₆. It seems clear from the analyses of rhodonite (excluding bustamite which is probably a separate species and not a variety) that more Ca than is expressed by this formula is not possible in this mineral; this conclusion is supported by the observation of Hallimon¹¹ that slags containing more than about 8% CaO crystallize to vogtite and not to rhodonite.

      If rhodonite is a mineral whose formula expresses a definite ratio between Ca and Mn then what is to be said regarding other ratios? Are any other ratios known among minerals?

      It seems possible that pyroxmangite¹² represents the essentially calcium free¹³ substance, though it contains much iron. Ford and Bradley very properly emphasized the fact that pyroxmangite is not the same as, nor a variety of, rhodonite in their original description of the mineral. It is positive and biaxial of rather small optic angle.

      Sobralite¹⁴ is complex in composition; it has been assigned the formula: CaMgFe₂Mn₄(SiO₃)₈. Since some analyses of rhodonite suggest that considerable MnMn₅(SiO₃)₆ is miscible in crystal solution in CaMn₅(SiO₃)₆  it might be supposed that sobralite represents such an isomorphous member of a rhodonite system, but, since sobralite differs optically and also in its X-ray pattern¹⁵ from rhodonite, such a conclusion is not warranted.

      Fowlerite¹⁶ has about the same tenor of Ca as rhodonite and may be regarded as Ca(Mn,Fe,Zn)₅(SiO₃)₆. The optic sign of fowlerite is different from that of rhodonite, but this may well be due to variation through 90° with increase of zinc, since the dispersion is also different.

      Bustamite¹⁷ has much more Ca than rhodonite and seems to be essentially CaMnSi₂O₆. The best analyses show little evidence of the existence of a gradation between rhodonite and bustamite. In crystallography, also, bustamite seems to differ distinctly from rhodonite.

      Vogtite¹⁸ is a substance found in slags which is similar to bustamite in optic orientation but seems to differ both from bustamite and rhodonite in composition since the formula is Ca(Fe,Mn,Mg)₂Si₃O₉. Hallimond¹⁸ in the original description showed that vogtite differs in essential characters from rhodonite, and it seems probable that it is also essentially different both in composition and properties (notably the cleavages) from bustamite.

      In summary, there is a group of triclinic minerals usually (but incorrectly?) referred to the pyroxene group whose formulas and mutual relationships are still uncertain. This group consists of metasilicates of manganese's and calcium in which the ratio between manganese and calcium seems to vary from 1 : 0 to 1 : 1. However, these minerals differ too much optically and crystallographically²⁰ to belong to an isomorphous series in the narrow sense of that term. The chief types thus far known are the following: