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Volume 12, pages 10-14, 1927
NOTES ON THE TRICLINIC PYROXENES
A. N. WINCHELL, University of Wisconsin.
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:
Babingtonite probably belongs to this group, but its formula is still under discussion.
1 System of Mineralogy, 1892 p. 344
2 Handbuch der Mineralogie, II, 1897, p. 960.
3 Mineralogische Tabellen, 1921 p. 88 and 108.
4 Am. Mineral., VIII, 1923, p. 215.
5 Mineralogy, London, 1902, p. 424.
6 Handbuch der Mineralchemie, II, 1875, p. 404.
7 Tsch. Min. Pet. Mitt., II, 1880, p. 198.
8 Tsch. Min. Pet. Mitt., XXXII, 1914, p. 264.
9 Am. Mineral., VIII, 1923, p. 215.
10 Centr. Mineral., 1914, p. 388.
11 Mineral. Mag., XVIII, 1919, p. 368.
12 W. E. Ford and W. M. Bradley: Am. Jour. Sci., CLXXXVI, 1913, p. 169.
13 This assumes that the CaO actually found (1.88%) is negligible because it is not essential.
14 J. Palmgren: Bull. Geol. Inst. Univ. Upsala, XIV, 1917, p. 109 and J. M. Sobral: Bull. Geol. Inst. Univ. Upsala, XVIII, 1922, p. 47.
15 Wyckoff, Merwin and Washington: Am. Jour. Sci. CCX, 1925, p 383.
16 E. S. Larsen and E. V. Shannon: Am. Mineral. VII, 1922, p. 95 and 149.
17 E. S. Larsen and E. V. Shannon: Am. Mineral. VII, 1922, p. 95.
18 A. F. Hallimond: Mineral. Mag. XVIII, 1919, p. 368.
19 With or without iron and magnesium.
20 As proved especially by their X-ray patterns: see Am. Jour. Sci., CCX, 1925, p. 383.