Volume 58, pages 1065-1068, 1973

     Hetaerolite from the Rodna Base Metal Ore Deposit: A New Occurrence

     Institute of Mineralogy, University of Heidelberg, and Department of Geosciences, Technical University, Darmstadt, West Germany

     The mineral hetaerolite (ZnMn₂O₄) which is chemically similar to the mineral hausmannite, was first described from the Sterling Hill deposit, New Jersey, by Moore (1877) and later studied in detail by Palache (1910). Other localities where hetaerolite is known to occur include the Franklin Mine, New Jersey, and Leadville, Colorado (Frondel and Heinrich, 1942). Recently Hewett and Fleischer (1960) have reported hetaerolite from the Contact Mine, Grant County, New Mexico, and the Lucky Cuss Mine, Tombstone, Arizona. They as well as Ramdohr and Frenzel (1956) before them, expressed the opinion that hetaerolite - as a product of weathering of Mn-bearing zinc ores - is much more abundant than previously thought. Now hetaerolite has also been found in the Pb-Zn-deposit at Rodna, Rumania, probably the first locality in Europe. The Rodna deposit belongs to the neogene metallogenetic province of the Carpathian Mountains (Giusca, Cioflica, and Udubasa, 1969) and consists of metasomatic ore-bodies in metasomatic limestones and of injections in andesitic explosion-breccias. The ores consist predominantly of pyrite, sphalerite, pyrrhotite, and galena (Udubasa, 1970). The hetaerolite discussed here was found in cavities within the crystalline limestones where it occurs in a matrix of limonitic ochre together with broken-off fragments of galena crystals, which have been partially coated by cerussite.

     Macroscopically the hetaerolite is usually stalactitic but occasionally botryoidal in appearance (Fig. 1). Under reflected light it is gray, and similar to magnetite. Its reflectivity measured with a Leitz-microphotometer is 17-19 percent (see Nichol and Phillips, 1965) and decreases considerably under oil immersion. The mineral is weakly pleochroic, even under oil immersion; reflection pleochroism can be observed only along zone or grain-boundaries. Changes in color and reflectivity were nevertheless observed frequently; they were attributed, however, to differences in grain-size and porosity. As a rule homogeneous, coarse grained zones tend to be enclosed by fine-layered ones (Fig. 2), which points to rhythmic separation from a gel. Hetaerolite is clearly anisotropic, but without colorplay. Of interest are the "pyramidal" microstructures observed under crossed nicols (Fig. 3), which are interspersed with fine-grained layers. This feature leads to an increase in anisotropy. Internal reflections were not observed.

     Experimental Results

     The hetaerolite was analyzed for Mn, Zn, and Pb with the electron microprobe (Table 1). Spectrally pure metallic manganese, synthesized zincoxide, and stoichiometric galena were used as standards. The corrections were carried out using the X-ray absorption coefficients of Heinrich (1966). Infrared-spectrometric study showed only 1.0 percent OH. Disregarding this small quantity of OH, the chemical formula for hetaerolite from Rodna may be reduced to:

     (Zn_1.26Pb_0.02) Mn_1.81O₄

     Hetaerolite coexists locally with a mineral which, based on microprobe analysis (MnO₂, 59.69 percent; PbO, 32.34 percent; ZnO, 5.33 percent), appears related to either coronadite or cesarolite. Its small quantities prevented its study by means of  X-ray analysis. Its optical properties could not be determined with exactness. Its reflectivity is greater then of hetaerolite (R = 22-23 percent) and its hardness less. The mineral appears to be weakly anisotropic.

    
FIG. 1. Hetaerolite from Rodna. Scale in cm.

    
(a)

    
(b)

     FIG. 2. Fine-layered hetaerolite encrusted with a weakly recrystallized zone of the same mineral. Polished section, oil immersion, magnification 930 X. (a) parallel nicols (b) crossed nicols.

    
FIG. 3. "Pyramidal" textured hetaerolite overgrowing a fine-grained zone. Polished section, crossed nicols, oil immersion, magnification 550 X.

     The thermal behavior of hetaerolite was studied with a Mettler DTA-analyzer using a heating-rate of 10°C/min in flowing air. DTA and TGA measurements were obtained simultaneously with this instrument. Hetaerolite shows a double reversible exothermic reaction at 460 and 480°C (Fig. 4). Both peaks possibly point to the oxidation of the manganese in the hetaerolite. The small (irreversible) peak (reaction) at 882°C is similar to the phase-change of hausmannite from the α-form to the ß-form (Agiorgitis, 1969). From this it may be assumed that the structure of hetaerolite already shows a certain amount of order prior to attaining the double peak, i.e., at about 480°C. A subsequent X-ray study of the heating end-product showed only hetaerolite (Table 2). Whether a phase-change from α-hetaerolite to ß-hetaerolite takes place at 882°C, as with hausmannite, is uncertain. However, the thermogravimetric curve permits us to assume a phase-change based on the continuous decrease in weight. The X-ray data for the starting material (I) and the tempered material (II) clearly show (Table 2) a difference in the intensity of the lines as well as a shift in the d-values between the non-tempered and tempered material. The differences in the intensity of the lines are not only the product of a difference in the crystallinity of both substances but possibly also a difference in a₀

     TABLE 1. Electron Microprobe Analysis of Hetaerolite from Rodna

     The bands occurring in the frequency range around 900 cm^-1 could not be identified with certainty. The fact that other Mn-compounds - for example KMnO₄ - show absorption bands in the 930-900 cm^-1 range would tend to indicate that a small fraction of the octahedrally coordinated Mn³⁺ was oxidized to Mn⁵⁺ or Mn⁷⁺. This assumption is supported by the observed continuous reduction in weight during thermogravimetric analysis.

     TABLE 2. Hetaerolite X-ray Data, before Heating (I) and after Heating (II)*