In vein material from the abandoned copper mine at Dhurode, County Cork, Republic of Ireland, the sulphosalt meneghinite is partly replaced by later minerals, notably a symplectite of galena and bournonite. Mineral analyses and proportions indicate that the bulk Pb/Sb ratio in the symplectite is almost identical to that of the meneghinite. It is inferred that Pb and Sb were the relatively immobile elements whose short-range segregation controlled the scale of symplectite intergrowth, during a diffusive replacement reaction in which Cu and S were added from the vein-forming fluid. This is the second sulphosalt-bearing symplectite for which immobile elements have been identified. In both cases, the inferred replacement reaction causes a volume increase, approximately 15% in the present example.

Tetrahedrite fahlores from the Coeur d'Alene mining district (Idaho) have been found to be enriched in Ag by the Ag–Cu exchange reaction

which occurred during cooling following galena mineralization. This solid–state reaction resulted in quantitative removal of Ag (in a AgSbS2 component) from galena and development of bournonite coronas on fahlore grains. The reaction produced a distinct population of high-Ag fahlores found in galena-rich samples and accounts for all of the bournonite mineralization. The most argentian of these high-Ag fahlores examined in this study (molar Ag/(Ag+Cu) = 0.303±0.011 and 0.336±0.011) are found in samples which achieved saturation with respect to other Ag-sulfosalts, namely pyrargyrite and polybasite and diaphorite, respectively. Multiple lines of evidence for the Ag-Cu exchange reaction are presented in this paper which applies mass-balance constraints and a thermodynamic database for sulfides/sulfosalts to microprobe analyses and textural observations, and to bulk production data. This solid-state reaction may explain why previous district studies have been unable to demonstrate convincingly primary fahlore zoning. Based on the Ag/(Ag+Cu) of fahlores coexisting with other Agsulfosalts and Fe-Zn partitioning between fahlore and sphalerite, we estimate that fahlore compositions were frozen in by 175°C. Examining the composition data for other Ag tetrahedrite fahlores found in the Coeur d'Alene district and elsewhere, we conclude that the thermodynamic database provides an accurate description of phase equilibria, except possibly for very Fe-rich systems where further studies of the Fe-Zn partitioning between fahlore and Fe-rich (Zn,Fe)S phases and of the thermodynamics of Fe-rich (Zn,Fe)S phases are warranted.

Reversed brackets on As and Sb partitioning between tetrahedrite-tennantite fahlores {∼CU10(Fe,Zn)2(Sb,As)4S13} and bournonite-seligmannite solid solutions {CuPb(Sb,As)S3} (400°C evacuated silica tubes, NH4Cl flux) indicate that the maximum nonidealities associated with the As-Sb substitution are about 165 (bournonite) and 250 (fahlore) cal/gfw on a per atom exchange basis. The experimental constraints are consistent with the following calibration of the As-Sb exchange reaction between these phases:

where kcal/gfw, X2 = Zn/(Zn + Fe) (or one-half the number of Zn atoms/formula unit) and X3 = As/(As + Sb) refer to atomic ratios in fahlore, and ΔG°23 = ΔH°23 = 2.59 ± 0.14 kcal/gfw (Raabe and Sack, 1984; Sack and Loucks, 1975; O'Leary and Sack, 1987; Sack, 1992).

Anisotropic opaque minerals viewed in reflected light microscopy show two sets of colours: the colours seen in plane polarized light which change as the section is rotated on the microscope stage, and the colours seen between crossed polars which change as the analyser is uncrossed. These latter colours are known variously as polarization colours or anisotropic rotation tints, but are here referred to as anisotropy colours. They are commonly a diagnostic aid to correct mineral identification. All these colours occur as a consequence of the dispersion of the relative permittivity (dielectric) tensor—the variation in the values of the tensor with wavelength of incident light and in low symmetry crystals, the variation in the directions of the principal axes of the tensor with wavelength.

In this paper, it is shown that the colour seen in plane polarized light and the anisotropy colours can be predicted for any orientation of section, at any stage angle, and for any degree of uncrossing of the analyser by calculations based on the dielectric tensor values, and these predicted colours compare favourably with the observed values. Three minerals are studied in this paper as examples: stannite, covelline, and bournonite.

Natural samples containing tetrahedrite–tennantite, bournonite–seligmannite and geocronite–jordanite from the Coranda-Hondol ore deposit, Romania, were investigated by Raman spectroscopy to determine its capability to provide estimates of solid solutions in three common and widespread sulfosalt mineral series. Raman measurements were performed on extended solid solution series (Td1 to Td97, Bnn25 to Bnn93 and Gcn24 to Gcn67, apfu). The tetrahedrite–tennantite and bournonite–seligmannite solid solution series show strong correlations between spectroscopic parameters ( position, relative intensity and shape of the Raman bands) and the Sb/(Sb+As) content ratio, while Raman spectra of geocronite–jordanite shows no evolution of Raman bands. In order to simplify the method used to estimate the Sb/(Sb+As) content ratio in tetrahedrite–tennantite and bournonite–seligmannite series, several linear equations of the first-order polynomial fit were obtained. The results are in good agreement with electron microprobe data. Moreover, a computer program was developed as an analytical tool for a fast and accurate determination of Sb/(Sb+As) content ratio by at least one spectroscopic parameter. These results indicate that Raman spectroscopy can provide direct information on the composition and structure of the tetrahedrite–tennantite and bournonite– seligmannite series.