JOSÉITE
Joséite – Bi4TeS2 – is a rare sulfide, typical of Bi-Te-Au parageneses. It occurs in Bi and Te ores, but it is never the primary ore mineral in a deposit.
It is possible that joséite is merely a Te-rich variety of ikunolite (see P. Bayliss: Am. Mineral. 76 (1991), 257-265). The literature mentions joséite-A, joséite-B, and joséite-C, none of which are officially approved.
Macroscopically, joséite resembles a dark mica, such as biotite or phlogopite.
1. Characteristics
Crystal system: Trigonal scalenohedral.
Color: Silver-white. Tarnishes to lead-gray, with iridescence ranging from steel-blue to iron-black.
Habit: Aggregates of tabular, platy, and foliated crystals, resembling micas.
Crystals up to 5 cm.
Cleavage: {0001} perfect.
Tenacity: Flexible.
Twinning: No.
Fracture: Micaceous.
Mohs Hardness: 2
Parting: No.
Streak: Gray.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 8.1 – 8.4
2. Geology and Deposits
Joséite is characteristic of high-temperature, bismuth-rich, gold-bearing hydrothermal deposits.
It also occurs in granular limestones (dolomites).
3. Mineral Associations
It occurs with common gangue minerals such as quartz and carbonates (dolomite).
It occurs alongside other Bi-bearing minerals, including bismite, eulytine, bismuthinite, galenobismutite, cosalite, laitakarite, ikunolite, hedleyite, and native bismuth.
Joséite is also found with secondary Bi minerals, such as pingguite.
It occurs with common sulfides, including molybdenite, arsenopyrite, pyrrhotite, galena, and chalcopyrite.
It is also associated with garnet (andradite), phlogopite, tetradymite, hessite, native arsenic, and native gold.
4. Transmitted Light Microscopy
Not applicable, as joseite is entirely opaque.
The literature states that joseite is uniaxial.
5. Reflected Light Microscopy
Sample preparation: The behavior of joséite during polishing is very similar to that of tetradymite, described below. Its polishing hardness is slightly lower than that of tetradymite or bismuthinite.
“Tetradymite is very soft, with a polishing hardness lower than that of galena or bismuthinite. Nevertheless, it takes a good polish, though often with some polishing scratches – especially when it occurs in association with harder gangue minerals. Basal sections are very difficult to polish.”
PLANE POLARIZED LIGHT – PPL
Reflection color: White to creamy-white.
Compared to the color of galena, the color of joseite is lighter and whiter.
Compared to the color of bismuthinite, the color of joseite is lighter and more cream-colored.
Pleochroism: No.
Reflectivity: 56.30 – 61.24%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Moderate anisotropy, ranging from greenish-gray to dark greenish-gray.
Other literature sources mention brownish-gray, or yellowish-gray to pinkish-brown and dark brownish-gray.
Basal sections are nearly isotropic.
Internal reflections: No.
May be confused with: Tetradymite, whose behavior under the microscope is very similar. It is not possible to distinguish joséite from tetradymite with certainty under the microscope.
In general, it is difficult to distinguish joséite from other Bi-tellurides.
General Characteristics:
Grain shape: Joséite occurs as thin, tabular crystals that are almost micaceous in appearance. It may form aggregates of small, intergrown crystals associated with native bismuth.
Polishing scratches are common due to its low hardness.
Perfect basal cleavage may be clearly visible.
A spindle-like (fusiform) texture, resulting from crumpling and folding, may be present.
Inclusions 1: Joséite may contain inclusions of ikunolite and bismuthinite.
Inclusions 2: Joséite occurs as an inclusion within native bismuth. It is also found as an inclusion along the cleavage planes of molybdenite, appearing as “augen” (elliptical) shapes alongside native bismuth, ikunolite, bismuthinite, and native gold.
Lamellae of joséite may occur along the cleavage planes of ikunolite.
