NAGYÁGITE
Nagyágite – [Pb3(Pb,Sb)3S6](Au,Te)3 – is a very rare sulfosalt, characteristic of Au-Te parageneses. It occurs in Au ores and constitutes Au and Te ore, but it is neither the only nor the main Au or Te ore mineral in the deposit. It is only very locally abundant.
It may contain Bi, Ag, and Fe.
Macroscopically, nagyágite crystals are quite similar to biotite. Under the microscope, they show some resemblance to molybdenite.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Black-gray.
Habit: Fine tabular crystals up to 1 cm, pseudo-tetragonal, often folded. Rarely granular. Massive.
Cleavage: {101} Excellent, {010} Perfect. Striations in (001).
Tenacity: Flexible, something malleable.
Twinning: Multiples in {001}, forming crosses.
Fracture: No information available.
Mohs Hardness: 1 – 1.5
Parting: No.
Streak: Lead gray black.
Lustre: Metallic
Diaphaneity: Opaque
Density (g/cm³): 7.35 – 7.49
2. Geology and Deposits
Nagyágite is typical of epithermal hydrothermal veins containing Au and Te. It is a rare telluride, abundant only in a few localities.
3. Mineral Associations
In the type locality (Nagyág, Romania), it is associated with native arsenic, arsenopyrite, other Te minerals (altaite, coloradoite, krennerite, petzite, sylvanite, stützite), native elements (gold, tellurium), sulfides (tetrahedrite, sphalerite), proustite, and rhodochrosite.
In addition to these, it may ocurr together with:
– other Te minerals (calaverite, tellurobismuthite, telluroantimony),
– other sulfides (pyrite, chalcopyrite, galena, alabandite),
– secondary Cu minerals (brochantite, linarite, covellite, malachite),
– some sulfosalts (bournonite, seligmannite),
– quartz and gypsum.
4. Transmitted Light Microscopy
This does not apply, as nagyágite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Nagyágite acquires a moderate polish despite its low hardness. Care must be taken not to leave the mineral with a very negative relief. The hardness upon polishing of nagyágite is low, lower than the hardness of sylvanite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Pale gray, light, may have a bluish tint.
Compared to the color of galena, the color of nagyágite is very similar, but has a pale lilac hue.
Compared to the color of petzite, the color of nagyágite is very similar.
Compared to the color of sylvanite, the color of nagyágite is much darker.
Compared to the color of hessite, the color of nagyágite is light brownish-white and lighter than hessite.
Compared to the color of krennerite, the color of nagyágite is gray with a creamy tint and darker.
Compared to the color of native silver, the color of nagyágite is much darker.
Pleochroism: Weak, distinct like molybdenite, between gray and brownish-gray.
Reflectivity: 37.28 – 41.23%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak but distinct anisotropy ranging from light gray (bluish) to dark brownish or greenish gray in sections that cross the cleavage.
In sections parallel to the cleavage, the anisotropy is very weak.
Indefinite extinction, almost parallel to the cleavage; often undulating.
Internal reflections: No.
May be confused with: some other minerals, but in general its recognition among the tellurides is relatively simple using its diagnostic optical features.
Molybdenite is similar, including because it exhibits good cleavage, lamellar crystals, often twisted, but the pleochroism and anisotropy of molybdenite are much stronger.
Sylvanite is lighter in color.
General Characteristics:
Grain shape: Nagyágite generally presents itself as fine tabular crystals, so much so that one of its old German names is “Blättertellur” (= tellurium in leafs). The crystals may be folded and twisted, as is common in molybdenite, for example.
Cleavage parallel to (010) is very visible.
Twinning: under careful observation it is often possible to observe the presence of polysynthetic twinning, in the form of lamellae arranged parallel to (001), which can be interpreted as mimetic twinning by (100). Sections parallel to (001) exhibit crossed twinning lamellae.
Altaite rims, white and luminous, are common in nagyágite crystals. In addition to altaite, sylvanite, petzite and hessite may occur at these rims.
Inclusions in nagyágite can be krennerite, calaverite, coloradoite, native gold, galena, bournonite, tetrahedrite, tennantite, and hessite.
Substitutions: nagyágite can be substituted by krennerite, altaite, sylvanite, petzite, hessite, bournonite, and tetrahedrite.
