SYLVANITE
Sylvanite – (Au,Ag)2Te4 – is a rare telluride, but one of the few Au ores besides native gold and calaverite. Thus, it is a secondary ore of Au and Te. As it contains some Ag, it is also a minor Ag ore. In addition, it may contain Pb, Cu, Sb, and Ni. Due to its characteristic dendritic/graphic habit, it is sometimes called “graphic gold” or “graphic ore”.
In polished section, the recognition of sylvanite requires experience and the additional use of other methods. One of the characteristics is the contact twinning, lamellar or penetration along {100}, assuming cuneiform (resembling ancient writing) or dendritic forms.
Like all Ag minerals, it is photosensitive and develops a dark-colored tarnish when exposed to light. It can also develop an iridescent tarnish.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Silver gray to silvery white tending towards yellow. Tarnishes to dark or colored colors.
Habit: Massive, skeletal, granular, columnar. Rare crystals up to 1 cm, short prismatic and thick tabular.
Cleavage: {010} perfect.
Tenacity: Brittle.
Twinning: {100} common.
Fracture: Irregular.
Mohs Hardness: 1.5 – 2
Parting: No.
Streak: From steel gray to silvery white.
Lustre: Metallic intense.
Diaphaneity: Opaque.
Density (g/cm³): 8.16
2. Geology and Deposits
Sylvanite occurs mainly in low-temperature, subvolcanic hydrothermal veins containing the Ag-Au-Te association. In these veins, it is the main gold-bearing mineral.
It is also typically found as a late mineral in medium- to high-temperature deposits. It is present in small quantities in many gold and silver deposits; but it rarely forms rich deposits or is of economic importance.
3. Mineral Associations
It occurs associated with common gangue minerals such as quartz (including chalcedony variety), carbonates (calcite, rhodochrosite) and fluorite.
It occurs with common sulfides such as pyrite, chalcopyrite, arsenopyrite, bornite, sphalerite and galena.
With rare sulfides such as seligmannite, tetrahedrite-tennantite, famatinite, bismuthinite, boulangerite and orpiment. Occasionally it is associated with laumontite and apatite.
In the specific paragenesis it occurs with native gold and Te minerals (native tellurium, melonite, stützite, tellurobismuthite, weissite, rickardite, altaite, coloradoite). Also with Au+Te minerals (petzite, nagyagite, calaverite), Au+Ag+Te minerals (krennerite) and Ag minerals (acanthite, pyrargyrite).
4. Transmitted Light Microscopy
This does not apply, as sylvanite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Polishing sylvanite is problematic due to its low hardness and the much harder minerals that are usually associated with it. The polishing hardness of sylvanite is much greater than that of acanthite and hessite, greater than that of altaite and nagyagite, and less than that of pyrargyrite, galena, native gold, and sphalerite. Sylvanite itself polishes well, but polishing scratches will always be present. Very small and heavily worn grains, located among other minerals, often appear quite dark.
PLANE POLARIZED LIGHT – PPL
Reflection color: White to light cream.
Compared with the color of galena, the color of sylvanite is creamy-white, lighter.
Compared to the color of nagyágite, the color of sylvanite is creamy-white, lighter.
Compared with the color of calaverite, the color of sylvanite is darker.
Compared to the color of altaite, the color of sylvanite is darker and somewhat pinkish.
Compared to the color of native tellurium, the color of sylvanite is slightly creamier.
Compared with the color of hessite, the color of sylvanite is light cream.
Pleochroism: Very distinct, ranging from light creamy-white to pale creamy-leather brown. Some literature mentions creamy-white to creamy-brown.
Pleochroism is very distinct in intergranular contacts and twinning planes.
Reflectivity: 51.14 to 61.65%
Bireflectance: Weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong anisotropy ranging from light bluish-gray to dark brown.
When rotating the stage, there is never a complete extinction position. At the maximum darkening position, it remains illuminated in dirty grayish-brown colors. Rotating the stage slightly causes it to lighten rapidly.
Some literature mentions grayish-brown, grayish-white, pinkish-white, and yellowish and bluish tones.
Internal reflections: No.
May be confused with: Characteristic features include high reflectivity, reflection pleochroism, intense anisotropy, and lamellar twinning.
Nagyágite is darker, finer, tabular, and has clear cleavage.
Krennerite does not exhibit twinning.
Calavarite does not exhibit twinning.
General Characteristics:
Grain shape: Sylvanite grains tend to be idiomorphic (euhedral) when grown in cavities. Sylvanite often appears in skeletal (dendritic) or wedge-shaped (graphic) crystals. It also occurs disseminated, in this case the crystals tend to occur idiomorphically.
Cleavage may be visible, but only rarely.
Twinning: lamellar twins (polysynthetic) are very abundant, dense, in large quantities, much more than chalcopyrite twins, for example. They are a characteristic of the mineral, they are always present. The lamellar twins at XPL have a spectacular development. The lamellae have very variable widths, sometimes they are so thin that they only become visible when immersed in oil. Twin lamellae are oblique to the extinction position and can be seen in isolated crystals that have grown on any substrate. Large grains may have their cores without any twinning lamellae, but their edges have a large amount of twinning; which could be misinterpreted as two different minerals.
Intergrowths can occur with hessite and nagyágite.
Inclusions 1: Skeletal gold inclusions are possible in the center of sylvanite crystals.
Inclusions 2: Sylvanite can occur as inclusions in acanthite, sphalerite, bornite, pyrite, chalcopyrite, and galena.
Alteration in the oxidation zone is common and occurs with the release of very finely powdered gold (“mustard gold”).
Substitutions: Sylvanite substitutes galena and hessite.
