THE MINERAL STEPHANITE

  • Chemistry: Ag5SbS4, Silver Antimony Sulfide
  • Class: Sulfides
  • Subclass: Sulfosalts
  • Uses: An ore of silver and as mineral specimens.
  • Specimens
Stephanite is an uncommon silver bearing mineral that is named after a onetime Archduke of Austria, Victor Stephan. Although not a well known ore of silver, it is never-the-less a locally important ore in some mines. Stephanite typically forms nicely shaped crystals that are tabular to short prismatic. Its crystals can have a pseudohexagonal outline making them look like crystals of polybasite, a closely related mineral. Stephanite lacks polybasite's rhombic striations on its pinacoidal faces, good cleavage and the red flashes.

PHYSICAL CHARACTERISTICS:

  • Color is an iron black to dark gray.
  • Luster is metallic.
  • Transparency: Crystals are opaque.
  • Crystal System: Orthorhombic; 2 m m
  • Crystal Habits include tabular or short prismatic crystals that are dominated by prominent pinacoidal faces. Crystals are sometimes intergrown and form pseudohexagonal plates. Also as disseminated grains and massive.
  • Cleavage is poor in two directions.
  • Fracture is conchoidal to uneven.
  • Hardness is 2 - 2.5
  • Specific Gravity is approximately 6.2 - 6.3 (heavy even for metallic minerals)
  • Streak is a black.
  • Other Characteristics: Dark coating can often form after prolonged exposure to light (can be removed by ultrasonic treatment) and crystals tend to be striated perpendicular to the prominent pinacoids.
  • Associated Minerals include silver, proustite, pyrargyrite, polybasite, sphalerite, quartz, barite, calcite, pyrite, fluorite, galena, tetrahedrite , acanthite and other silver sulfide minerals.
  • Notable Occurrences include Sonora, Mexico; Freiberg, Saxony, Germany; Colorado, California and Comstock, Nevada, USA; Atacama, Chile; Cobalt, Ontario, Canada; Cornwall, England and Bolivia.
  • Best Field Indicators are good crystal habit, density, softness, association with other silver sulfosalts and lack of cleavage.
STEPHANITE specimens:
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STEPHANITE specimen stp-1
$ 26.00
Dims: 0.7" x 0.7" x 0.2" (1.8 x 1.8 x 0.5 cm)
Wt: 4.0 g
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
What appear to be 3 different crystals make up this Stephanite cluster. They are flat and almost tabular in their habit, and have a definable hexagonal shape. However, each of these crystals are actually composed of several smaller, intergrown, orthorombic crystals which appear almost tabular. They are intensely intergrown, so it is difficult to determine their individual shapes or sizes, but there are very many different faces and angles visible. All of the crystals have a dark gray color and a metallic luster. There seems to be some damage visible, but due to the crystals' intergrowth, it is difficult to tell whether it occurred before or after the specimen's harvest. The specimen is affixed inside a domed plastic thimble box.
no photo
stp-1 ($ 26.00)
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
STEPHANITE specimen stp-2
$ 31.00
Dims: 0.6" x 0.6" x 0.3" (1.5 x 1.5 x 0.8 cm)
Wt: 5.6 g
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
At least 2 Stephanite crystals are intergrown on this specimen, though one of them appears to make up the bulk of the piece. Both are in relatively good condition, though there is some noticeable breakage areas on one of the larger flat surfaces. Their edges are rather well-defined and their faces are rather disjointed, but the largest one is very smooth, making me think that it belongs to a single large crystal. The crystals have a dark gray color and a metallic luster that is a bit dull. The specimen is affixed to the inside of a domed thimble box.
no photo
stp-2 ($ 31.00)
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
STEPHANITE specimen stp-3
$ 41.00
Dims: 0.9" x 0.7" x 0.5" (2.3 x 1.8 x 1.3 cm)
Wt: 9.7 g
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
Though this specimen seems to be made up primarily of 2 large crystals, each one of these "crystals" is in turn made up of many smaller orthorombic crystals. These smaller crystals doubtless have an orthorombic form, but they are so intensely intergrown that their size and shape are indeterminate. There is an area on the cluster that seems to be made up of a few very thin intergrown tabular crystals that may be made of polybasite, but I cannot be certain. There is also a considerable amount of damage to the specimen, but I cannot tell if it is human-induced. All the crystals have a dark gray color and a slightly dull metallic luster.
no photo
stp-3 ($ 41.00)
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
STEPHANITE specimen stp-4
$ 74.00
Dims: 1.2" x 0.8" x 0.7" (3.0 x 2.0 x 1.8 cm)
Wt: 10.1 g
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
One of the most impressive examples of Stephanite that I have seen, this piece is made up of several thin orthorombic prismatic crystals that are clustered against a flat section of what appears to be a crust of polybasite that is coated with a very thin layer of pyrite or chalcopyrite. They all have excellent form with well-defined edges and clean faces. The entire specimen is in very good condition, showing only little damage. The largest of these Stephanite crystals measures about 0.6 x 0.2 x 0.1" (1.5 x 0.5 x 0.3 cm). All of them have a dark gray color and a metallic luster. Accompanying the Stephanites are at least 4 pyrargyrite crystals that have excellent trigonal prismatic form and a bright subadamantine luster, and show a dull red glow in bright light. The largest measures about 0.3 x 0.2 x 0.2" (0.8 x 0.2 x 0.5 cm). It barely fits inside a domed thimble display box.
no photo
stp-4 ($ 74.00)
level 530, Santo Nino vein, Fresnillo, Zacatecas, Mexico
STEPHANITE specimen stp-5
$ 35.00
Dims: 8.0 x 7.4 x 6.8 mm
Wt: 3.75 ct. (0.75g)
530 Level, San Guillermo vein, San Luis shaft, Fresnillo, Zacatecas, Mexico
This specimen consists of three intergrown stephanite crystals, all showing a pseudohexagonal shape and clear striations. The crystal shapes are very good. There are also small amounts of acanthite on this specimen. This crystal was found loose in a cavity of a larger acanthite specimen. A second (much smaller) crystal was also present, and is included in the shipment of this crystal.
no photo
stp-5 ($ 35.00)
530 Level, San Guillermo vein, San Luis shaft, Fresnillo, Zacatecas, Mexico
STEPHANITE specimen stp-6
$ 90.00
Dims: 1.46x0.91x0.55" (3.7x2.3x1.4cm)
Wt: 0.32oz. (9.1g)
Fresnillo, Zacatecas, Mexico
While this specimen is mostly acanthite by weight, the best crystals are stephanite, and there are numerous crystals of polybasite-pearceite. The stephanite crystals are relatively large, are well (although complexly) shaped, and are black in color. These crystals also present well, as they are close to the extremities of the specimen. The polybasite/pearceite crystals are relatively small, and have strong striations. The acanthite is lighter in color, provides the structure of this specimen, and the crystals tend to be distorted, although there are many fine crystal faces.
no photo
stp-6 ($ 90.00)
Fresnillo, Zacatecas, Mexico
STEPHANITE specimen stp-7
$ 39.00
Dims: 0.91x0.47x0.31" (2.3x1.2x0.8cm)
Wt: 26.6ct (5.32g)
Fresnillo, Zacatecas, Mexico
At first glance, this specimen is just two large acanthite crystals, linked by more acanthite (all pseudomorphs after argentite). A closer examination reveals a nearly perfect pseudo-hexagonal crystal of silvery-black stephanite. Another of these is nestled under the specimen, as several nice (but very tiny) chalcopyrite crystals with a nice irridescence (visible only with a loupe). There is also a plate of translucent red polybasite intergrown with the largest stephanite.
no photo
stp-7 ($ 39.00)
Fresnillo, Zacatecas, Mexico
 

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