The Mineral ACANTHITE/ARGENTITE

  • Chemistry: Ag2S, Silver Sulfide
  • Class: Sulfides
  • Uses: An ore of silver and as mineral specimens.
  • Specimens

Acanthite is the most important silver ore next to galena. Actually there is an argument to be made that acanthite could be the most important silver ore. Galena is a lead ore that often contains enough silver (usually about 1%) that when it is processed for its lead content the by-product silver supplies the majority of the world's needs. But most, if not all the silver in argentiferous galena is thought to actually be microscopic inclusions of acanthite crystals.

Acanthite is often confused with the name argentite and it is no wonder. Several mineral guides interchange the names or combine the names as is done here. But the proper mineral name when referring to Ag2S at room temperatures is acanthite. Argentite is a name applied to one polymorph (meaning many shapes) of Ag2S. Acanthite and argentite have the same chemistry , Ag2S, but different structures. Argentite has an isometric structure and is only stable at temperatures above 173 degrees Celsius and if cooling from a melt, will form isometric crystals such as cubes, octahedrons and dodecahedrons. Upon cooling to below 173 degrees Celsius, argentite transforms from its isometric structure to the monoclinic structure of acanthite. The transformation often distorts the crystals to unrecognizable shapes, but some will still have an overall isometric crystal shape. These crystals are called pseudomorphs (false shapes) as they are actually acanthite's crystals in the shape of argentite's crystals. Argentite has been historically used when referring to these crystals, but the acknowledgment of the true identity of the mineral requires the naming of these crystals (at lower temperatures) as acanthite.

Acanthite, in addition to the crystals discussed above, forms interesting groupings of crystals. The crystals themselves are often distorted, but they group together into some intricate arborescent (branching) structures.

Argentite comes from the Latin word for silver, argentum, from which the chemical symbol for silver, Ag, is also derived. Acanthite comes from the Greek word for thorn, acantha, in allusion to its typical crystal habits.

Acanthite is generally easily identified although it may appear like galena and other silver sulfide minerals. The crystal habits discussed above are usually diagnostic enough, however the sectility test may be needed in some cases. Acanthite is sectile which means that it can be cut into by a knife just like lead. Acanthite is generally a very valuable mineral specimen, due mostly to the high silver content and the rarity of good crystals. It is a pleasure to own fine acanthite specimens once they are finally procured.

PHYSICAL CHARACTERISTICS:

  • Color is lead gray to black.
  • Luster is metallic.
  • Transparency: Crystals are opaque.
  • Crystal System: Monoclinic; 2/m below 173 degrees Celsius (acanthite) and isometric; 4/m bar 3 2/m above (argentite).
  • Crystal Habits include rarely well formed pseudo: cubes, octahedrons and dodecahedrons. Non-argentite crystals (those that formed below 173 degrees Celsius) of acanthite tend to be of a slender prismatic habit. More commonly found massive and as coatings or as arborescent (branching) and reticulated groups.
  • Cleavage is absent.
  • Fracture is conchoidal.
  • Hardness is 2.5 - 3
  • Specific Gravity is approximately 5.5 - 5.8 (slightly heavy even for metallic minerals).
  • Streak is a shiny black.
  • Other Characteristics: Sectile, meaning it can be cut with a knife like lead and fresh shiny surfaces will eventually form a dull coating after prolonged exposure to light (can be removed by ultrasonic treatment).
  • Associated Minerals include silver, quartz, bornite, gold, galena, proustite, pyrargyrite, stephanite and other silver sulfide minerals.
  • Notable Occurrences include Guanajuato, Mexico; Freiberg and Saxony, Germany; Cobalt, Ontario, Canada; Comstock Lode, Nevada and Butte, Montana, USA; Cornwall, England; Chile; Peru; Bolivia and especially Kongsberg, Norway.
  • Best Field Indicators are crystal habit, density, softness, sectility, association with other silver sulfosalts and color.
ACANTHITE specimens:
(hover for more info)
ACANTHITE specimen aca-1
$ 45.00
Dims: 0.7" x 0.6" x 0.3" (1.8 x 1.5 x 0.8 cm)
Wt: 1.5 g
Sirena Mine, Guanajuato, Mexico
This small thumbnail specimen consists of a cluster of cubic Acanthite crystals. I don't exactly understand the concept, but apparently these crystals are pseudomorphs after argentite, a mineral with the same chemical composition as Acanthite, but with a different structure. Needless to say, they have a definitely cubic form. The cluster is rather flat and shaped like an inverted triangle; it has a branching habit that appears nearly arborescent in some ways, but mostly resembles the form of laboratory-grown bismuth crystals. Together, the crystals form a stepped "hopper effect". None of the individual crystals exceed 3 mm in any dimension, and all have a metallic luster and a dark gray color with subtle tinges of colorful iridescence. I have affixed the specimen in a domed plastic thimble box for display and protection purposes. It is a superb little piece with no detectable damage.
no photo
aca-1 ($ 45.00)
Sirena Mine, Guanajuato, Mexico
ACANTHITE specimen aca-2
$ 60.00
Dims: 0.7" x 0.6" x 0.5" (1.8 x 1.5 x 1.3 cm)
Wt: 3.4 g
Sirena Mine, Guanajuato, Mexico
About 24 cubic Acanthite crystals make up this specimen. They are intergrown to form a small cluster that appears to be coated with a thin layer of chalcopyrite or possibly marcasite near its base. Though these crystals do not exceed 3 mm in any dimension, they are in excellent condition, showing no detectable damage, and have equally good cubic form. Their color is a dark gray and they have a metallic luster. A subtle iridescence is visible on parts of the small cluster, which has been affixed inside a domed thimble box for display.
no photo
aca-2 ($ 60.00)
Sirena Mine, Guanajuato, Mexico
ACANTHITE specimen aca-3
$ 25.00
Dims: 0.9" x 0.4" x 0.3" (2.3 x 1.0 x 0.8 cm)
Wt: 5.5 g
level 490, Santo Nino vein, Fresnillo, Zacatecas, Mexico
At least 4 Acanthite crystals are intergrown on this specimen. All have a definite monoclinic prismatic form that is somewhat warped (likely due to intergrowth) and a bit rounded, but free from human-induced damage. All are long and slender, and the largest measures about 0.5 x 0.3 x 0.1" (1.3 x 0.8 x 0.3 cm). They all have a dark gray color and a metallic luster. The specimen is affixed inside a domed thimble box.
no photo
aca-3 ($ 25.00)
level 490, Santo Nino vein, Fresnillo, Zacatecas, Mexico
ACANTHITE specimen aca-4
$ 28.00
Dims: 0.8" x 0.6" x 0.4" (2.0 x 1.5 x 1.0 cm)
Wt: 5.4 g
level 490, Santo Nino vein, Fresnillo, Zacatecas, Mexico
This cluster consists basically of two Acanthite crystals. Both show a considerable amount of warping in their form; the material towards their collective base is nearly amorphous and tapers down to a thin "stem". They have excellent monoclinic form near the top of the cluster, however, with reasonably well-defined edges and smooth faces that show their gray color and metallic luster. One of the crystals intrigues me a bit, though, as it appears to be twinned; there is a definite line of symmetry that runs through it. The crystals show no human-induced damage and, besides their amorphous areas, are in excellent condition.
no photo
aca-4 ($ 28.00)
level 490, Santo Nino vein, Fresnillo, Zacatecas, Mexico
ACANTHITE specimen aca-5
$ 25.00
Dims: 1.0" x 0.5" x 0.5" (2.5 x 1.3 x 1.3 cm)
Wt: 7.7 g
level 490, Santo Nino vein, Fresnillo, Zacatecas, Mexico
Likely the most intriguing aspect of this specimen is its form. The specimen consists of a cluster of very warped monoclinic Acanthite prisms that are heavily intergrown. Upon close examination, the cluster shows a curious blend of amorphous and crystalline tendencies, as if the largest crystal at the center of the cluster melted, and then started to crystallize in areas as smaller crystals, while most of the surface area "froze" in an amorphous state. Like other specimens that I have seen, this piece's base tapers down to a thin "stem". The visible crystals on the piece have relatively good form, with clean faces and edges that show off its metallic luster and dark gray color. There is a small spot of fresh breakage near its base where it was separated from its place of formation. Otherwise, there is no damage.
no photo
aca-5 ($ 25.00)
level 490, Santo Nino vein, Fresnillo, Zacatecas, Mexico
ACANTHITE specimen aca-6
$ 38.00
Dims: 0.7 x 0.5 x 0.3" (1.8 x 1.3 x 0.8 cm)
Wt: 2.0 g
Guanajuato, Mexico
This small thumbnail specimen consists of a formation of tiny Acanthite cubes that has a suspiciously stalactitic appearance. The largest of these crystals measures 0.4" (1.0 cm) in diameter and is incomplete- it has the form of half of a cubo-octahedron whose edges are truncated. The smaller crystals generally have a very good cubic form, with well-defined edges and clean faces. Their color is the standard moderately dark silvery-gray, and their luster is a slightly dull metallic. Thin dustings or crusts of at least two other minerals are present, but I cannot identify them and they are miniscule, anyway.
no photo
aca-6 ($ 38.00)
Guanajuato, Mexico
ACANTHITE specimen aca-7
$ 25.00
Dims: 1.9 x 1.9 x 0.9" (4.8 x 4.8 x 2.2 cm)
Wt: 1.6 oz. (46 g)
Guanajuato, Guanajuato, Mexico
This small hand specimen consists of several small Acanthite clusters and formations that rest on a gray limestone base. These formations are mostly massive in habit, but a few tiny crystals are visible, along with some semicrystalline formations. The crystals have excellent pseudocubic form, and one area shows some thin, prismatic forms. The mineral shows the standard dark silvery-gray color and a dull metallic luster.
no photo
aca-7 ($ 25.00)
Guanajuato, Guanajuato, Mexico
ACANTHITE specimen aca-8
$ 225.00
Dims: 1.7 x 1.2 x 0.9" (4.3 x 3.0 x 2.2 cm)
Wt: 23 g
San Mateo Vein, San Luis Mine, Fresnillo, Zacatecas, Mexico
A cluster of several intergrown Acanthite crystals rests on the calcite base of this small hand specimen. These crystals are small, not exceeding 0.3" (8 mm) in diameter and are in moderately good condition, as several are noticeably damaged or broken. Where intact, however, they have excellent pseudocubic form that takes on the shape of octahedrons and cubo-octahedrons. They have the standard dark silvery-gray color and dull metallic luster and are intergrown with a bed of similarly-sized calcites that have a milky-white color.
no photo
aca-8 ($225.00)
San Mateo Vein, San Luis Mine, Fresnillo, Zacatecas, Mexico
ACANTHITE specimen aca-9
$ 115.00
Dims: 1.5 x 0.8 x 0.7" (3.8 x 1.9 x 1.7 cm)
Wt: 16 g
Guanajuato, Guanajuato, Mexico
This specimen consists of an Acanthite formation that pseudomorphed from a cluster of heavily-intergrown pyrargyrite crystals. Apparently, the Acanthite completely replaced this other silver mineral, and grew a few pseudocubic crystals, besides. This actually means that the trigonal prismatic pyrargyrite was originally replaced by cubic argentite, which then pseudomorphed into Acanthite! The crystals are well-formed, and all of the material has the standard dark gray color and slightly dull metallic luster of the specie. A small amount of what appears to be white quartz is present.
no photo
aca-9 ($115.00)
Guanajuato, Guanajuato, Mexico
ACANTHITE specimen aca-10
$ 63.00
Dims:1.2x0.9x0.5" (3.0x2.3x1.3 cm)
Wt: 0.7oz. (21g)w/base
Level 590, La Sirena Mine, Guanajuato, Mexico
This cute little specimen consists of crude crystals of acanthite that are covered with a thin layer of microcrystalline chalcopyrite. The overall effect is that the acanthite crystals have a shimmering golden-bronze appearance that is quite appealing. There are also a few tiny rhombohedrons of calcite scattered about the base of the specimen. The only damage to this specimen is the tip of one of the crystals has been cleaved. This specimen is firmly glued to a clear acrylic base.
no photo
aca-10 ($ 63.00)
Level 590, La Sirena Mine, Guanajuato, Mexico
ACANTHITE specimen aca-11
$ 110.00
Dims:1.2x0.9x0.9" (3.0x2.3x2.3 cm)
Wt: 0.6oz. (17g)w/base
Lvl 590, La Sirena Mine, Guanajuato, Mexico
This little thumbnail consists of several intergrown crystals of acanthite, with no matrix present. These crystals are in the form of monoclinic prisms, to 0.2" (0.5cm) in length. This is an excellent example of the species. There is no damage to this specimen. It is firmly glued to a clear acrylic base.
no photo
aca-11 ($110.00)
Lvl 590, La Sirena Mine, Guanajuato, Mexico
ACANTHITE specimen aca-12
$ 75.00
Dims: 1.3 x 1.1 x 0.6" (3.5 x 2.9 x 1.5 cm)
Wt: 13.8 g
Rayas Mine, Guanajuato, Mexico
Like most acanthite specimens, this one is a pseudomorph after argentite. It shows excellent form with a somewhat arborescent habit, dark silver color and metallic luster. While most of the crystals show the distortions caused during the conversion, there are some nice, well formed cubic crystals present. Also, there are bits of quartz and (likely) pyrite as well.
no photo
aca-12 ($ 75.00)
Rayas Mine, Guanajuato, Mexico
ACANTHITE specimen aca-13
$ 60.00
Dims: 2.13x0.55x0.24" (5.4x1.4x0.6cm)
Wt: 22.6ct (4.52g)
Rayas Mine, Guanajuato, Mexico
This is an excellent specimen of acanthite. It is a pseudomorph (after argentite), showing several crystal forms characteristic of that mineral, including distorted cubes and dodecahedrons. The specimen has an intricate crystal form, as it is a long crystal with many small crystals attached - I would not call it arborescent, because none of the "branches" are long, the longest is about 3mm in length. The acanthite is the usual metalic silver in color, opaque, and with a metallic luster. Apart from the base, it is undamaged. THere is a small amount of a slightly pink mineral on the base. I am not sure of its composition, but it looks like a low grade (massive) rhodochrosite.
no photo
aca-13 ($ 60.00)
Rayas Mine, Guanajuato, Mexico
ACANTHITE specimen aca-14
$ 60.00
Dims: 0.83x0.47x0.43" (2.1x1.2x1.1cm)
Wt: 25.8ct (5.16g)
San Juan de las Rayas Mine, Guananjuato, Mexico
This is a pseudomorph of acanthite after argentite. The argentite crystal shapes were distorted in the conversion, as is often the case. The form is that of a stem with several blocky crystals attached. The acanthite appears grayish-black and metallic, with its luster dull in most places from a brown tarnish.
no photo
aca-14 ($ 60.00)
San Juan de las Rayas Mine, Guananjuato, Mexico
ACANTHITE specimen aca-15
$ 53.00
Dims: 1.13x0.63x0.42" (2.88x1.61x1.06cm
Wt: 28.20ct (5.64g)
San Juan de las Rayas Mine, Guanajuato, Mexico
This acanthite specimen has a rather unusual appearance. It is a dark gray-blue in color, tending towards green and purple in some areas, due to an oxidized surface layer. However, a few tiny spots of damage show the mineral's silver appearance, and there are many very tiny silvery spots that reflect a point light source. A 15x loupe reveals that these are little crystals rising above the tarnish layer, and furthermore they are all aligned (reflecting the light source at the same angle), indicating that they are just more recent crystal growth of the same single large crystal comprising this specimen. There is also a tiny colorless transparent crystal, probably calcite but it does not show enough crystal form for me to be sure.
no photo
aca-15 ($ 53.00)
San Juan de las Rayas Mine, Guanajuato, Mexico
ACANTHITE specimen aca-16
$ 29.00
Dims: 0.78x0.39x0.34" (1.98x1.00x0.86cm)
Wt: 9.40ct (1.88g)
Proana mine, Fesnillo, Zacatecas, Mexico
This is a typical specimen of acanthite, although it has excellent form. There are numerous distorted crystals in a branching tree, providing an excellent (but small) specimen of this mineral.
no photo
aca-16 ($ 29.00)
Proana mine, Fesnillo, Zacatecas, Mexico
ACANTHITE specimen aca-17
$ 25.00
Dims: 1.17x0.52x0.28" (2.96x1.32x0.72cm)
Wt: 0.14oz (4.1g)
San Luis Shaft, Fresnillo, Zacatecas, Mexico
This specimen is acanthite, showing little crystal form. It is likely a distorted pseudomorph after argentite, as it does show hints of crystal form. One side of the crystal is all rounded, while the other side has many pits and an area that looks like a fracture pattern with a pleasing texture.
no photo
aca-17 ($ 25.00)
San Luis Shaft, Fresnillo, Zacatecas, Mexico

 


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