ACANTHITE
Specimen aca-4

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.