Mexico is a mosaic of different realities and beauties.
Enrique Pena Nieto
My last post commented on several specimens of calcite crystals that came up from Mexico via a grandfather and granddaughters. Exact collecting information was very fuzzy; however, I expect they came from one of the mines at Santa Eulalia in Chihuahua. My purchase plan was to give these specimens to some younger rockhounds.
I did come back from Big Piney with some nicer specimens containing collecting information. The first is a really nice barite matrix covered with gemmy orange wulfenite crystals associated with green botryoidal mimetite. I have written about these minerals before but here is a partial repeat.
Mimetite is a lead chloro-arsenate [Pb5(AsO4)3Cl] often found with pyromorphite and/or vanadinite. In the former mineral the phosphate radical, PO4, replaces the arsenate radical (AsO4) while in the latter the vanadate radical (VO4) replaces the arsenate. However, mimetite is also closely associated with wulfenite [(Pb(Mo4)] since both form as secondary minerals in the oxidation zones of primary lead minerals.
Mimetite seems mostly yellow in color although other colors are common—green, red, colorless, brown, orange-yellow and shades in-between. It usually appears as small barrel shaped prismatic crystals but may be tabular, mammillary, granular, rounded, botryoidal and others.
Wulfenite is a lead molybdate with colors ranging from orange (common) to yellow, yelow-orange or red. Crystals typically are tabular and transparent, at times opaque, but may form pyramids or stubs.
The specimen I brought home came from the La Morita Mine, Ascención Municipality, Chihuahua, Mexico. This former polymetallic mine is well known for its luscious silky wulfenite crystals and the unique mimetite. MinDat noted that since 2018 the Mine has been operated for specimen production rather than metallic ores. However, my specimen was collected/mounted on the 2nd of January 1972, about 50 years ago.
The second specimen I brought home exhibits pointy scalenohedron crystals of calcite covered by a secondary calcite druse with all sitting on a base of small gemmy quartz crystals. It almost appears that the quartz is/was slowly replacing the calcite forming quartz ps. calcite. The specimen was collected from a mine in Guanajuato, Guanajuato Municipality, Guanajuato, Mexico. I am guessing either the La Sirena or Valenciana Mine.
Scalenohedron crystals of calcite covered with secondary calcite druse on a base of gemmy quartz crystals. Width FOV ~5.3 cm.
An original calcite crystal covered by a secondary shiny druse of calcite. Width FOV ~1.5 cm.
The Guanajuato mines are classified as “epithermal vein deposits” where a variety of minerals are located in veins that are probably the result of previous fractures and cracks in the host rock. The hydrothermal brines, especially sodium-calcium-chloride brines, are effective solvents and are able to dissolve the primary sulfide ores and circulate through the host rocks. As these brines begin to cool, usually at shallow depths, deposition of minerals take place and often these veins of minerals are quite rich. In fact, epithermal gold deposits are some of the richest gold deposits in the world. At Guanajuato mineralization consists dominantly of silver sulfides and sulfosalts (a metal + semi-metal + sulfur), base metal sulfides (metal + sulfur) mostly chalcopyrite, galena, sphalerite, and pyrite, and electrum (alloy of silver and gold). Gangue minerals (the waste rock) are generally quartz and calcite; the host rocks (holding the minerals) are Mesozoic sedimentary and intrusive igneous rocks and Tertiary volcanic rocks (Morgan and others, 2014).
The Rayas Mine, one of the richest of the Guanajuato mines, was located atop the mother lode, or veta madre, set prominently in the hills above the city. In fact, the mine still partially operates and produces ore, as well as specimens. The Rayas has a long history of bringing to grass specimens of acanthite, a silver sulfide [Ag2S] that crystallizes in the Monoclinic Crystal System. It is a low temperature sulfide and is the stable form below 177 °C. The high temperature silver sulfide is argentite, crystallizing in the Cubic Crystal System, and as it cools below 177 °C it transforms to acanthite. Rockhounds still may find old specimens labeled as argentite; however, the only form stable in “normal” air temperature is acanthite.
Acanthite is a common ore of silver, lead grey in color although it tarnishes black (as on Sterling Silver), has a metallic luster, and is soft (~2.0-2.5 Mohs). What make acanthite such a nifty display specimen is that it often appears as a “stack” of pseudo-octahedral and/or pseudo-cubic crystals (formed during transformation from cubic argentite).
A “stack” of pseudo-octahedral and/or pseudo-cubic crystals of acanthite. length of "stack" ~2.0 cm.