MORE CHLORIDES: SYLVITE & CHLORARGYRITE

As noted in the previous blog, the halide minerals include those in which the halogen anions (chlorine, bromine, fluorine and iodine), with a negative charge, combine with metal cations (positive charge).  Many halide minerals seem to have low specific gravities, are essentially non-conductors of electricity, some good cleavage, are transparent to translucent (mostly depending upon impurities), and are soft (2-3+ Mohs).

Atacamite, described previously, is a chloride as are sylvite, halite, and chlorargyrite.  The initial two minerals are known as evaporites since their sedimentary depositional environment is usually a restricted circulation and drying basin.  Chlorargyrite, a silver chloride (AgCl), is much different and is a secondary mineral found in rocks that produce silver (similar to atacamite). 

In the United States the best known marine evaporites, described in my well-used Glossary of Geology (distributed by the American Geological Institute) as “water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution” are from the great Permian Basin.  The Basin was part of  a broad and shallow cratonic (inland) sea that extended from Mexico to southern central Canada; Permian rocks are well-exposed and much studied from west Texas north through the Plains states and Rocky Mountains.  As the Permian continued (that is going from older Permian time to younger) the world’s continents begin to coalesce into the vast end-of-Paleozoic supercontinent termed Pangaea. This event had the effect, in the later Permian, of driving off the earlier Permian shallow seas and creating very restricted circulation basins where water evaporated, the seas became more “saline,” and evaporitic minerals began to form.  In my native Kansas, where Permian rocks are well-exposed in the eastern one-half (and southwestern quadrant) of the state, observant rockhounds can literally see the rocks move from fossiliferous limestones and shales to inhospitable (for life) beds of red shale, halite (subsurface only), gypsum, and anhydrite.  However, the thickest and best known of the Permian rocks are located in the Permian Basin, a Permian subsistence basin occupying west Texas and parts of southeastern New Mexico. 

The Delaware Basin is a subsection of the greater Permian Basin and includes the section around Carlsbad, New Mexico, where redbeds and evaporites are common and gypsum crops out over a wide area.  Halite, anhydrite and potash (potassium salts) are widespread in the subsurface.  In addition, the Delaware Basin is a major producer of hydrocarbons.

The Carlsbad Potash District produces rock salt from dry mines, brine fields, and solar-salt operations at 18 locations; gypsum is mined at 13 sites; potash is produced from five underground mines; and sulfur is produced by the Frasch process at one site (Johnson, 1997).

 

Permian Basin.  Public Domain map.

Klein (2002) noted that in marine evaporitic basins the minerals precipitate out in a select order, and in the reverse order of their solubility.  The first to come out, and therefore the most common minerals in the rock column, are the carbonates: calcite [CaCO₃], and dolomite [CaMg(CO₃)₂] when evaporation reduces the original sea water by ~50%; gypsum [CaSO₄-2H₂O] and/or anhydrite [CaSO₄] when ~20% of the original volume is left (anhydrite, rather than gypsum, with higher salt concentration and higher temperatures); halite [NaCl] when ~10% is left; and finally the much rarer magnesium and potassium sulfates langbeinite [K₂Mg₂(SO₄)₃], polyhalite [K₂Ca₂Mg(SO₄)₄-2H₂O], kieserite [MgSO₄-H₂O], and chlorides sylvite [KCl] and carnallite [KMgCl₃-6H₂O].  Although rare in most deposits, sylvite can form thick deposits and is mined extensively (for potassium) in the Carlsbad Potash District. 

 

Sylvite from the Carlsbad Potash District.  Width ~5.2 cm.

I have a mineral from the District that is somewhat difficult to identify—sylvite or halite!  Both minerals are found in similar environments and actually may be found together in the same specimen. Both have very similar physical characteristics: isometric with a cubic habit, usually found in massive granular masses, varied color from colorless to others due to impurities, soft (~2.5 Mohs), white streak.  In other words, they look alike. 

The major differences seem to be that sylvite has a “salty” taste but is more bitter than halite, and does not fluoresce under UV light (halite is commonly reddish orange under short wave and reddish to green-orange under long wave UV).  Sylvite from the Carlsbad Potash District.  Width ~5.2 cm.

OK, it is not wise to do a “taste test” but---I scraped a small amount of powder and unwisely put it on my tongue immediately rinsing with copious amounts of water.  I found it to be very bitter.  In addition, the specimen does not fluoresce. So, I pronounced it sylvite, a very late forming potassium chloride.

A nondescript sample of chlorargyite with arrows pointing at blackish "balls" of the minerals.  Other dark-colored areas may also be chlorargyrite.  The light-colored (blueish) vugs are some sort of a clay mineral.

 

 

A photomicrograph of chlorargyrite with the arrow pointing at blackish "balls" of the minerals.  Width of view ~5 mm.

Chlorargyrite is also a chloride mineral (AgCl) but forms in a much different environment than the evaporitic chlorides.  Chlorargyrite is a secondary mineral and is found in the oxidized zone of sulfide deposits.  At a “typical” sulfide ore body meteoric water dissolves and leaches out several minerals as it percolate downwards.  This action has an oxidation effect on rocks above the water table but oxidation stops at the top of the table.  As this percolating solution reaches the water table mineral sulfides (secondary) begin to precipitate and a zone of mineral enrichment develops (the supergene). However, at times the water table fluctuates up and down, the primary surface (gossan) minerals (lots of quartz, iron oxides) dissolve, and  the percolating solutions drop new (secondary) metals in the zone of oxidation.  For example, chlorargyrite is the silver mineral found in the oxidation zone whereas acanthite [Ag₂S] is the silver mineral of the supergene deposits.  Azurite [Cu₃(CO₃)₂(OH)₂], malachite [Cu₂(CO₃)(OH)₂] and chrysocolla [(CuAl)₂H₂Si₂O₅(OH)₄-nH₂O)] are copper oxidation minerals while chalcocite [Cu₂S] and bornite [(Cu₅FeS₄] are supergene minerals.  Both the supergene and oxidized zones are greatly enriched with minerals (chlorargyrite can be 75% silver), are usually fairly close to the surface, and therefore easy to extract. Generally they are/were the first to be mined. 

Chlorargyrite is a very soft mineral (1-2 Mohs), crystallizes in the isometric (hexoctahedral) system but rarely is found as yellowish crystals.  Mostly it is massive, sometimes columnar, and is usually dark brown to dark purple to almost black in color.  The specimen in my collection has patches of tiny, almost black, melted together, “balls.”  It matches photos shown on MinDat.  At times bromine or iodine ions partially substitute for the chlorine with resulting bromian chlorargyrite (also known as embolite) or idoargyrite.

My small purchased specimen was collected from the “Turquoise District—Courtland-Gleeson District” Cochise County, Arizona.  Mineralization at Courtland-Gleeson-Pearce is of several types: (1) copper carbonates and oxides in irregular blanket deposits where the Cambrian quartzite is thrust over Mississippian limestone creating a fault breccia (broken rock) close to a contact with an igneous intrusion; (2) lead and zinc carbonates, lead sulfates and zinc silicates with silver chloride, manganese and minor copper and gold in irregular ore bodies in Pennsylvanian-Permian limestones along fractures and faults; (3) turquoise in near-surface stringers and lenses in altered granite and quartzite—solution in fracture zones; (4) manganese oxides in irregular masses along fractures in limestone; and (5) spotty base metal ores with gold and silver values in veins located in intrusive rocks (MinDat, 2011).  What all this means is that faulting in the area created fracture zones that allowed heated (from the igneous intrusions) and mineralized solutions to travel through and deposit the metallic ores.

So, that the story.  Attach a chloride anion to something like a potassium cation and a salt is produced.  Sylvite is an important for use in fertilizers (the K) in the formula.  Attach the chloride to a silver cation and a very rich silver ore results.  I have not noticed a town by the name of Sylvite; however, I have visited Chloride, Arizona, an old silver mining town!

REFERENCES CITED 

Johnson, K.S., 1997, Permian evaporites in the Permian basin of southwestern United States: Prace - Panstwowego Instytutu Geologicznego, Issue 157, pt. 2.

Klein, C., 2002, the 22nd edition of the Manual of Mineral Science: John Wiley and Sons, New York.