PERICLASE AND GENTHELVITE: CONNECTION TO COLORADO

  Meteorite shower over Chelyabinsk on February 15, 2013.

Взрыв метеорита над Челябинском   SOURCE

At the corner of Oracle and Lester (the “main road” leading into Mineral City) is a show venue termed La Fuente de Piedras.  The name comes from the former eating establishment that once occupied the space.  Housed within were 14 dealers including MinDat, Jeff Scovil Photography, Alfredo Petrov (of MinDat fame), and part of the Rock Currier Collection of Fine Minerals.  The latter (270 specimens exhibited) was then moved to auction on February 7 by Heritage Auctions (Dallas).  The specimens I observed were top quality and priced accordingly (with estimated selling prices).  As I understand, some specimens have been sold in the past and others wait for a new year. MinDat had a small space selling used geology and mineral books.  Prices were fair and the variety of titles wide. Jolyon was often available to visit and talk about MinDat.

 

  

 

 

 

 

 

Nest door to La Fuente was a newer looking establishment with a big Arkenstone sign on the outside, the business (Arkenstone Gallery of Fine Minerals) of Rob Lavinsky.  There were perhaps a half dozen or slightly more sellers in the building including a friendly young man from the Czech Republic who had something like 2000 minerals “for sale” including a huge selection of thumbnails and micromounts.  Unfortunately, I did not get his business card and cannot remember his name; however, we had great chats about Czech foods since my close relatives bake really great kolaches and play accordions!   I also picked up a number of nifty minerals including periclase and genthelvite.

Periclase, a rare magnesium oxide (MgO), was collected from coal mine dumps near Kopeysk in the Chelyabinsk Oblast, Russia.  The Oblast is located on the eastern slope of the southern Ural Mountains.  The name Chelyabinsk rang a small bell in the back recesses of my mind, but it could not quite make it to the surface—so to the Web. In mid-February 2013 a large meteoroid, perhaps 10,000 tons, exploded over the Oblast injuring several hundred people and damaging several thousand buildings.  Ding ding.

Until the end of the 20^th Century coal was the main economic driver of the region; however, all mines are now closed due to the high costs of production and transportation, and the low quality of the coal (soft and brown).  The region was infamous for spontaneous fires that raged through the mines and dumps for months or years and reached temperatures of 1000-2000 degrees F. Somewhere near 50 new mineral compounds have been described from these dump/mine fires; however, MinDat believes that only 8 of these compounds are accepted as legitimate minerals.

One of these somewhat rare mine/dump minerals is periclase, a mineral that is usually found in dolomites that have been cooked by metamorphic heat (contact metamorphism).  But in addition, it can form in coal (such as in Russia) or wood fires. Garvie (2016) noted drill cores taken in basins around Phoenix, AZ, have produced a suite of minerals from ash associated with burning of Palo Verde trees: - fresh ash (before raining): bütschliite, calcite, fairchildite, lime, periclase, traces of other K salts.
- after rain: arcanite, fairchildite, kalicinite, magnesian calcite, magnesite, sylvite, 'unnamed (Hydrated K Carbonate).
- after another rain: calcite, magnesian calcite, periclase.
- ash collected after 2 years: calcite, magnesian calcite, nesquehonite.

The mechanism behind this periclase formation by fire remains a mystery to me (beyond my pay grade).

Yellow-brown periclase with a few fairly defined crystals.  The black tiny crystals may be or may not be strebrodolskite, a calcium iron oxide.

The best specimens desired by collectors are small (mm size), sub-vitreous, and brightly colored (green, yellow, black, off white, brownish yellow, and colorless) octahedral or cubic crystals (Isometric Crystal System). Crystals are hard (~5.5 Mohs) but brittle and leave a white streak.  Less desirable specimens are grains, either individuals or “fused” together.  Some specimens are massive, never large but the grains or crystals are tough to spot.  Either way periclase was a nice addition to my collection since I was unfamiliar with the mineral.

For a little trivia: 1) under “normal” conditions of temperature and humidity periclase often alters to brucite, Mg(OH)_2.   

A second mineral purchased was the beryllium zinc silicate sulfide genthelvite [Be₃Zn₄(SiO₄)₃S]. I really was not very familiar with the mineral as it seems rather uncommon (but widespread).  However, its Type locality is close to home: West Cheyenne Canon, Cheyenne Mining District (St. Peters Dome Mining District), El Paso County, Colorado.

In 1961 Edwin B. Eckel of the USGS, after 30 years of work, authored Bulletin 1114, Minerals of Colorado: A 100 Year Record.  The Bulletin was a major contribution to understanding mineral resources of Colorado and was popular with both amateur rockhounds and professional geologists.

The earliest record in Eckel’s Type list is from 1874 when F.A. Genth described schirmerite, a silver-lead-bismuth sulfide, from the Geneva District located about two miles southeast of the mining community of Montezuma (located a few miles east of the Keystone Ski Resort). The next record is 1877 when Genth described the mercury telluride mineral, coloradoite, from Boulder County.  Genth was a professor of mineralogy at the University of Pennsylvania and evidently loved wandering around Colorado looking for minerals. In 1892 Genth described a new mineral from the Saint Peter’s Dome area but did not recommend a name. It took 50+ years before the “described mineral” was formally named by Glass and others (1944).  Genthelvite was named after Genth, and the mineral’s relationship to the mineral helvite [Be₃Mn₄(SiO₄)₃S] with which a solid solution series exists.

At the Type Locality genthhelvite is found in the   miarolitic cavities of granitic pegmatites associated with the Pikes Peak Massif; at other localities genthelvite is an accessory mineral in alkaline granites and syenites, greisens, and skarns. The specimen I purchased is from the famous Poudrette Quarry (broad sense) that is located in the East Hill suite of the Mont Saint-Hilaire alkaline complex (igneous rocks containing a high concentration of sodium and potassium, more so than in other igneous rocks and therefore containing feldspathoid minerals [lots of nepheline]) (Normand and Tarassoff, 2006).  See Blog postings March 10 and March 29, 2021.

 

Cluster of genthelvite crystals from the Poudrette Quarry.  Width FOV ~5.0 mm. 

Genthelvite crystals are usually vitreous, transparent to translucent, fairly hard at ~6.5 (Mohs) but brittle and come in a variety of colors: colorless to white (most common) pink to red, yellow, green but often weather to a brown to black. It imparts a green color under both short and long UV. Genthelvite belongs to the Isometric Crystal System and good specimens contain pyramidal crystals. The Canadian specimens seem much “nicer” that the darker crystals I have seen in Colorado Collections.

REFERENCES CITED

Garvie, L.A.J., 2016, Mineralogy of paloverde (Parkinsonia microphylla) tree ash from the Sonoran Desert: A combined field and laboratory study: American Mineralogist, v. 101.

Genth, F.A.,1892, Contributions to mineralogy; No. 54. American Journal of Science: v. 44.

Glass, J.J., Jahns, R.H., Stevens, R.E.,1944, Helvite and danalite from New Mexico and the helvite group: American Mineralogist v. 29.

Normand, C. and P. Tarassoff, 2006, GEOLOGICAL ASSOCIATION OF CANADA, MINERALOGICAL ASSOCIATION OF CANADA, 2006 JOINT ANNUAL MEETING MONTRÉAL, QUÉBEC

FIELD TRIP 4A: GUIDEBOOK, MINERALOGY AND GEOLOGY OF THE POUDRETTE QUARRY, MONT SAINT-HILAIRE, QUÉBEC

Microsoft Word - GuidebookMSHfinal2-JP2.doc (mcgill.ca)