Monday, January 23, 2017

CYLINDRITE: A CIGAR-ROLLED SULFIDE



Today, in the middle of Kansas, is a small town with a population of “around” 400 persons and maybe a hundred more if one counts free-range dogs.  I left the town ~55 years ago when the population was about 400 people plus the dogs.  It was a pretty "normal" farming community where a “typical farm” ranged from a section quarter to perhaps an entire section.  Larger acreages usually included some land rented by the farmer.  In addition, each farm usually raised a few head of livestock—hogs, cattle, chickens—for sale and for personal use.  Dogs abounded as did the free-range cats that were called “mousers.”  The cats were “just there” and were often quite wild.  In addition, because of inbreeding there were several of these animals that were not the sharpest pencil in the box!
 
Free-range dogs appreciate attention, especially after a long hike.  The broken clavicle?  Just a little friendly wrestling match!
I distinctly remember Saturday evenings when the outlying occupants came into to town for the weekly “trading” (as my mother called it) at the two grocery stores.  The trading was usually in the form of chicken eggs that a farmer might bring in and “trade” for a few groceries.  My father operated a gasoline (plus the usual small scale mechanic work) station and rarely took in trades unless he figured out that commodities would be his only way to collect a debt.  One time we had enough butter to last a year, and to a small kid that staple was magic compared to the terrible tasting oleomargarine that usually stocked our pantry.

One of the great things about growing up in such a small town was the fact that kids were like the local dogs---free-range. We simply wandered all over town and had a boundary of about one mile in any rural direction to explore.  Just be home by supper.
 
Old friends after a tough day of hunting for turdites.  That Kansas sun can be hot so don't laugh at the hat!
I bring this up since I was a rockhound at an early age and always picked up and hauled home anything that caught my eye.  But, I also named many of these special rocks—nothing serious since I didn’t have the slightest idea what they really were.  Therefore, my names were like crickite (we pronounced the streams cricks) for finding the stone in a creek, roundyites (shape), slaberoos due to the layering, and turdites, a young boy’s favorite rock name (giggle, giggle), etc.  The names I conjured up generally were due to either shape or location.  Today, I suppose most new mineral names come from honoring a person.  However, one mineral I could have named as a child is the sulfosalt cylindrite (FePb3Sn4Sb2S14).  What appears as crystals in cylindrite specimens are actually rolled up flat sheets in the form of a cylinder, kind of like rolled tobacco leaves in a cigar.  Unfortunately, I never had a chance to collect the mineral as most specimens come from Bolivia.
 
Massive cylindrite mixed with the typical rolled-up sheets.  Longest cylinder in center is ~1.1 cm.
 
Photomicrograph (black & white) of above specimen.  The shiny metallic luster does not work well with my scope. Again, the longest cylinder is ~1.1 cm.
Cylindrite belongs to the sulfosalt minerals, a member of the sulfides group.  Sulfosalts contain a metal (mostly lead, copper, iron or silver although a few others, mercury, zinc, vanadium may be present), a semi-metal like arsenic, germanium, bismuth, antimony, or the metals tin or vanadium, and then sulfur (Richards, 1999).  

Because of the interesting habit of cylindrite, I found a specimen last February at one of the Tucson dealers (Shannon and Sons Minerals). The mineral is black to dark gray in color with a metallic luster.  It is quite soft at ~2.5 (Mohs) and has the traditional opaque diaphaneity of metals and the malleable tenacity. Although cylindrite appears in massive or cylindrical forms, Triclinic crystals are present but tiny (beyond the limits of my microscope).  I really don’t understand the mechanism behind the formation of cylinders.  Evidently, cylindrite originally occurs as stacked and layered structures but these flat sheets begin to deform from pressure and “roll up” into smooth cylinders.  MinDat states that “layer curving is one of the ways of an accommodation of the dimensional sheet misfit by a cylindrite crystal.”  That is about all that I can understand!  However, I do note that several references refer to cylindrite rolls as crystals; however, the large rolls are not individual crystals but rolled up sheets and the sheets are composed of individual crystals.

Cylindrite is a fairly rare mineral and most specimens come from mines in Brazil; my specimen was collected from the Itos Mine, Oruro City.  There are several tin-silver mines in the area and some produce cylindrite.  The mineral seems to form as the result of hydrothermal solution next to an igneous intrusion. For reasons beyond my comprehension, there are numerous studies that have been completed on synthesizing cylindrite and relatives (mainly franckeite).  During these studies microprobe analyses indicate there may be some solid solution series between lead and tin in cylindrite. I have read a number of these papers without really understanding much—see Jiuling and others, 1988.  I simply picked up my specimen due to the nifty habit of rolled up cylinders.

OLD FRIENDS
Can you imagine us years from today,
Sharing a parkbench quietly
How terribly strange to be seventy
Old friends, memory brushes the same years,
Silently sharing the same fears.
                             Simon and Garfunkel  

REFERENCES CITED
Jiuling, L., H. Jiashan, Z. Kezi and Z. Guilan, 1988,  An experimental study on three quaternary phases in the Fe-Sn-Sb-S System: Pb-free Franckeite, Pb-free Cylindrite and (Fe, Sb)-Ottemannite s.s.: Acta Geologica Sinica, v. 1, no. 4.

Richards, J.P., 1999, Encyclopedia of Geochemistry in Encyclopedia of Earth Sciences Series, C.P Marshall and R.W. Fairbridge, eds.: Springer Netherlands.