Sunday, November 24, 2013

"ZINNWALDITE": MICA PIKES PEAK BATHOLITH



Mr. “Rockhounding the Rockies” is a pegmatite digger here in the Pikes Peak area and has collected fine specimens of smoky quartz and amazonite along with some nice fluorite and topaz.  He also has pulled out some really great pieces of petrified wood from the Cretaceous and Tertiary rocks along the Front Range.  The other night he gave me, not one of those beautiful blue-green microclines crystals, but a somewhat nondescript piece of zinwaldite, something that “clutters up” his digging holes in the Precambrian pegmatite.  I accepted it with gratitude since it is one of those strange and weird minerals that so fascinate me.  But wait, zinnwaldite is not really a valid mineral species!

Zinnwaldite from Lake George Ring Complex.  Note brass-colored sheets.  Arrows point to three "faces" of pseudohexagonal crystal.  Maximum width ~ 3cm.
 Zinnwaldite is a lithium-iron mica [KLiFe+2Al(AlSi3O10)(F,OH)2] that is common as euhedral, pseudohexagonal books in miarolitic cavities (pockets) of pegmatites of the Precambrian (~1.05 Ga) Pikes Peak Batholith (Eckel and others, 1997; Foord and Cerny, 1995).  In these cavities the zinnwaldite occurs with the more desirable minerals such as  smoky quartz, topaz and amazonite. It is hard to describe the physical characteristics of zinnwaldite other than to say the “mineral” consists of thin sheets of a brass-colored substance that in side view is a rather non-descript black-brown color.  To me, in a side view, the specimen resembles a fissile black shale.  The sheets of this mice are not as elastic as either muscovite or biotite; however, they may be peeled off and almost appear as a brassy colored shale.  I did notice, however, that other described specimens range in color from brown to green to violet.  The hardness varies from 3-4 (Mohs) and in other localities zinnwaldite sometimes appears as bundles of vitreous sheets forming rosettes.  Zinnwaldite belongs to the Monoclinic crystal system but the individual crystals appear as hexagonal in shape and are termed pseudohexagonal.
 

X-section view of specimen.  Notice layering of sheets.  Width ~3.6 cm.
MinDat.com noted that the International Mineral Association (IMA) has discredited zinnwaldite as a valid mineral and that it is now classified somewhere in the series between siderophyllite (iron rich) and polylithionite (lithium rich) mica.  OK, but what about the Lake George Ring Complex specimens—what are they?  The 1997 Minerals of Colorado (Eckel and others) list them as zinnwaldite.  MinDat.com notes that both siderophyllite and polylithonite are found at Lake George; however, their photos of Lake George specimens are listed as zinnwaldite!  To make it more confusing, at least to me, are the numerous (and definitive) studies by E. E. Foorde and co-authors on Pikes Peak Batholith rocks.  In a 1995 article, Foord noted that the pegmatites contained a number of different micas with the earliest micas (the iron-magnesium micas) forming tapered columnar crystals growing toward, and adjacent to the miarolitic cavity zone which contains the later crystallized micas (the lithium-fluorite micas).  And, that most cavity-grown zinnwaldite crystals show a decrease, from core to rim, in total Fe and Mg, whereas Si, Li and F increase and Mn, Rb, Cs and Na are essentially constant.

So, my question is still not answered.  Is the zinnwaldite collected in rocks of the Lake George Ring Complex classified as siderophyllite or polylithonite?  Or perhaps just put into the “series”, especially since Foord and others (1995) noted that the amount of iron and lithium varied within the same specimen from rim to center! 

I love geology (and life), especially when there are so many questions suddenly appearing out of “nowhere”.  For example, in the middle seat of an airplane—which arm rest is yours?

ADDENDUM 11 June 14:  More "zinnwaldite" from the gut pile of Mr. Rockhounding the Rockies, AKA the resident meteorologist of Lake George.

Pseudohexagonal crystal.
A stack of mica.  Height ~2.6 cm.
Crystal on matrix.  Width of broken crystal ~1.6 cm.
Large broken crystal; stacked micaceous plates.  Width ~7.2 cm.
ADDENDUM 6 MAY 2014:  Mr. Rockhounding the Rockies just presented me with another piece of "zinwaldite."  In his urgent quest for amazonite, he simply tosses some specimens in the "gut pile."
Zinnwaldite from Lake George Ring Complex.  Note pseudohexagonal crystal.  Maximum width ~4.5 cm.

REFERENCES CITED
Eckel, E. B. and others, 1997, Minerals of Colorado: Denver      Museum of Natural History, Denver. 

Foord, E. E., P. Černý, L. L. Jackson, D. M. Sherman, and R. K. Eby, 1995, Mineralogical and Geochemical Evolution of Micas from Miarolitic Pegmatites of the Anorogenic Pikes Peak Batholith, Colorado:  Mineralogy and Petrology, v. 55, Issue 1-3. Large broken crystal; stacked micaceous plates.  Width ~7.2 cm.
 

No comments:

Post a Comment