"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.

 

ARKANSAS BAUXITE

Pisolitic bauxite from Arkansas.  Width ~4.1 cm.

Many CSMS members have been to the State of Arkansas on a collecting trip, most likely to collect quartz crystals and dabble in the diamond mine.  I would be interested in meeting someone who actually has collected a diamond in the State Park!  However, there are many other really interesting minerals in Arkansas that offer some collecting opportunities.

One of these “minerals” that Colorado rocks do not offer is bauxite.  Now, I don’t know much about bauxite except that it: 1) is an ore of aluminum; 2) forms in a tropical to subtropical environment; 3) occurs as small to large oolites; and 4) is mined in Arkansas (and elsewhere).  I learned all of these facts in a grade school geography class!  What I later learned as a geology student is that bauxite is not really a “mineral” but is a composite of several different aluminum-rich minerals: [gibbsite—Al(OH)₃, diaspore-AlO(OH), boehmite-ALO(OH)],  a couple of iron oxides [hematite-Fe₂O_3,  goethite- FE³O(OH)],  and a clay mineral [kaolinite-Al₂(Si₂O₅)(OH)₄].

Bush (2007) described the formation of Arkansas bauxite as lateritic weathering and leaching of alumina and silica from intrusions of nepheline syenite (lots of nepheline and sodium/potassium feldspars) exposed during the Tertiary in what is now Pulaski and Saline Counties in the Gulf Coastal Plain.  These rocks (nepheline syenite), rather rare in the U.S., are alumina-rich and quartz-poor and evidently were intruded during the Late Cretaceous as the old Reelfoot Rift was reactivated.  Later in the Tertiary, intense weathering in a tropical environment allowed precipitation of the alumina-rich minerals.  Actually bauxite is an ancient soil and Harder (1948) noted its stratigraphic importance as marking “long periods of emergence, quiescence, nondeposition, and subaerial weathering”.

Bauxite ore has been mined in Arkansas since ~1896 and reached peaks during World War I and II. Major companies abandoned mining in ~1991 but a smaller company still operates a small mine (I think).

I once collected several pieces of bauxite; however, I mostly added them to a university collection that was lacking specimens and only one small piece remains.

Bauxite specimen from Pulaski County, Arkansas collected late 1960’s. Width ~6.5 cm.

OK, here is the story that goes with bauxite.  The mineral nepheline (Na₃KAl₄Si₄O₁₆), last seen in my blog on Black Hills phonolite (9-5-13), is a feldspathoid, a fairly rare group of minerals with a very low silica content.   Nepheline is related to leucite, lazurite, sodalite and others. As I understand it, nepheline does/can not exist with free quartz in nature (as with all feldspathoids).  Rockhounds usually find nepheline in the above-mentioned intrusive rock, nepheline syenite, and its extrusive equivalent, fine-grained phonolite.  I really never understood, in my mineralogy and petrology classes, the genesis of nepheline-rich rocks!  Were they the result of a segregated magma or did they form in a completely separate magma?  Of course, I don’t understand much about even stranger feldspathoid rocks such as the leucite-bearing, lamproite volcanic rocks in the Leucite Hills of southwestern Wyoming!  A field trip leader tried to explain it to me one time in my youth.  However, I recollect it was a very hot and windy day and my mind was on a cold adult beverage waiting for the group in Rock Springs.  At any rate, the U.S. has those aluminum-rich rocks in Arkansas because of the presence of nepheline syenite.  

   

The other related story concerns the Reelfoot Rift, an ancient rift system that is currently buried under what is now the Mississippi River from about Cairo, Illinois, south to the Gulf.  The rifting of the early Paleozoic proto- North American continent was probably most active in the Cambrian.  The exact cause of rifting is not well understood but perhaps that area of the continent was situated over a hot spot—a mantle-derived plume developed along the boundary of two old Precambrian terranes (Csontos and others, 2008).   As the continent split the resulting down-dropped rift basin filled with several thousand feet of Early Cambrian clastic, non-marine sediments overlain by a thick sequence of Cambrian and Ordovician marine rocks.  Then some weird things begin to happen and these “things” lasted throughout the Paleozoic.  Although the rift seemed to fail in the early Paleozoic, the basin still accumulated Paleozoic marine sediments during times of continental “quietness” but every time some tectonic mass along the east coast (current direction and location) bumped into the continent the rift area was squeezed and somewhat pushed up.  Geologists know these bumps as the Taconic (mostly Ordovician), Acadian (mostly Devonian) and late Paleozoic Alleghenian (or Appalachian) orogenies.  So, many of the mid- to late Paleozoic rocks are missing from the rift basin (absence also partially due to later erosion in the early/mid Cretaceous).  Sometime in the mid- to late Cretaceous the rift was reactivated and marine waters reached far to the north (~Cairo) and essentially received sediments, off and on, throughout the late Mesozoic and Cenozoic—the Mississippi Embayment or Mississippi trough was born.  Paleocene (Midway) and Eocene (Wilcox, Claiborne, Jackson) marine incursions  also reached far to the north while Oligocene, Miocene and Pliocene marine rocks are restricted to more southern states like Louisiana and the modern Gulf Coast. 

 

Diagram Reelfoot Rift.  Courtesy USGS.

There are a number of faults associated with the rift system and they seem the cause of the many seismic events in the region (for example the New Madrid earthquake).  In addition, the faults allowed magma to rise toward the surface and some of that magma cooled into nepheline syenite.  So, if it were not for a failed rift my grade school geography class would not have produced a lesson on mining aluminum in the U.S.  Although this factoid may seem trivial, lessons like this whetted my appetite for learning more about rocks and minerals and eventually resulted in a career.

REFERENCES CITED

Bush, W. V., 2007, History of Bauxite in Arkansas: Arkansas Geological Survey AGES Brochure Series 003.

Csontos, R., R. Van Arsdale, R. Cox and B. Waldron, 2008, Reelfoot Rift and its Impact on Quaternary Deformation in the Central Mississippi River Valley: Geosphere, v. 4, no. 1.

Harder, E. C., 1948, Stratigraphy and Origin of Bauxite Deposits: Bulletin of the Geological Society of America, v. 60, no. 5.