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!
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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.
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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!
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.
