Colorado has seen some absolutely gorgeous weather
in Fall 2015. Although it seems
unusually dry, especially after the local weather forecasters promised us a
Godzilla El Nino wet fall, the days have allowed rockhounds to scour the
countryside for treasures. However, not
all rockhounding is created “equal.” For
example, some of my friends spent the second weekend in October near Moab,
Utah, (Yellowcat) collecting petrified woods and agates. I spent this same weekend pretty much
confined to my chair or using a cane to hobble around! In early May I had my right knee replaced and
in late September I was the recipient of a brand spanking new right hip. It appears that as I age the body parts begin
to wear out; this may reflect the many hills, mountains and streams I crossed
during my active geology field days, or it simply may be genetics! Of course I have absolutely no regrets about
that career, and was unable to really do much with my parental genetics. The good news is that I am healing and
hopeful for a complete recovery.
This time at home allowed me to read a voluminous
amount of material, including a rereading of one of my favorite authors, Bernd
Heinrich, an ecologist from the University of Vermont. In A Year
in the Maine Woods Heinrich sort of summed up my philosophy for this
summer: We all have the capacity to
wonder. We all use it. But most of us
must restrict it to the immediate things that affect our well-being. I am currently living a life of luxury
(maybe intellectually but not physically) in
that I can spend hours per day wondering about useless things, like: why is
Mr. Rockhounding the Rockies fascinated with smoky quartz, how do the hummingbirds
at my feeders find their way to Central America, how loud was the “roar” of a
dinosaur, how was stereoscopic sight developed in mammals, etc.?
So, in order to get in a single leaf peeping trip
before the recent hip replacement, I decided to take a little day trip to the
west just enjoying the scenery, the fall colors, and maybe pounding on a few
rocks. I grabbed a large coffee, a
couple of sweet rolls, water, and made it all the way to Leadville before
retreating toward home.
One of the places that I wanted to explore was the
geology in the Trout Creek Pegmatite District (TCPA) situated along US 24/285
east of Buena Vista in the Mosquito Range. I had visited the area
perhaps nine years ago hunting for uncommon minerals and always vowed to
return. Well, the years just sort of
moseyed along and I decided that visiting the area now would be an easy chore
for an old guy limping along.
 |
|
Location map showing the Trout Creek Pegmatite Area
situated east of Buena Vista. Map from
Hanson and others (1992).
|
The TCPA, at first glance, seems nothing spectacular
with much of the area exposing sections of a large granodiorite (granite-like
rock with large amounts of plagioclase feldspar) pluton(s). Hanson and others (1992), in a major
description of the pluton mineralogy, termed the unit the Denny Creek
Granodiorite. However, the Colorado
Geological Survey (Wallace and Keeler, 2003) chose not to formalize a name for
the plutonic rocks and mapped them as Early Proterozoic (Precambrian) Xgd dated
at ~1.7+- Ga. Whatever the case, there
are several intrusive pegmatites and these are the source of numerous
Rare Earth Minerals (REM) containing a variety of Rare Earth Elements
(REE). The TCPA is perhaps the most accessible
area in Colorado where rockhounds can hunt for, and collect, some fairly uncommon
minerals containing REE.
Los Alamos National
Laboratory's Chemistry Division Presents a
Periodic Table of the Elements
Period
|
Group
|
||||||||||||||||||||||
1
IA 1A |
18
VIIIA 8A |
||||||||||||||||||||||
1
|
2
IIA 2A |
13
IIIA 3A |
14
IVA 4A |
15
VA 5A |
16
VIA 6A |
17
VIIA 7A |
|||||||||||||||||
2
|
|||||||||||||||||||||||
3
|
3
IIIB 3B |
4
IVB 4B |
5
VB 5B |
6
VIB 6B |
7
VIIB 7B |
8
|
9
|
10
|
11
IB 1B |
12
IIB 2B |
|||||||||||||
------- VIII -------
------- 8 ------- |
|||||||||||||||||||||||
4
|
|||||||||||||||||||||||
5
|
|||||||||||||||||||||||
6
|
*
|
||||||||||||||||||||||
7
|
**
|
||||||||||||||||||||||
Lanthanide Series*
|
|||||||||||||||||||||||
Actinide Series**
|
|||||||||||||||||||||||
The rare earth elements (REE) are a group of ~18 elements
that are not really rare, but have strange sounding names that are tough for
many people to pronounce. On a periodic table, the REEs occupy a special line
with atomic numbers from 57-71 (the lanthanides: lanthanum, cerium,
praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, and lutetium) plus #21scandium
and #39 yttrium. Together these elements are relatively abundant and may “make
up” about 200 parts per million (PPM) in rocks of the earth’s crust. In contrast, tin is a little over 2 PPM and
silver is approximately .07 PPM. Yet in
2012 the world mined about 230,000 tons of tin while ~9000 tons of yttrium were
produced. In fact, the individual
crustal abundances of cerium, yttrium, lanthanum, and neodymium are about the
same as crustal abundances of lead, nickel, zinc and tin. What gives?
It appears the answer is concentration!
Many valuable metallic elements are concentrated in rocks of the earth’s
crust and companies have developed the ability to mine elements with only a few
PPM in the ore. The REE, although
“common,” are scattered in the rocks and are rarely concentrated in economically
mineable amounts. However, the REEs are
a valuable commodity and US companies are rapidly trying to ramp up mining
permit applications and develop new and better methods for extraction. If the country is not successful in this
endeavor, then the nation will continue to be held hostage by other successful
countries. How will we make electric/hybrid car batteries, or even cell phone
batteries, or night vision goggles?
The REM then are minerals that contain one or more
REE. The USGS (2015) noted the principal
economic sources of rare earths are the minerals bastnäsite (carbonate
containing cerium, yttrium, lanthanum), monazite (phosphate mineral containing cerium,
lanthanum, niobium, samarium, gadolinium), loparite (oxide containing cerium,
niobium) and the lateritic ion-adsorption clays. The first three minerals are
commonly found in igneous rocks of varied composition. The ion- absorption
clays are found in tropical or subtropical environments where there has been
intensive weathering of bedrock. Over
250 lateritic deposits containing REE are known and in fact these clays are an
important and critical source of commercially mined REE (Cocker, 2012).
Although Colorado has a large number of sites across
the state containing REM/REE, the TCPA has open access (I think) collecting
areas close to a major highway.
There are several pegmatites intruding into the granodiorite (the best
exposed are the Yard, Crystal No. 8, Tie Gulch, and the Clora May); many
contain REM, especially aeschynite.
 |
One of the small pegmatites exposed in the TCPA.
|
Actually aeschynite is referred to as the Aeschynite
Group and contains minerals that are a combination of various REE plus oxygen
(O), or oxygen and the hydroxide radical (OH).
One of the reasons that REE are “rare” and difficult to extract is that
the REM contain a variety of REE and lack the concentration of just a single
REE. At least that is my understanding,
and it may be a little thin! One of the small pegmatites exposed in the TCPA.
 |
|
Aeschynite-Y collected from the Clora May Mine,
TCPA. Width ~2 cm.
|
At the TCPA the major REM is aeschynite-Y with a
complex formula of [(Y,Ca,Fe,Th)(Ti,Nb)2(O,OH)6]; yttrium
is the dominant REE. The mineral would
be somewhat difficult for me to identify if I had not known the area was a major
producer. It sort of looks like slag or some
sort of a shiny metal (it is a metal).
My specimen has a metallic to submetallic luster although some pieces
may have a waxy or resinous luster. The color of aeschynite-Y is all over the
place with mine being black; however, Webmineral.com lists other colors as
brownish-black, brown, brownish-yellow, and yellow. There are some neat photos on the web showing
very yellow specimens. It is hard to
measure the hardness but somewhere between 5 and 6 (Mohs). Crystals are indistinguishable and the
mineral usually appears as massive. My
specimen is covered by small conchoidal fractures and because of the mineral’s brittleness,
a small fragment has spalled off.
As a softrocker/paleo person I am sort of baffled by
the entire REE thing, but especially their formation. For an explanation I refer the readers to Hanson
and others (1992) who noted that at the TCPA “the formation of …aeschynite-Y as
a late-stage pegmatite mineral requires the rare combination of Ti, Nb, Y ...enrichment coupled with low concentration of fluorine in the final
stages of pegmatite formation.”
There are several small mines in the TCPA pegmatites
where evidently feldspar (and perhaps other minerals) was mined several decades
ago. Of these mines, the Clora May is probably the best known. Nine years ago I hiked up to the Clora May
and found the specimen of aeschynite-Y. On
this trip I was limited to about 10 steps from my pickup and was unable to make
the hike.
Thanks to the Los Alamos National Laboratory for the use of the Periodic Table.
Thanks to the Los Alamos National Laboratory for the use of the Periodic Table.
REFERENCES
CITED
Cocker, M. D., 2012, Lateritic, supergene rare earth
element (REE) deposits: Arizona Geological Survey, Special Paper 9, Chapter
4.
Hanson, S.L., W.B. Simmons, K.L. Webber and A.U.
Falster, 1992, Rare-earth-element mineralogy of granitic pegmatites in the
Trout Creek Pass District, Chaffee County, Colorado: Canadian Mineralogist, v. 30.
United States Geological Survey, 2015: www.minerals.usgs.gov/minerals/pubs/commodity/rare_earths/
Wallace, C.A. and J.W. Keller, 2003, Geologic Map of
the Castle Rock Gulch Quadrangle, Chaffee and Park Counties, Colorado: Colorado
Geological Survey Open-File Report 01-1.
And finally in closing, back to Bernd Heinrich: I may not have noticed it, but since “growing
up” I’d been constantly taking on more and more responsibilities and
projects. To have any chance of getting
them done, I’d gradually been speeding up and finally I arrived in the fast
lane. The landscape had become a blur;
after a while, I did not really see it any more. I may still have known it was there, but only
because I recognized the cues from previous experience. Perhaps life was flashing by like a tape
player speeded up. I recognized the
sound, but I did not hear the music. Perhaps
that is why I walked off a job I dearly loved the day I turned 63. Perhaps that is why I spent so many hours
this summer doing “nothing” but looking at the mountains, thinking about rocks
(and sometimes mortality) drinking coffee, reading and sometimes doing
nothing. I do not think it has been wasted.