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Minerals & Earth Science
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February-March 2003
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In this issue:
I. Some interesting reading
II. Mineral spotlight
III. Lab project: A bead test for Manganese
IV. News, and the future
I. Interesting reading: "Mineral Analysis and Analysts" by M. H. Hey
The
article appears in
the March 1973 issue of Mineralogical Magazine. It's actually a
transcript of Hey's lecture and traces the development of mineral
analysis all
the way back to gold and silver assays of ancient Egyptian times. The
"simple" assays, which we overlook today in favor of procedures like
energy-dispersive spectroscopy (EDS), were at one time very important
and
useful. Before
modern
instrumental analysis, there was exclusive reliance on gravimentric
analysis
for quantizing the components of a mineral sample. The term
"gravimetric
analysis" really includes all the chemical procedures that go along
with
weighing the final sample and quantizing the elements or ions present.
The
analysts of yesterday had to have a detailed knowledge of chemistry;
knowing
how to operate machinery and interpret the readings wasn't enough. It's
appropriate that the author comments, "With the development of
instrumental methods of analysis … it is not uncommon to meet a young
analyst who has never done a simple gravimetric analysis in his life." One of the
most
interesting areas of this article is the discussion of organic and
organometallic reactions for detecting mineral constituents.
Dimethylglyoxime
(for determination of Ni) was used in mineral assays as far back as
1905,
according to Hey. He points out that organic and metal-organic
compounds such
as sodium succinate were used for mineral analysis for as much as a
hundred
years before that.The
article does explore
the more "modern" analysis methods such as atomic absorption and
electron microprobe. M. H. Hey is no Luddite, but he doesn't fully
endorse
these methods in the article- at least not as replacements for
traditional
"wet-chemical" analysis. He laments the "neglect of borax-bead
and microcosmic-salt-bead tests and of blowpipe analysis - a versatile
tool in
experienced hands." He goes on to discuss the future of wet-chemical
methods.One need
not be a
professional chemist or mineralogist to appreciate Hey's viewpoint. In
the
final paragraphs of the article he points the reader to M.H. Klaproth,
a German
mineralogist who in 1795 noted the lack of knowledge of so many
"mineralogists" and "chemists". The translation is provided
in Hey's endnotes:
One may well doubt one's own eyes when one reads how one chemist sought to dissolve a gemstone in distilled water, how a second boiled an ore in potassium ferrocyanide solution for several hours in order to determine its iron content…
Finally,
"Mineral
Analysis and Analysts" can be a real treasure just because of its
bibliography- it cites a wealth of analytical chemistry and mineralogy
works
covering a span of two-hundred years. Some are famous in mineralogy
circles,
some not- but Hey emphasizes that an "old" analysis is not
necessarily flawed, while a "new" one is not necessarily correct. Most major
university
libraries should have bound volumes of Mineralogical Magazine and
related
publications. It's definitely worth a look if you get the chance.II. Mineral spotlight… Fluorite
This issue
we'll revisit
the realm of more common (and more accessible) minerals. This time
we'll take a
quick look at fluorite.Sure, the
mineral texts
and field guides already have a wealth of information about this
mineral. Yes,
we all know fluorite's crystal morphology isn't that varied: cubes,
octahedrons, some tetrahexahedron faces here and there- maybe some
interpenetration twinning. The origin of its name and its connection
with the
word "fluorescence" are just about common knowledge in the collecting
world. Nearly every mineral enthusiast is familiar with fluorite's
formula
(CaF2), its luster, and its perfect octahedral cleavage. So
just
what is it that makes this "fluor-spar" so appealing?Some of
the reasons are
right there in the photos of your favorite field guide-or better yet,
right in
front of you. The colors! The well-formed crystals! In some specimens,
that
fluorescence-- stunning! No mineral collection looks right without at
least one
fluorite. There are quite a few collectors out there who are so
enamored of the
mineral that it has its own shelf or has even displaced most other
species from
the collection. Admittedly, a shelf of fluorites is almost mesmerizing
to look
at while relaxing on the couch: Illinois purples and yellows, New
Hampshire
greens, Blanchard Mine blues, Tennessee violets, Ohio tans, European
rose-pinks… so many fluorites, so little shelf space…Then there
was the Big
One. I saw it at a well-known rock shop in the Midwest a couple years
back. I
don't even know how two people could have carried it. A tub was barely
large
enough to contain it. It should've been out on a shelf for passers-by
to look
at, but then maybe it would've collapsed any shelf it sat on. The
person who
found this huge specimen of Illinois purple fluorite must've been
wild-eyed
with excitement… you know the feeling if you've ever uncovered a "big
find" (a relative term, admittedly). It doesn't have to be at a mine or
rock cut, either; a nice fluorite can exert a similar power from the
far side
of a crowded mineral show. Fluorite
has
fascinating, hidden sides too.While
hammering on some
rocks at "The Buckwheat" in Franklin during one of the Franklin
Mineral Museum's night digs, we found some that contained the variety
of
fluorite known as "chlorophane". What was especially captivating that
night was its behavior under the hammer: every time the rock was struck
it gave
out an eerie "mist" of glowing aqua-green light. A 16-lb sledge made
a luminous but fleeting cloud of color that lasted no more than a split
second.
Was it triboluminescing dust ejected from the rock? Was it from
thermoluminescence caused by localized heating at the hammer impact
site? We
weren't sure, but it was beautiful. Fluorite from some localities is
even known
to emit small traces of uncombined fluorine on breakage- this extremely
reactive gas instantly converts some of the air's oxygen into ozone,
which has
a characteristic odor.While much
of it
wouldn't be news to today's professional mineralogist, the earth
science
literature of the past few decades houses quite a bit to interest the
teacher,
amateur mineralogist, or mineral collector.In Volume
13 of Geochemistry International (1976) there appears an
article
by two
Russian Mineralogists who surveyed the rare-earth element (REE) content
of
fluorite from various localities. Since the article was written in the
days of
the former Soviet Union, one can guess why the authors' itinerary
didn't
include sites outside the USSR. Nonetheless, the enormous area of the
former
Soviet Union has a vast wealth of rock types and mineral resources,
which
evidently proved more than adequate for the study.The
authors of the
article, Ganzeyev and Sotskov, determined that the amounts of trace
REE's
corresponded with the conditions under which the fluorite crystallized.
As one
might expect, fluorite from granite pegmatite occurrences had the
highest REE
content, followed by that from alkaline intrusive formations. Basaltic
fluorites had the lowest; a little higher was fluorite of
low-temperature
hydrothermal origin. Overall, the trace REE's encountered in the
highest
concentrations were ytterbium (Yb), cerium (Ce), samarium (Sm), and
lanthanum
(La), in that order. Also found in fluorite were europium, terbium,
lutetium,
and strontium (not a REE, but tested for anyway). Fluorite's
own chemical
properties are evident, at least in part, with tests that don't require
complex
machinery to perform. One procedure in particular is dangerous because
it
produces hydrofluoric acid (HF), and so it is presented only for
discussion
purposes. A piece of fluorite placed in a glass test tube, covered with
concentrated sulfuric acid, and then heated gently will evolve HF.
(Again:
because of its danger, this should not be attempted, except by a lab
instructor
or chemist - and in a fume hood, for that matter.) The HF in turn will
attack
the glass, etching it. A sufficient quantity of HF will dissolve the
glass
entirely. The amount produced from a tiny crumb of fluorite won't
breach the
test tube wall, but it will ruin the test tube and any other glass it
contacts… not to mention what it can do to the experimenter! A danger
of
HF is that it rapidly penetrates the outer, dead layer of human skin
that
normally protects us. It then causes severe damage to living tissue,
often
before pain is even felt; it is also highly poisonous in addition to
its
corrosive properties.Fortunately,
fluorite
itself is very stable under normal conditions and presents no chemical
danger
to the collector. Its corrosive little secret can remain locked away
safely in
an ionic vault as those pretty crystals sit upon their shelf and exert
their
charms. If fluorite weren't so stable, in fact, it might not occur in
the wide
variety of rock types and environments that it does. Even the
sedimentary rocks
of North America's geologically stable interior can contain fluorite
specimens
(just as with calcite, discussed in issue #1). If fluorite were
chemically
unstable or easily taken up into solution, its fluoride anions when
liberated
would "grab on" to the nearest metal cations and be re-deposited all
over the place. Who could even guess how toxic the groundwater would be
with
all the fluoride ions it would then carry? If seawater contained as
much NaF as
it does NaCl, "sea salt" certainly couldn't be sprinkled on
foods… Calcium
fluoride's
near-insolubility is unusual, especially among the alkali and
alkaline-earth
metals (Na, K, Mg, Ba, etc.). Obviously, the right conditions allow for
some
dissolution of CaF2 (otherwise we'd have no hydrothermal fluorite
deposits),
and it's true that some areas have naturally high fluoride ion
concentrations,
but think of how much fluoride is locked safely in the countless tons
of
fluorite buried in the earth. As you can
see, there's
a lot to know about fluorite… yet we've left out quite a bit; this has
been only a superficial look! Hopefully it will prompt you to
investigate more
about this fascinating, beautiful mineral.Update:
a
reader pointed out a property of fluorite that we'd forgotten to
mention; while
other minerals will either melt or simply not respond to the torch
flame, a
small piece of fluorite will decrepitate violently, sending fragments
in all
directions. It makes for an interesting demonstration if everyone in
the area
has eye and face protection... III. Lab project: Qualitative test for manganese
Supplies: Safety goggles / face shield, heavy gloves, platinum wire set in glass handle (available from our on-line store at http://www.crscientific.com), sodium carbonate, potassium nitrate (or sodium nitrate), a manganese-containing mineral (a tiny piece, ca. 5 mm long), propane torch, metal tongs / forceps.
General procedure: As with all experiments, you alone are responsible for safety. Protective eyewear and leather / canvas gloves must be worn. Do not perform this experiment near papers or other flammable materials. It is best done outdoors.
1. Crush a
small piece
of the manganese-containing mineral sample. If it is suspected to
contain
arsenic or arsenates, it must be roasted on charcoal with the oxidizing
flame
to volatilize all "As" before the sample is allowed to contact
platinum wire. Do not inhale the fumes produced!2. Combine
the crushed
(or crushed and roasted) sample with approximately 4 times its volume
of
Na2CO3 and add a small amount of KNO3
or
NaNO3. Mix very thoroughly.3. Heat
the Pt wire loop
in the flame briefly. This will burn away skin oils and other
contaminants
which may contain sodium ions. While the loop is still hot, touch it to
the
mixture in order to pick up a bead-sized amount.4. Put the
Pt wire loop
back to the flame until the mixture fuses to a glass. Observe the
resulting
bead color. If it is bluish-green , especially where the bead contacted
the
platinum wire, this suggests manganese; continue to step 5. If the
coloration
does not appear at all, manganese is not present. If the bead is black
or
otherwise too dark to discern the color, try the test again with a
smaller
amount of mineral sample relative to the bead.5. When
the fusion
cools, remove it from the platinum wire loop and pulverize it finely.
Place
this powdered material on the spot plate or in a test tube. Add enough
dilute
acetic acid or HCl to dissolve the solid. If manganese is present, the
solution
will turn reddish-violet and then a brownish precipitate will form on
standing.Conclusions & Discussion: The simple procedure discussed above are useful for determining the presence of manganese in a mineral sample.
Manganese
occurs in
nature most commonly as either Mn+2 (as in rhodochrosite,
MnCO3) or Mn+4 (as in pyrolusite, MnO2).
The
Mn+3 state also occurs in minerals fairly often; examples
include
braunite and hausmannite.Potassium
nitrate
releases oxygen when heated. This in turn oxidizes the manganese to a
higher
(colored) state through chemical combination with Mn to form
"manganate" ions. The
greenish coloration
obtained upon heating the bead suggests oxidation to Mn+6,
as
encountered in the manganate (VI) ion (MnO4-2).
The sodium carbonate used in the bead provides an alkaline environment
that
lends stability to Mn+6. Dissolving the fusion in dilute
acid lowers
the pH to where this oxidation state of manganese is no longer stable.
Some of
the manganate (VI) is oxidized to Mn+7 as manganate (VII)
ion
(permanganate, MnO4-). The rest is reduced to
Mn+4, which would explain the brownish precipitate. This
process of
simultaneous oxidation and reduction of an element is called disproportionation.
The presence of Mn+4 (as
manganese
dioxide) then catalyzes further decomposition of MnO4-;
if let stand long enough, nearly all of the permanganate will decompose
to
manganese dioxide.As in
other bead tests,
there are likely some unidentified parallel reactions and highly
unstable
intermediates during the heating phase. There can easily be more than
one redox
"couple" happening at once. These unforeseen "extra" redox
couples can make it tough to predict outcomes, especially in an
environment
where flame is applied to something like potassium nitrate and a
mineral
sample. The more complex a chemical environment, the larger the variety
of
possible reactions - consider, for example, the destructive
distillation of
coal (a discussion we can save for a future issue…) Even if we
can only make
conjectures about the chemical mechanisms, we've explored an
interesting,
"classic" test for manganese that can be done with very simple
equipment. Update:
One very simple, additional test I hadn't mentioned is specific for
MnO2 or pyrolusite; throwing a piece into hydrogen peroxide
will
produce bubbles of oxygen from the catalytic decomposition of
H2O2. While there are other compounds that do
this,
pyrolusite is the only commonly-occurring mineral I could think of that
should
give this result.IV. News and Future Events
Thus far,
the newsletter
has been almost entirely mineral-related. we've received requests for
microbiology and microscopy articles, and we plan to add some coverage
of this
in future issues of the newsletter. We'll see how it goes- if the
microbio /
microscopy ends up branching off into a separate newsletter, then we'd
consider
making separate sign-up sheets for the mineral and for the bio /
microbio
lists. Alternatively, we may include links to new articles in the
newsletter
and put the articles themselves on the website to save file size. While some
people enjoy
a very narrow field of study and all the specialized knowledge that
dwells
therein, others find science generally so enthralling that they can't
help but
dabble in several disciplines at once. (This is a fair description of
yours
truly...) Besides, there's no shortage of connections between biology,
chemistry, physics, and mineralogy. As for the
website (http://www.crscientific.com),
it has a
number of new items available, including a changing inventory of
mineral
specimens and a growing list of laboratory ware. One fairly recent
addition to
the site is the Alpha
1501
fiber-optic illuminator; we've personally tried one and are very
pleased
with it. The Alpha 1501 is a great aid when viewing minerals under the
stereo
inspection microscope… its "cold" light gives no danger of
shattered crystals that can occur in the heat of a desk lamp.The
CR-Scientific
website also has new articles and a growing list of simple experiments.
We're
planning to archive past issues of this newsletter on the site as well.
Finally, we are constantly trying to improve the site's organization
and
layout. Your feedback is always appreciated. That concludes this issue of the CR-Scientific newsletter.
Until next time, stay safe and have fun.
Email: sales_AT_crscientific_DOT_com
(replace the "_AT_" with an @ symbol and the "_DOT_" with a period. In order to thwart spam-bots, I'm no longer providing a direct link or easily-harvested address.)
While the information in this newsletter is thought to be accurate to the best of the writer's current knowledge, it is not guaranteed to be free of errors or to be suitable for any particular use. The procedures and experiments outlined within can be dangerous or even fatal if carried out improperly. If you choose to attempt any of them, you proceed entirely at your own risk.
You may distribute this newsletter freely, provided the contents of the file are not altered in any way. Please email the address given above if you find any errors or omissions or would simply like to make a suggestion.
Articles Index
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Suggested reading:
Ganzeyev, A.A., and Sotskov, Yu. P. "Rare Earth Elements In Fluorites of Different Origin". Geochemistry International, 13: 51-56 (1976).
Hey, M.H. "Mineral Analysis and Analysts". Mineralogical Magazine, 39: 4-24 (March 1973).
Smith, Orsino C. Identification and Qualitative Analysis of Minerals. Princeton: Van Nostrand, 1953.
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