Interview with an expert
Interview with an Expert
U-Haul spoke with Paulus B. Moore, Ph.D., Professor Emeritus, Department
of Geophysical Sciences, University of Chicago, for his insight into the
unique mineral deposits at Franklin and Sterling Hill. Dr. Moore spent
32 years at the University of Chicago and was one of the youngest faculty
members ever to become a full professor at the university. Dr. Moore's
passion for minerals began when he was 7 years old.
U-Haul: How was the unique mineral deposit at Franklin-Sterling
Dr. Moore: To begin with, I must explain that in mineralogy (mineral
chemistry) we have to work backward from the minerals we culled from a
locality to determine their origin. I like to think of a mineralogist
as a natural inorganic-products chemist. He essentially works with natural
substances, that is, crystalline substances that are formed independent
of man, that is, formed independent of conscious intervention.
Now, the problem with mineral chemistry is that we have the crystals,
the minerals. But we don't really have anything that came before
them. We have to infer that. So that means that we are making assumptions
along the way. Now this is
basically the risky part. We don't have all the evidence,
especially the origin of these things. But from the chemistry of the minerals,
the chemistry and the texture of the assemblage that is the marble that
envelopes the ore deposit, and of the texture of the ores themselves,
we can come to certain limiting conditions, occasionally even conclusions.
To me, it seems that the origin of what I call the proto-ore or the proto-marble,
what came before the big event of crystallization, was sedimentary, because
everything we see (it's remnant in what we see today), is in the
form of bands, everything is banded. We see the ores themselves are banded,
even the marble and the accessory minerals in the marble form bands. And
the best way to explain this is to assume you have something like a shallow
sea or a basin where, over time, you had precipitation of material. And
sedimentary rocks almost invariably are banded, because of the fact that
you get precipitation of material in time, and the chemistry also changes
somewhat in time.
We essentially have three major metallic elements and they're all
elements of the first transition series. The most abundant element is
iron, and next to that is zinc and next to that is manganese.
U-Haul: How did we get a combination of iron, zinc and manganese?
Dr. Moore: The most recent theory, which is the one I like, is
that you had a volcanic source and the volcanic source spewed out rather
high concentrations of fluids with manganese, iron and zinc. One common
example we have today is deep-sea vents on the ocean floor. That seems
to me to be a reasonable model, the rift model. Remember that the
Franklin Marble is 1 billion years old and evidence suggests that
1 billion years ago the whole area was a rift valley, possibly a bashing
of two continents together. As a rift valley, you probably had a lot of
volcanogenic activity with the spewing out of manganese, zinc and iron
U-Haul: How do you get these tremendous concentrations of ore?
Dr. Moore: These ores were not only rather tightly banded, they
were tightly bound together in a kind of hook-shaped mass which dipped
down about 60 degrees to the east and went for a depth of about 3,000
feet; something like 2,500 feet across and 60 feet in thickness. And that's
an incredible mass of ore -- and for both deposits, they're very
One way, and again this is all inferential, but so far I haven't
been able to find any contradictions, is that you had all this stuff spewing
out into a shallow sea or a lake. How do you concentrate this into an
ore deposit, because the ore deposit clearly is not evenly distributed
throughout the whole Franklin Marble? Well, further winnowing or concentrating
by organisms. We had microorganisms
about one billion years ago and one way they got energy was by exploiting
transition metals. Transition metals are the ideal way of getting
energy through electron transfer. A good example of this is our blood
hemoglobin (iron). Hemoglobin is in fact the transport mechanism for oxygen
and that involves iron, a transition metal.
Here we have manganese, iron and zinc, three transition metals and they
were concentrated by these organisms. You could imagine the situation
where you had a local concentration, a rather huge quantity of zinc, manganese
and iron oxide. Then it's just a matter of taking this proto-ore
and shoving it down a Benioff Zone, a collision of two continents where
one continent goes under the other. And if you go down deep enough, lets
say 20 to 30 kilometers, that's going to be heated up to high enough
temperature and receive enough pressure that everything will recrystallize
to form a rather high-grade rock. In fact the Franklin Marble is a high-grade
coarsely crystallized rock.
From external laboratory evidence we have been able to get numbers. We
know that the Franklin Marble crystallized somewhere between 600 and 800
degrees centigrade at pressures between five to six kilobars. So this
seems to make good sense. That's about roughly the distance down
where it probably originally was. Then, owing to calcite being less dense
than basalt, it rebounded back up toward the surface, and since then it
became more and more exposed by erosion with time and was discovered as
these deposits, probably back in the days of the Lenni Lenape Indians,
even before the white man was there.
U-Haul: Why do we have so many minerals, about 350 species?
Dr. Moore: First of all, one thing we observed is this: most of
the minerals of Franklin-Sterling Hill are low in water (usually hydroxylated
only) and this is an important tip-off. It seems that in the process of
recrystallization this proto-material was desiccated so much that there
wasn't much water remaining and the conditions were almost anhydrous.
The major ores and calcite are anhydrous. And so what we did have was
just enough water present, which in the final squirt when everything was
consolidated, at lower temperatures probably -- you're talking only
about maybe no more then 1% or one tenth of one percent of total mass
consisting of water --percolated through cracks in the system and hydrothermally
(water at temperature, as in a pressure cooker) reworked the minerals
that were already there through back reactions.
This started, let's say, around 500 degrees and then gradually cooled
down, to 400, then to 300 degrees and so on. So, going down you get a
succession of crystallizing minerals. Minerals -- their fields of stability
in pressure, temperature and composition -- are very dependent on these
factors. And if you change the pressure
and temperature a little bit, you get a different mineral. The
most famous example of this is diamond. If you take graphite, which is
one of the softest substances known (graphite pencils!) and if you squeeze
it under high enough pressure you get diamond, which is the hardest natural
If you have this process continuing over a long period of time with an
unusual chemistry involving manganese, zinc, iron and some other minor
elements, elements which commonly occur from volcanic sources, it reinforces
the argument that perhaps with almost certainty the original source was
I have come to some very strong conclusions about the condition under
which these minerals formed, from the standpoint of their crystal structures.
Most of the minerals at Franklin, which I call secondary or hydrothermally
reworked minerals, occur in cracks or fissures. They occur as little crystals
and they're highly esteemed by collectors. These are the minerals
that collectors go gaga over. These are the crystals that really made
Franklin famous from the standpoint of the amateur and professional mineralogist.
Some of these species are unique to the deposits, names like holdenite,
roeblingite, mooreite, gageite, chlorophoenicite, and many others.
Well, one interesting thing about these minerals is that they occur in
these crack systems. I have looked at their structures in great detail
(work I've done and work other people have done) and have come to
the conclusion, on the basis of how the atoms are arranged, that these
structures could only have formed under highly basic or alkaline conditions.
They could have only formed under a brine, which was so basic, like lye,
that if you put a body in it, it
would dissolve in an hour or two. That's what we are talking
about, strong base not acid. And this, to me, is the key as to why Franklin
is so unique -- that so many of these minerals have formed only under
the basic or alkaline conditions.
The outer crust of the Earth is acidic; just as with our atmosphere,
everything oxidizes, rusts. We rust, everything rusts in time. So, we
have an acid environment; oxidation leads to the formation of acids. Bases
or hydroxides are in many respects the opposites of acids; they are unusual.
Basic environments are rather unusual on the surface of the Earth, another
reason why Franklin is so interesting.
We're talking about a very strongly basic environment. It is very
unusual. In fact you may ask (this question is often asked) and of course
I wracked my brains over it; why is it so basic? I mean how do you get
such basic situations? It's easy to understand acid situations: you
only need oxygen. But how do you get such extremely basic assemblages?
Well, I have a mechanism for that: intracrystalline auto-oxidation reduction
U-Haul: What is intracrystalline auto-oxidation reduction reaction?
How does it work?
Dr. Moore: Intracrystalline means within the crystal; auto-oxidation,
that's the self-oxidation of the iron; reduction, that's the
splitting off of a hydrogen atom. What this means is that there are certain
mineral structures, certain mineral species, that if you heat them up,
the metal's valence state in the mineral changes. It's usually
iron, for example. In this case let's use ferrous iron (Fe2+)
as an example. If we take a ferrous iron mineral, a water-bearing ferrous
iron mineral, and we heat that up, the iron oxidizes from a ferrous state
to a ferric state, but the structure remains intact. How can the mineral
do that because the metal ion charges changed? Well, the way it compensates
is for a water molecule to split off a hydrogen atom so you have Fe2+
going to Fe3+. H2O going to OH-.
There you have a perfect balance. See what I mean, Fe2+
plus H2O, you still have Fe2+ because
H2O is neutral. But Fe3+ plus OH-
leads to the same charged balance (2 + 0 = 2, 3 + -1 = 2). Owing to the
presence of the water molecule, we would expect such a mechanism to occur
at only relatively low temperature.
U-Haul: And how did this reaction play a role in the formation
of the mineral deposit at Franklin-Sterling Hill?
Dr. Moore: The hydrogen that splits off during the reaction streams
out as a gas, and hydrogen is a
very powerful reducing agent. And all you need to have is hydrogen
gas bubbling through these cracks, bubbling through the fluids and it
will react to reduce more oxidized minerals around, forming very basic
assemblages. For example, native lead or lead metal, is known from these
reduced veins at Franklin. Remember acid implies oxidation, base implies
reduction. Things get reduced in basic environment and they get oxidized
in acid environment.
So what we have is a very basic assemblage. We have a vast number of
species. My interpretation of that is the cooling process. The lowering
of temperature and pressure probably occurred over a long period of time.
It was a relatively slow process. It started at a fairly high temperature
and a fairly high pressure to begin with. And so what we got was a whole
sequence or paragenesis (associations in time and space) of coexisting
minerals with time. But once they were formed, they couldn't go back.
They were formed; they were stuck there. It took too much energy to rearrange
them and form minerals that would be stable in those later conditions,
so they just remained behind as metastable entities. That's why we
have rocks. In most of the rocks on the surface we have today, the minerals
are out of equilibrium. But the reason they remain behind is it takes
too much energy for atoms and bonds to rearrange to be where they should
be at lower temperature and pressure. We don't see what they presently
should be; we see the final phases, formed at some higher temperature,
which just can't rearrange at lower temperature.
U-Haul: What makes the fluorescent minerals at Franklin-Sterling
Hill unique and so sought after by collectors?
Dr. Moore: Well, there are certain minerals at Franklin which
have become famous in their own right for their intense fluorescence response
to short-wave ultraviolet radiation. It is well-understood today that
the major activator, what essentially
causes this fluorescence effect, is not usually a major element
but an element that occurs in small amounts, anywhere from one tenth of
one weight percent to maybe a few weight percent. The presence of that
element can lead to fluorescence response. The element which has been
implicated in the vast majority of minerals at Franklin-Sterling Hill,
which causes their fluorescence, is manganese. Mainly manganese (2+),
divalent manganese, or Mn2+.
Here are some examples. Willemite; willemite's beautiful green fluorescence
is due to manganese (2+). The smaller the amount of manganese you have
present, the more intense the fluorescence. In fact if you have about
one tenth of one weight percent of manganese present, the willemite actually
looks colorless. But, it not only fluoresces intense green, it also phosphoresces
green. Even when you turn the lamp off, the thing still emits green radiation.
This is often the case with fluorescence. If you have a very tiny amount
of activator, it gives an even stronger response. You might say why is
that so? It's a phenomenon called quenching. If you have enough atoms
of this activator present, the electrons bounce around as on a pool table,
the fluorescence dissipates and get lost, so to speak, and never make
it out to produce its visible response. Certain elements themselves are
notorious quenchers, iron for example. The presence of a few percent iron
usually destroys fluorescence.
Another example is calcite. Calcite at Franklin contains about one or
two weight percent on average of manganese. Calcite fluoresces a beautiful
red to orange color. The most famous one probably is esperite, a lead-calcium-zincosilicate,
which fluoresces a lemon yellow. There are probably about two dozen or
so examples of Franklin minerals which have intense fluorescence as the
result of manganese (2+).
U-Haul: Why are people so interested in studying Franklin-Sterling
Dr. Moore: Because of its uniqueness. I believe that human
curiosity and the fascination of the natural world are the only things
that count. The serious-minded human being, the scientist (amateur
or the professional), is always motivated by the unique. It doesn't
matter what it is. The driving criteria which has made Franklin-Sterling
Hill worth studying, right from the beginning, is the unusual mineralogy
-- the uniqueness of it all. The second thing is the fluorescence; that
came later. And the third thing, not to be sniffed at, is that some of
the minerals are downright beautiful. There are some very attractive minerals
that did occur at Franklin, especially manganese-bearing minerals that
tend to be rose, flesh pink and purple in color. But most of all, bear
in mind that only a tiny fraction of humans have any interest in Franklin-Sterling
Hill. It is hardly a national pastime.
U-Haul: What have we missed?
Dr. Moore: I can't overemphasize how important it is to preserve
the historical past of the area, the importance of it. There aren't
too many mines or miners left. There are two museums, two good museums
that are doing this very well, one in Franklin and one in Ogdensburg.
And research must continue, as evidence toward understanding the curious
origin of these deposits is still far from complete.