New Jersey

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 Hill created?

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 oxide.

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 similar.

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 substance known.

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 volcanogenic.

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 reaction.

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 Hill?

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.