CR-Scientific Minerals & Earth Science newsletter

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April / May 2003
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In this issue:
I. A Slide Heater for Microscope Work
II. Mineral spotlight
III. Lab project: Pyrite, Arsenopyrite, and Pyrrhotite
IV. News, and the future



I. A Slide Heater for Microscope Work
This is essentially a makeshift device to speed the evaporation of test solutions on a microscope slide. Direct heating of a slide over an open flame can spatter the droplet, decompose the test compound, or crack the slide. Even if these don't happen, the evaporation is usually so fast that the crystals are too small to study.
Many, if not most, operations where slide heating is performed require a temperature no greater than the boiling point of water. This improvised slide heater uses steam heat to warm the slide without the undesirable effects of heating with an open flame.
The slide heater consists of a flat copper piece soldered to one end of a copper pipe fitting. The fitting itself has a hole drilled in the side, out of which protrudes a curved piece of copper tubing to serve as a vent. This, too, is soldered in place. The other end of the pipe fitting is wedged onto a stainless steel funnel and glued in place with high-temperature epoxy or stove putty. Whatever is used, it must have a working temperature greater than 100 C (212 F).
The apparatus is placed over a beaker of boiling water; it is best held in place with a ringstand and clamp. A slide with a test drop can then be placed on top of the flat copper piece and warmed at any time. See photo, below:

Makeshift slide heater
Evaporating a drop of tartrazine solution with the slide heater.

The only liquid in the beaker (the source of steam, not visible in the photo) should be water. Salt may be added to alter the boiling point, but alcohol or other liquids must not be used.
For best results, a hot plate should be used to heat the beaker of water- not an alcohol burner. It becomes too easy to forget there's an open flame with a reservoir of flammable liquid-- it's best not to take the chance. Some of the steam also recondenses in the funnel and drips back down, so it's a good idea to use a beaker slightly larger in diameter than the funnel, or else make sure there are no exposed electrical fixtures nearby... A WWI-era electric hotplate the author has (that's correct, WW One) is not a safe heat source for this because of some exposed wires underneath. Modern plates are of course sealed.
There are no assurances regarding the safety of this slide heater, but so far there haven't been any disasters with it.


II. Mineral spotlight… Monazite
Although many collectors are already familiar with the origin of monazite's name (from the Greek monazein, "to be alone"), some may find it puzzling why certain of us can become all but obsessed with this mineral. Do we go off to search for this interesting mineral because we also seek a kind of monazein - an expression, perhaps, of rarity? Who knows? Here's a brief article on the mineral itself so you can reach your own conclusions.
Truly, monazite is not all that rare, and its crystals don't have to occur alone. Furthermore, "monazite" is not just one mineral; it's currently a family of three: monazite-(Ce), monazite-(La), and monazite-(Nd). The most common of these is the Ce or cerium-predominant type, while the hardest to find is undeniably the Nd or neodymium-predominant type. Cerium-rich monazite also houses thorium in many cases, though the amount varies greatly. Enough is present in some samples that carrying them in the pockets can be unwise.
Typically, the most radioactive monazites are either in massive form or have crystals of a dull, fractured, or eroded appearance. Samples from the Betafo-Antsirabe pegmatites of Madagascar are red and have a dull to waxy luster reminiscent of some kind of rubber or plastic compound. A stick of jeweler's rouge also comes to mind. Monazite specimens the author discovered in some pegmatites in western New Jersey also have a red color; at least one collector of radioactive minerals has suggested this is due to accumulation of hematite in and around the monazite (personal communications, 1999-2000). The answer to this may be on the pages of a professional mineralogy journal somewhere.
In contrast to the radioactive, poorly-crystallized monazites are those whose crystals are sharp, translucent (or even transparent), and virtually free from thorium. Grey dolomite cast on the dumps during former mining operations at the Buckwheat Open Cut at Franklin, New Jersey can house tiny, yellow crystals of monazite-(Ce) that resemble minute gemstones. Though they're not common, there's a good chance that a persistent collector at the Buckwheat Dump can find them with the help of a 10x loupe or a stereo inspection scope. True to the mineral's name, these crystals are often alone in the dolomite. Vugs with sphalerite, albite, pyrite, rutile, hemimorphite, etc. will usually not have monazite crystals in them, though somewhere on that dump there's a chunk of dolomite waiting to teach this author a lesson about making generalizations.
The Siglo Veinte tin mines in Llallagua, Bolivia have produced tiny crystals of monazite-(La)* which analyses of the time have shown to contain no measurable thorium (Gordon, 1939). These crystals are nearly colorless in fluorescent light, but sunlight or incandescent light cause them to appear rose-pink. The Bolivian monazite crystals, like their Franklin counterparts, are usually quite small and require a stereo inspection scope or a good loupe to view. However, the rocks of this locality were also host to a few larger crystals here and there, on the order of half a centimeter or so.
Since it's a phosphate, monazite will respond to a fairly simple chemical test that's very helpful in eliminating other suspects. Dissolve a small amount of the mineral in about 1 mL of strong sulfuric acid; it may need to stand in the acid for a few hours if heat is not used. Next, prepare a solution of ammonium molybdate. Add this to the solubilized monazite sample, then add a few drops of strong nitric acid and allow to stand for some time at room temperature. A yellow precipitate of ammonium phosphomolybdate will form if phosphate is present. The only source of confusion here is that arsenate will also form a yellow complex with ammonium molybdate; however, Chamot and Mason (1940) indicate that the arsenomolybdate complex will precipitate only if the sample is heated. If both arsenate and phosphate are present in a sample, the phosphomolybdate complex will precipitate first (1940).
This has been only a cursory look at monazite from a collector's standpoint, with a superficial bit of chemistry thrown in. For more information on monazite's mineralogy and crystallography, texts such as Dana's Mineralogy are a good place to start. There is also some interesting information on the web about monazite. Much of it relates to the use of microprobe analyses to establish the age of granitic rocks using lead, uranium, and thorium content. Monazite Comparison Samples has microprobe analysis results for monazites from four different localities, including summaries of the elemental composition (Ce, La, Nd, Th, Gd, and others). This is just one example of available web articles; the search engines can yield quite a bit of interesting reading about monazite.


III. Lab project: Pyrite, Arsenopyrite, and Pyrrhotite

Supplies: Strong magnet; hammer; powdered sulfur; ammonia solution (ammonium hydroxide); lead acetate or lead nitrate solution; spot plate; crucible; dropper; safety goggles / face shield; heavy gloves; propane torch; crucible tongs; forceps; a fragment of pyrite, arsenopyrite, or pyrrhotite.

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. A small fragment of the mineral should first be pulverized with a hammer into fine particles. Using as strong a magnet as you can obtain, see if the tiny particles cling to it. A rare-earth magnet (Sm-Co or Nd-Fe) is preferable because of its high field strength. Pyrrhotite fragments will adhere to the magnet, but pyrite and arsenopyrite will not. If this test is positive, you need not perform the rest of the steps unless you suspect close associaton with the other "pyrite" minerals.
2. This step MUST be done outdoors. If no fragments adhere to the magnet, place the pulverized mineral sample in a crucible with an equal volume of powdered sulfur. Mix thoroughly and put the cover on the crucible. Heat the crucible with the torch until fumes just start to form. Do not heat the crucible too long or it will drive off the arsenic that might be present.
3. When the fusion cools completely, pour ammonia solution into the crucible and let it stand. You may warm the crucible slightly. With a dropper, take some of this solution and put it in a clean spot plate well.
4. Add two or three drops of lead acetate or lead nitrate solution and watch for a black precipitate of lead sulfide.
5. (Alternate test): warm the mineral fragments in a solution of 3 molar NaOH or KOH. Take some of this solution and place it in a clean spot plate well. Add 2 or 3 drops of lead acetate or lead nitrate solution and look for a black precipitate of lead sulfide.

Conclusions & Discussion:
These three sulfide minerals are fairly common worldwide and can be confused easily, particularly when they occur in massive form. Their "textbook" colorations don't always occur in the field; tarnish and other factors can contribute to this confusion. The can also be so closely associated that it's useless to rely on visual identification.
While some minerals require highly involved, quantitative or semi-quant analyses to distinguish between them, the "pyrites" give their identities away with fairly simple qualitative tests that can be done with minimal time and equipment.
Feigl and Anger (1972) note that pyrite does not react with molten sulfur to produce FeS, while arsenopyrite does. The latter gives off As2S3 in the process. Adding ammonia to the fusion produces a mixture of (NH4)3AsSO2 and (NH4)3AsS2O, which upon addition of aqueous lead ions will give a black precipitate of PbS (1972).
Other minerals containing arsenic with our without sulfur can in theory give reactions with these tests. Loellingite (FeAs2) is one example. The author hasn't tried the sulfur-ammonia-lead ion test on loellingite, but it and similar minerals are abundant enough at specific localities (such as Franklin-Ogdensburg, NJ) to cause some confusion. In the case of loellingite, the absence of sulfur would be proof that it's not arsenopyrite- though this is a separate chemical test. Associations are another clue, but only in a sample that's "typical" for the locality.
Some way or other, an atypical-looking assemblage always seems to surface on the rock piles or the swap-and-sell tables. Add to this the fact that well-formed crystals are found much more often in textbooks than in real life, and it becomes clear why chemical tests are so helpful in many cases.

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The "pyrite" minerals are often found in massive form. It would be so much easier for the collector to identify specimens if they all had crystals as distinct as the arsenopyrite shown above.



IV. News and Future Events
There are a couple of new articles on the site which may interest newsletter subscribers: a simple mineral test / experiment involving barite is on this page, and an enzyme assay with the spectrophotometer is on this page.
New centrifuges have arrived- they're available in the on-line catalog. The Ultra 8V centrifuge is a great all-around unit for tabletop use. It has variable speed, timed shutoff, a locking lid, and suction cup feet. It holds up to 8 tubes of 15 mL capacity.
the Ultra 8V centrifuge

There have been many inquiries about the Belomo loupe. They're unfortunately still not back in stock, and it's not certain now when they'll arrive. There has been some difficulty getting them from Belarus where they're made. As soon as they do become available again, the site will be updated. We really hope to get them back in by, say, June- there have been a great many inquiries about the loupes, and we know many customers are patiently waiting until we get them again.
We're constantly trying to improve the site's organization and layout. We're also open to suggestions for this newsletter. Your feedback is always appreciated. Feel free to comment on what you'd like to see more of, what you'd like to see less of, what you find too detailed, what you find not detailed enough, etc.


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.


Notes:

* Gravimetric analysis (Gordon, 1939) shows slightly more lanthanum than cerium; however, it's close enough that there are probably some samples that could be rightfully called monazite-(Ce). Some yttrium was also present in the samples tested (1939).


Suggested reading:

Anger, V., and Feigl, F. Spot Tests in Inorganic Analysis. Amsterdam: Elsevier, 1972.

Chamot, Emile, and Mason, Clyde. Handbook of Chemical Microscopy. London: Chapman & Hall, 1940.

Gordon, Samuel G. "Thorium-Free Monazite From Llallagua, Bolivia". Notulae Naturae of the Academy of Natural Sciences in Philadelphia 2 (1939).

Hillebrand, W., and Lundell, G. Applied Inorganic Analysis. New York: John Wiley and Sons, 1929.

Smith, Orsino C. Identification and Qualitative Analysis of Minerals. Princeton: Van Nostrand, 1953.




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