Specimen Identification... maybe.
Let's start with something "easy". I found some tiny but interesting-looking crystals in a piece of coarse-grained granite one day. Luckily, there were one or two that were well-formed enough for me to make this drawing (using a microscope). They appear to be tetragonal system... or is it orthorhombic? Note from the drawing that the crystal's cross section would not be square - it would be rectangular...
Let's skip this, uh, "evidence" for a moment. Back to the things we are sure of: they're brown, subtranslucent to opaque, have a vitreous to resinous luster, a nearly colorless streak, and a hardness greater than 5 1/2. A couple of the broken crystals appeared to have cleaved along the prism faces, in two directions. The sum of this evidence suggests zircon. Perhaps. Some of the crystals do appear to have the requisite square cross-section, but then there are others that don't. What is going on here?
I'll now answer the mystery for you: All this time I've known, with near 100% certainty, that the crystals were zircon.
Distorted or atypical crystals that don't seem to "fit" their crystal system are actually not uncommon in nature. There are "wire" pyrites, pseudocubic calcites, and many more. Pyrite is of the isometric system, leading us to the false expectation that pyrite must always be cubic-- or at least equant (e.g., regular-dodecahedral). This is not the case. To those who pay attention, it is one of the many small messages from our Creator, saying: "your limited understanding does not constrain me."
Zircon is of the tetragonal system, leading us to the equally false expectation that zircon crystals must always exhibit a square cross section. Usually, they do. It's just that sometimes one encounters a rectangular one instead. If the crystals experienced uneven pressures during their growth or plastic deformation afterward, you might not even see rectangular ones. The cross-sections could present uneven sides, non-perpendicular axes, and so forth.
Let's suppose you're collecting in a place such as Franklin, New Jersey, where there are at least a couple of hundred mineral species. Of those couple hundred, you will probably encounter at least 50, especially if you're a micromounter. Of those 50 minerals or so, there will be plenty of groups of 4 or 5 (or 10) that look exactly alike. Worse, there is plenty of overlap between the groups. As just one example, massive gahnite is often confused with diopside here; the good news is that chemical tests can distinguish them without too much difficulty. Furthermore, the crystal habits are different.
It's a bit frustrating when you don't have access to a quantitative analysis lab or expensive test equipment. There are companies that offer instrumental analysis services for "unknown" mineral specimens, but this can get expensive when one has a whole shoebox full of unknowns.
That brown to black pyroxene mineral that pops up so often at Franklin, Sterling Hill, and elsewhere: what is it? The clinopyroxenes are fairly common here, but the problem is that they all look pretty much alike.
The local names "schefferite" and "zinc-schefferite" refer to something in the clinopyroxene group which appears to be a zinc / manganese-bearing form of aegirine, diopside, augite or hedenbergite. The pyroxene-group minerals are well nigh impossible to distinguish from each other by mere sight, at least at Franklin-Sterling Hill (a clue, however, is assemblages; if yours is the same as a piece that has been analyzed, well...) You could just label your unknown pyroxene as a "pyroxene" or "pyroxene-group mineral" if you're not sure.
When I don't know exactly which pyroxene it is, it usually gets labeled "diopside" until I find out otherwise.
How can you be sure what you've found? If you get one of the "easy" ones like fluorite, calcite, sphalerite, or willemite, then that's no problem. Outside that realm, however, it's very tough be sure. Even franklinite has thrown sight-ID people; a friend of mine collected a specimen that was dull green (like gahnite) to black (franklinite), but turned out to be neither. It proved to be magnetite, a fact that lay unknown until he decided to put a magnet to it several months later.
Just remember that locality information is much more important than specimen I.D. Feel free to put a label on it that says "UNIDENTIFIED", as long as you make sure to specify the place you found it (and the date, preferably). That's good enough for me. Mineral identity can be established with tests; locality can't be recovered if you failed to write it down. Trust me, your memory will fail you on that.
It is useful to do some basic tests to narrow the identity of a mineral. For silicate-group minerals, though, streak and hardness tests are all but useless. (Actually, that's a bit of an exaggeration; they do have marginal usefulness, even with silicates). As for crystal form... What crystals??
We often expect a particular mineral sample to part or cleave neatly as we're told in the mineral textbooks; however, another difficulty with real-life specimens is that fine-grained, compact aggregates do not behave this way. A case in point is willemite; much of the time it's in compact masses that just fracture randomly. The only reason we'd have any inkling that it's willemite is because of its fluorescence. Come to think of it: Lawson Bauer was said to have a box of specimens he'd show people-- the minerals in said box having practically every shape, habit, and color-- and after visitors were thoroughly stumped for a while, he'd reveal that they were all willemite.
Borax bead tests... These were a staple of old-time assayers and mineralogists. They still work well. A bit of borax is melted and a loop of platinum wire is dipped in this. Platinum wire is ideal, because Pt is fairly inert and does not contaminate the bead with any color of its own.
The molten borax bead is then dipped in a bit of powdered mineral sample and re-heated to redness. When the bead cools, the color that results can identify certain elements that were present in the sample. "Salt of phosphorus" (sodium ammonium hydrogen ortho-phosphate, or just 'sodium ammonium phosphate') is also used for bead tests. It produces a different set of colors.
Fusibility or blowpipe tests... another favorite of the old-time experimenters. Since every mineral has a definite composition, it also has a definite melting point that can be observed and compared to other minerals. It's generally qualitative, though. In other words, is the sample easy, normal, difficult, or impossible to melt in a blowpipe flame? Perhaps an even more useful question: is there any special behavior after melting it- i.e., does the sample become fluorescent, change color, or bubble and expand? Does it decrepitate, crumble, or become incandescent?
Consider Franklin, where you can narrow down your silicate mineral to only 4 to 5 choices (and the 6th, 7th, and 8th choices which of course you never thought of). Some assemblages, though, are distinctive... even if they appear to the newcomer as a dull, drab chunk of generic rock. For example, the "petedunnite assemblage" is highly distinctive.
Associations can be the key to figuring out what you have. Maybe. For example, the petedunnite assemblage starts out looking pretty plain, but an experienced eye can pick it out right away. Once you learn to recognize this assemblage, it really does look different. With practice, you will find its appearance distinct enough that you can identify it in the field.
Crystal structure can be of great help... that is, if you can find well-formed crystals. Just because a mineral appears in your favorite book as sharp, damage-free, six-inch, doubly-terminated crystals doesn't mean you're going to find ones like that. You might find some crystals good enough to allow ID of a specimen if you use a low-power microscope, though.
Fluorescence can be a useful tool in a place like Franklin. However, for every fluorescent mineral that can be readily identified, there are probably ten that have ambiguous or no fluorescence, and no distinguishing characteristics. What should you do if this is the case?
"Expert consensus" can be wrong. On the other hand... sometimes it can be right. If somebody gives you an ID that might be wrong, be polite but try to find out how they arrived at it. Many a conundrum can be settled with a couple drops of carefully-applied HCl. The good, old fashioned fusibility test can also help.
Analysis Revisited: There's no substitute for chemical & instrumental analysis. Even the pros of sight-I.D. can make mistakes. I've seen amethyst quartz labeled as "calcite" by collectors who should have known better. One or two simple tests would have immediately distinguished quartz from calcite; the HCl "fizz test" and a hardness test come to mind first. Calcite is much softer than quartz and will bubble when treated with acid; quartz is highly resistant to most acids and cannot be scratched easily.
Although some minerals are very hard to distinguish with chemical tests, these tests provide a great way to narrow the possibilities.
I hope you've enjoyed this article.
Rock Types (More on Identification)