FMF - Friends of Minerals Forum, discussion and message board

[Twigg]

I'm not sure if this is the right place for this.

I have been trying to learn to identify minerals in self-collected rocks from my yard, but it's a lot more difficult than I thought. Just an aside, I've made amateur attempts to learn to identify plants, insects, mushrooms, and more but nothing else is as thoughtful and subtle as studying rocks. I imagine I'm learning the concepts of mineralogy and geology from the wrong starting point, but I'd like to know what I'm seeing more than what I should be.

Out of around a hundred rocks, I feel confident that I have properly identified less than twenty rock types and (more important for this forum) about four minerals. I think my main troubles may be related to concepts and definitions about crystals and mineral properties (which I hope I could get some help with here). I keep asking myself whether I am considering the right data points to make such a claim about the rock/mineral or whether I am focusing on the right details and such. Introductory guides to petrology and mineralogy seem to focus on only the simplest aspects and examples, and the advanced ones seem to be more like what I'm looking for (in that I feel like the author knows exactly my questions) but take a very intensive bottom-up approach that doesn't help me much.

Specifically, how does one confirm that they are observing an actual crystal in a rock? How do you know if you are looking at a sample whose crystallography is disguised by its habit? For example, I have one rock with a matrix that forms an almost perfect shape symmetrical on one lateral axis. It is convex with five lateral faces. Two faces come to an acute edge at the "top," opposite another face on the "bottom." The two top faces each joins with a "side" face at an obtuse complementary angle. The two "sides" are parallel and join with the "bottom" face at right angles. All faces come to a point at one end and join with an ill-formed, stubby replica of the shape on the other end. Thus, the plane intersecting the "top" edge and the lateral bisector of the "bottom" face is the only axis of symmetry. I'm tempted to think of it as a crystal because of this clear geometry, but it appears to be made of grains. What observable characteristics distinguish a crystal from a coincidence, or is the geometry enough to conclude that the sample is crystalline? I'd think that's not true, because minerals are cut into all sorts of shapes for jewelry.

Secondly, another question about crystals. Let me give a hypothetical situation. You have a sample of an unknown mineral in a matrix. On close observation, you find a few faces and an edge or two that look like they could be part of a well-formed crystal. The matrix (or intergrown crystals) obscure the rest of the crystal. What do you do (or maybe I should ask, can you do) if you're bent on identifying this unknown?

That's all I can think of now, but if I have more related to this, I will post them here in replies rather than make a new thread. Thank you guys so much in advance.

[Pete Richards]

I'm sorry to say it, Twigg, but you're trying to tackle perhaps the most difficult aspect of specimen geology for a new-comer to handle. Rocks are by definition made up of grains of minerals. Minerals are naturally occurring chemicals with a specific chemical formula (ignoring some subtleties). "Grains" of a mineral are individual crystals.

The grains of minerals in rocks tend to be randomly oriented, though sometimes there is a preferred orientation. Most of the grains in most rocks will not have any discernible boundaries corresponding to the external form of crystals that grow into a void - the minerals that most of us collect most of the time. Even if a mineral grows with crystal boundaries, for example a feldspar growing early in the solidification of a granitic magma, when you see the product in your back yard, you can't tell from the surface what the orientation is of the view that happens to be present, so imagining the shape of the entire crystal is very difficult.

Most grains of minerals in most rocks are too small or too confined in the rock to do many of the standard tests we do to try to identify minerals - cleavage, hardness, streak, etc. So getting any information is difficult.

Petrologists study rocks in thin section (thin slices of rock about the thickness of a piece of paper or thinner) that allow light to come through, and they use all sorts of advanced microscopic techniques involving polarized light and mineral optical properties to study hundreds of grains in the rock, and can thereby identify the minerals present. Modern sophisticated technologies like x-ray crystallography, scanning electron microscopy, and electron microprobe studies provide valuable information about chemical composition and crystal structure. But few people have access to such technology.

So, sadly, you may be able to identify the occasional mineral in the occasional rock, but many will be unidentifiable without at least a microscope and some considerable patience, study, and cleverness.


On the other hand, rocks in a particular region tend to be the same, so a geologist at a local college might be able to identify many of them for you, based on the regional geology. But if the glaciers brought most of the rocks you find, the "region" from which they came could be very large, and positive identifications may be more difficult.

[Pete Modreski]

You are right, Twigg, "it can be a lot more difficult than you thought" to identify rocks and minerals that one finds in the field.

There is just no quick and easy guide or method to doing so; it is simply going to take a fair amount of experience and knowledge gained thereby, to be able to recognize and identify minerals with confidence. You can do this by a combination of reading and browsing through any field guide to rocks and minerals, plus viewing actual specimens whenever you have the opportunity, in museums and at mineral shows or dealers' shops.

I guess one could say, that it is because of the considerable variety and diversity in color and appearance of each given rock or mineral, that they can be so hard to identify; unlike living species of plants or animals, which "by their very nature" will always conform to their own particular size, color pattern, and other characteristics.

However, once you aquire some familiarity, you will realize that your task is simplified by the fact that only a limited number of minerals and rock types are going to be among what you encounter locally. Even in an area with glacially transported rocks (probably the most diverse situation one can encounter), you will find that granite, gneiss, quartzite, sandstone, limestone, and metamorphosed basalt will likely comprise the bulk of every "rock" that you find; and likewise with minerals, quartz, feldspar (most of which commonly is microcline), muscovite and biotite mica, and hornblende will constitute the bulk of what one finds. If you think you've already been able to identify as you say "20 rock types and 4 minerals", you are really doing quite well and have probably identified the bulk of what is present around you.

Something you'll soon learn, is that most of the "special" minerals (the other 4444 besides the 4 you have so far recgonized) are not just found lying about the ground wherever you might be, but are found in particular, local places which become known to mineral collectors, and are not likely to be found unless you follow someone's published or orally passed on directions to places of particular interest as a "mineral locality". Even for a relatively "common" mineral such as pyrite--if one just started out on one's own and scoured the local plains and mountains looking for pyrite, you might spend days or weeks or months in the field before you necessarily found any! But by following directions to some local abandoned mine or notable mineralized road cut, in most areas a collector could probably go right to, and collect specimens of, pyrite with little difficulty and without traveling any great distance.

And, just to agree with what Pete Richards said and with what you yourself have been observing, most minerals that one finds are impure, intergrown, with poorly developed crystallization, and are just not very easy to recognize by the "textbook" criteria. But again--you are helped by the fact that 75% of them are going to be quartz, and 75% of the rest are going to be feldspar!

Best of luck to you,
Pete Modreski