FMF - Friends of Minerals Forum, discussion and message board

[Robson Vieira]

Hello there. Now I bring an ugly chunk of rock from a gabbro quarry. The piece has an interesting kind of a crystal.
Color: greenish/yellowish gray. The luster can show golden metallic iridescent patterns
Luster: vitreous and pearly at some faces
Streak: yellow to gray
Hardness: 3 to 3,5 cause it was scratched by a fluorite chunk
Cleavage: good in two directions
Crystal form: elongated but with a fibrous behavior as well
Density: not possible to measure.
The crystals show golden iridescent colors at some faces and beautiful cats eye effect as well.
First of all, I thought it would be a pyroxene or a feldspar, but the cleavage and hardness doesn't match. Now I would say enstatite var. bronzite but the hardness is lower as well. What do you think?

[Bob Carnein]

When testing hardness, it's a good idea to try what I'd call "reciprocal" hardness. What I mean is, try scratching the unknown mineral with something, then try scratching the "something" with the mineral. If your sample is fibrous, the hardness test with a chunk of fluorite may just be breaking bits off both samples, rather than truly scratching the sample. If you can, see whether the unknown mineral scratches fluorite or calcite (and I'd also try a knife or steel needle). Also, if it's fibrous, you won't get an accurate hardness for the same mineral with a non-fibrous habit (can you scratch bits loose with your fingernail?).

[SteveB]

Just to further Bob's comments, It's worth while purchasing a set of hardness scribes. You'll get a consistent experience that you don't get using pieces of materials as the reference, since you can never be certain the piece you choose is uniform in hardness at the contact point. Using alternatives is alright in a pinch as just one data point to help your identification, but in itself it's never proof though. Something else I'd suggest is pop to a hardware store and pick up a small bottle of hydrochloric acid and if you can find a small plastic bottle with a top you can squeeze a drop out of (NOT a bottle with eyedropper and rubber bulb as the acid will eat it.) Put 50/50 water and HCL in the bottle and you have a good safe pocketable tool to take with you into the field or conduct a safe acid test at home. That will give you another good data point to help with identification, just note the reaction over time and under magnification. some minerals fizz rapidly immediately, some don't appear to but generate bubbles after a while.

Identification is a scientific process of gathering data to eliminate possibilities, not gather a few data points and make an accurate conclusion. You've done better than many others expecting a blurry photo to provide an ID. It may also require time to make a conclusive determination, so keeping notes of what you've done to test and the results etc. is a good idea. And something you can add to at any time to refine and improve accuracy. for example when you can afford or borrow a good hardness scribe set you can note the hardness you measure from that and learn if your alternative technique is good or not. When you have a mineral in mind you can research it in regards to tests you can undertake, so maybe you think its mineral X which reacts fast to hydrochloric acid but yours show no reaction at all then it can't be mineral X. Plus there are many varieties, as minerals are altered in their environment so you may be close but its a lesser known variety. Though crystals tend to be the pure form so test results are pretty good.

When you know for certain where the mineral came from you can research the location, say with Mindat, and look into the listed minerals for that location as possibilities to investigate and test against what you have. It can be a tedious process but very easy and interesting to go through. Just be patient and totally push out of your mind thinking you have a diamond, or emerald or whatever valuable mineral. It may be, but until you eliminate the more common alternatives the doubt is always there. Also location references don't always show every single mineral in that area, it depends on the research and source data they used.

Sorry I can't help with this specific crystal, but the above will help you in the best direction.

cheers

[Robson Vieira]

Thanks Bob, your idea is really helpful. I'm in doubt about the mineral exactly for the hardness data, because the other information about the sample could conduct to a final conclusion. I'm going to test as you have stated, even though is difficult to test hardness in such kind of material, because the crystals are very small.

Thank you again

[Robson Vieira]

Yeah Steve, it makes sense. A hardness scribe would be helpful, because these kinds of pieces are hard to test, and the scribes have the essential: accuracy.
I tend to avoid home materials, like knife blade and copper coin, that may have not homogeneous physical properties as reasonable scribes would have.

I'm grateful for your comment.