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Guidance rather than identification
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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AngelL




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PostPosted: Dec 31, 2021 14:25    Post subject: Guidance rather than identification  

Good day everyone. My ignorance is legion but I'm working on it. I have spent some time working toward identification of a sample and have run into some difficulty. I am not looking for help identifying the sample but rather I'd be grateful if anyone could guide me toward getting reliable data so I can do it myself (and hence, be able to identify the next hundred-thousand).

This particular sample may have been partially tumbled but since I've never tumbled a rock and have no intention of doing so, I don't know what a partially tumbled rock would look like. I have googled 'partially tumbled rocks' and saw some pictures but the images I'm seeing aren't illuminating the issue for me - I'm unsure if that's what's causing the effect I'm looking at. The flat, most exposed parts of the sample are shiny. After examining examples of luster, I'd guess adamantine. However, the most exposed parts of the sample that are NOT flat are what I would refer to as dull. It looks surprisingly like the last car I had after the clear coating started to peel.

The color is also the same color as that car - a deep red to burgundy.

I am awaiting a streak plate but I used a known piece of raw hematite to convince me that I could use the unglazed underside of the porcelain sink in the bathroom. Using that, the streak produced was red to light red.

Also awaiting hardness scribes but, as you may have guessed, patience is not my forte. The sample easily scratches steel. However, when I moved up to a known sample of quartz, I got dust residual from both samples (the unknown sample as well as the quartz). This would leave me to believe that I've hit 'hardness equilibrium' but the first 4,923 webpages I've visited on the subject don't explain what neither scratches the other means and I really hate to assume. It has proven itself to be a barrier to learning other disciplines.

Lastly, the specific gravity was measured at 2.57. I do understand that due to impurities and scale sensitivity this isn't as helpful as I'd like, but I can report that this was determined using distilled water and a scale with a sensitivity of 0.01 grams.

Oh! Perhaps an unrelated question, but while I'm here. I ordered two books earlier in the week which came today. I was wondering if anyone might be familiar with them and could recommend using them or replacing them with something else. They are: "A Field Guide to Rocks and Minerals" by Frederick Pough and "Simon & Schuster's Guide to Rocks & Minerals" Thank you for any guidance you can offer on these questions.
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Jesse Fisher




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PostPosted: Dec 31, 2021 14:49    Post subject: Re: Guidance rather than identification  

Both of the books you mention are good "pocket guide" types and have been in publication for quite a while. For a more detailed introduction to the science of mineralogy and mineral identification I would recommend "Mineralogy" (originally published as "Mineralogy for Amateurs") by John Sinkankas.
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rweaver




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PostPosted: Dec 31, 2021 15:04    Post subject: Re: Guidance rather than identification  

Jesse Fisher wrote:
Both of the books you mention are good "pocket guide" types and have been in publication for quite a while. For a more detailed introduction to the science of mineralogy and mineral identification I would recommend "Mineralogy" (originally published as "Mineralogy for Amateurs") by John Sinkankas.


Can you even find John's book anymore? I got my copy in 1973 from John and signed by him.
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Jesse Fisher




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PostPosted: Dec 31, 2021 16:04    Post subject: Re: Guidance rather than identification  

My copy is the 1986 reprint. I suspect it is out of print but I see 9 copies currently available through the Abe Books website.
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AngelL




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PostPosted: Dec 31, 2021 17:10    Post subject: Re: Guidance rather than identification  

I appreciate the feedback on the guides I picked up; thanks.

As for the sample - I noticed upon my return that I didn't actually ask any questions regarding the sample - demonstrating my unfamiliarity with what a good question looks like in this context, I suppose. I rather expected that the deep-red/burgundy color would be instrumental in narrow this down - is/was that a reasonable assumption?

Would my supposition that a rock that was partially tumbled produce the effect I mentioned - shiny in parts and dull in others?

Would the untreated underside of the bathroom sink work? Or would it give me a skewed result? Incidentally, I tried it on the underside of the cover to the toilet's reservoir as well, and the streak matched the sink's under belly.

When I went on mindat(dot)org I began putting everything I suspected I knew about the sample in but the results weren't even in the same zip code. So I backed everything out except for the color, thinking I'd go sample by sample. That brought up another question. Does anyone have a suggestion about entering subjective values in such searches? i.e. am I better off using 'red' rather than 'burgundy', or 'pink' for the streak rather than ''light red'?

I am aware that a minerals specific gravity is only as useful as the purity of the sample (and the equipment) but another feature of the sample I'm playing with is that it seem, to the naked eye (as well as through a 10x lens) to be homogenous. I'm confident that appearances can be deceiving, but did assume that if I can't see composite materials then the specific gravity reading I took would be closer to whatever mineral is giving it that deep-red/burgundy color than a sample that was covered in flecks or specks consistent with another mineral. Is that a generalization that can be made? I ask because most of the candidates that came up on mindat were way, way off. i.e. 2.57 vs. 3.58 or worse.

It might be helpful to point out that 'm using a 64.2 gram sample. I'm thinking the smaller the sample, the more likely it is to make compounding errors. Am I better off with larger samples (to a point)? My assumption was that this size would be fine but a < ~10 gram sample would need to be treated differently.

Last, I just want to point out re: the hardness test attempt I made that those websites I referenced DID tell me that if neither scratched the other, they were equal (or at least very close). I said it really poorly... Both samples - the unknown and the quartz had very fine pieces/dust break off when scratching each other. So although neither left a scratch, there was no mention of what this meant.

So, to clarify: I'm trying to correct my thinking and my methodology (within practical limits). Obviously my methodology would improve greatly as soon as I pick up an x-ray spectrometer and an electron microscope - but that won't be happening any time this year - or next, since that's less than ten hours away. The question is primarily referring to my methodology based on the tools I had at my disposal.
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AngelL




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PostPosted: Dec 31, 2021 19:26    Post subject: Re: Guidance rather than identification  

Feel as if I've blown up the board today. I've learned something and with that information I'm confident that I have the rock identified. I wouldn't bet the kids; but maybe dinner. Anyhow, I doubt this will be valuable to anyone but me; I'm sure you all know already but...

I put the name of the mineral I suspected in the mindat system and a handful of variants popped up - including red ones with information pages that specifically said 'red'. But when I kept the name of the mineral there and added 'red' as the color, it returned 'no matches found'. That was unexpected and resulted in me not being able to identify it. Wiser now. :)

Specific gravity for the sample was 2.57 while a pure sample is listed as 2.58.
The hue matches perfectly.
I had concluded that the hardness was ~7 and the fact sheet says 6.5 - 7.
Luster was problematic as the fact sheet says its dull vitreous to greasy - and neither vitreous or greasy were options on the 8-Luster chart I was using.
I said transparency was opaque - and that's a checked box.
The only thing I found issue with was streak. I came up with pink and the fact sheet says 'white'. I'm going to chalk it up to, 'I shouldn't have used the underside of the sink and toilet'.

It's Red Jasper.

It took me most of the day because we've got a 70x50x45 mm 240-gram sample that looks nothing like it, and I didn't take a range of appearances into consideration. Live and learn - I'll be less likely to make that mistake again.

I spelled out my process here, not because I thought it would be of interest - I trusted that I was working with something common and mundane. I did it so someone could point out any obvious errors in the way I tackled it. I realize everyone has lives but if there are any glaring issues you want to point out - I'd be grateful. Thanks!
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Pete Modreski
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PostPosted: Jan 01, 2022 02:40    Post subject: Re: Guidance rather than identification  

I believe you've studied the specimen well and done the tests correctly, Angel, congratulations! Your concluded answer is what I would have said from your descriptions.

I had been writing a longer response to you, with a few comments about doing the tests, but the text I was typing seems to have disappeared on me. If I find it I'll resend it, or, I'll rewrite it to you later.
Pete
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SteveB




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PostPosted: Jan 01, 2022 08:19    Post subject: Re: Guidance rather than identification  

I also feel you’ve done a reasonable job of your “process”, there are pitfalls easy to make in most ways and assumptions which people take leading them to incorrect conclusions. Dirst the shin/dull lustrues on a tumbled surface tend to indicate a non-uniform material, ie a rock composing of multiple minerals or a single mineral that was formed from a nonuniform mix and underwent non-uniform heat & presure processes. Hardness and specific gravity lists are aimed at “pure” perfect mineral specimens (typically what you get from a crystal of the mineral) so in reality what we often are trying to identify can give us varying results. Think about the practicality involved in the creation process of rocks and minerals as being like cooking, if the ingredients are porly mixed they form obviously different blobs in the mix as too does the heat in the oven effect t he result where say the surface can be burnt and dry while the interior is cool and wet, You cant assume every specimen underwent the perfect mixing or heating conditions . So When testing hardness you are best using hardness scribes which are a standaridised tool also you hard you press to scratch will differ as does where on a specimen you test a flat crystal surface gives the best feedback for you to test, while the rough surface of a broken crystal doesnt give good feedback. There is an aspect of the feedback of making the scratch that I find vital to determine if I got a good scratch or not to use as a value. You only get a value of this hardness made a solid scratch and by working up through each scribe point from soft to hard you end up with a whole number range for hardness, this is the best precision you can get at home. So try to test a variety of places all around your sample of what appears to be the same material.rocks lie granite are made of a number of minerals (eg Quartz, feldspar and tourmaline) so test each mineral thats large enough to see as separate instead of running a scratch down the surface of the specimen which gives you a useless result for the specimen as a whole. Sedimentary rocks are by this nature difficult to test as the fine grains could be all sorts of minerals. So while you can get a number range by scratching it doesnt help to identify. Understanding this is vital for any chance of accurately identifying. Similarly specimens with cracks and voids become difficult to get an accurate specific gravity value for as the sample is no uniformly solid material. But it can give you a minimum density provided you’re confident its uniform mineral all throughout. When uniformity of minerals is in doubt SG is a diffeicult value to measure and assign for certain. Something like a granite which contains several minerals you ideally want to crusah untill you have the purest mineral pieces to measure and make several measurement of hardness and SG on each you can, its ok here to use colour to help you separate the minerals and be reasonably certain they are identical mineral and likely to have formed under the same conditions and give the same value. A granite boulder broken up each rock fragment can be expected to have the same enough distribution of its minerals with the same sizes and overall a uniform density, but a specific gravity table listing granite is derived from a lot of granite samples so it may be represented as a single figure based on the average or maybe a range (maybe range values are averages to begin with , unless the table has a reference you dont know). So just take tables as guides, especially from a generalised book, while a reference paper should outline the specifcs of how they got the values. But I’ve found the values to be pretty close. Using tap water will give you different values of specific gravity when using a water immersion method to measure specific gravity. Having cheap scales especially those high precision ones you find on ebay you can’t be certain of their accuracy as you never get supplied with a calibrated mass to weigh and a method to calibrate the scale itself with. So investing in a quality scale really helps, You should be able to turn the scale on and off and re-zero and measure a mass ten times and get identical result every single time, not close but exact. Also find yourself a clear container you can place on your table top covering your scale and the spefic gravity rig entirely so air currents from you, air-conditioning, your breath etc don’t effect the scale measurement, you want the scale to be on a perfectly level surface and isolate from moving air and give the rig the time it needs to settle after you place your specimen in it, Record your weighed value once everything is settled. Its an easy mistake to record your value while leaning over the scaleand breathing down on it, thus giving a false value. Its slow going but care to have a repeatable process you can be certain about is important for your confidence. Another big pitfall is people getting a value and seeing its close to several minerals on a list and saying “I recognise that mineral name, it must be it”. Colour is pretty irrelevant as most minerals come in the same range of colours, Each measurement is only a single datapoint towards identification and you need a bunch to sometimes narrow down the possibilities, and people commonly see something rare and valueable in a book and say my specimen looks pretty close and so whatever measurements they take might be closish and so they convince themselves they have that mineral. Dreams of fortunes really clouds peoples judgement. Its tricky too because often we are dealing with less than ideal specimens and ones we dont want to damage, crushing and being analysed in a mass spectrometer is about the only accurate way to analyse the element in a mineral which you then compare to known chemical makeups of mineral to get an accurate identification. Thats a very costly exercise and destructive to the specimen XRF analysis can help but they only cover a subset of the chemical elements so there room for error in identification there as its aimed at the mining industry who want to know how much gold, silver, lead, alumminium etc there is per cubic ton of this piece of land and thus work out the cost of mining the area vs the expected yield to see if its profitable, its a tool for large scale rather than home or reference applications, and again its an expensive tool. I have seen promising developement towards consumer level devices but not have resulted in anything for earth science areas, they ended up for agricultural and food industries testing organic compounds so not any use for rock hounds.
Crystaline structure are important in mineral identification as is cleavage planes, and with hardness and specific gravity and streak (yes the underside of a porcelain sink is good for streak tests). If you can measure all those datapoints you have a good chance of narrowing down the posibilities.Note weighing and phots are not mentioned as they tell you nothing really, though of course there are many minerals that are obvious to identify on sight and in time you get to be able to spot many rocks and minerals in the field by various means. A clear crystal of topaz held in the hand for about five seconds whould feel cold not warm when touched to your upper lip or cheek, white a similar sized quartz crystal will warm up under the same test. Some veins of white material are soft enoug h to chew in the field strongly suggest something like gypsum and certainly not quartz over time and with exerience you get to learn the feel, smell and taste of various minerals to help guide your identification (like the taste of high iron content or copper) but be careful about this as arsenic and lead and mercury can be absorbed into your blood and are found in rocks so don’t just go crushing and eating everything out there. But often just sniffing a rock or touching to the tongue gives you something you can take note of . Of course the outside of a rock chemically reacts with the wether and can change appearance over time so when collecting ni an area make sure to have some samples you can break open to observe and measure from the internal surface ( eg serpentinite is typically greenish but an outcrop exposed to the weather becomes orange) some more delicate and reactive minerals can oxidised at home in your display case and change colour .

Colour is a big area people can get stuck on. The same photo shown on 100 different random computers will show different colours and that even before we take into account wha I mean when I say reddish orange actually is compared to what you mean when you say reddish orange. You firstly need a correctly colour balanced camera to take a photo, The image needs to be stored with a suitably wide range accurate colour space not just one that only store 90% of the possible range of colours, and then viewed on a colour calibrated screen. Likewise a printed photo, The same colours look different in all these places to varying degrees and then there’s the lighting conditions of the original object when its photo was taken or the lighting conditions of where you view a printed photo. Minerals often appear different colour hues or shades when viewed outside in sunlight compared to indoors under energy saver lighting, So in practice colour is not an identification method but it can be taken in generality, Reds are associated with iron rich rocks/minerals and greens/blues associate with copper rich ones. Doesn’t mean every copper rich mineral will be green or blue, or that everything green or blue is rich in copper. Its only a broad generalisation, its helpful out in the field to help you choose which outcrop to go investigate.
For home identification and specimen photography you could look into getting a desklamp with a Full Spectrum bulb, colours show up as they should, some mineral that look vibrate outdoors can appear dull under indoor lighting but look great again under a full spectrum bulb again, because the light spectrum of light bulbs are rough aaproximations of white light, I also collect stamps and some rare stamps are hard to identify because of their colour and you need a full spectrum indoor light to be able to pick them from the other similiar colour shaded copies of the same stamp. Similarly some gemstones need various light sources to be able to separate from other gems them can be mistaken for and often found with in nature.

Cheers
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