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aga
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Rock Molester's Club
With the Huge demand for mineral analysis amongst the members (i count about 5 max) this thread is dedicated to the discussion of rock/mineral
identification, and also the chemical analysis thereof.
The first suggestion is cation identification after digesting a rock sample in an acid or other solvent, then subjecting the liquor to various tests.
Given that this is Amateur science, Qualitative testing is probably the simplest way.
http://www.public.asu.edu/~jpbirk/qual/qualanal/confirm.htm#...
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aga
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Here's the photos of 3 samples taken, and digested in HCl.
Remarkable how different the colours are.
Liquor B looks like FeCl2 yet has not changed shade of green in 3 days, which would have been expected.
   
All samples are taken from within a few miles of where i live in southern Spain.
Sample C is very friable - you can crush it up in your hands easily. A and B are more like 'rocks'.
[Edited on 25-7-2015 by aga]
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blogfast25
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Many pure minerals have multiple cations. Beryl (a favourite of mine)
Be<sub>3</sub>Al<sub>2</sub>Si<sub>6</sub>O<sub>18</sub> is a case in point.
But minerals in nature are rarely pure, but usually accompanied by 'gangue'. Iron is something you'll find in most gangue.
Also, most minerals don't really respond to dilute acids anyway, you need harsher treatments to unlock their 'secrets'!
Did these sample dissolve? Any CO<sub>2</sub>?
[Edited on 25-7-2015 by blogfast25]
[Edited on 26-7-2015 by blogfast25]
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aga
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Sample B showed the most activity with HCl and H2SO4, evolving lots of gas with HCl (untested - doh).
It has dissolved in both of those acids, leaving a grainy grey/black mush.
One of the Great things about this particular endeavour is that there is plenty of fresh (and free) raw material available in case something is
forgotten or is not done right.
I'll go and hack some more from the earth and do it again to find out what the gas was.
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j_sum1
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If I get your purpose here aga, it is to gather together in one thread (a) methods for digesting rocks, (b) qualitative methods for identifying
components of the resultant chemical soup, (c) results and discussion of experiments and (d) identification of the rocks and minerals by whatever
means.
Colour me highly interested. This deserves to be a sticky. So many practical applications.
I'll add my thoughts and contributions soon.
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blogfast25
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Be prepared for lots of alkali fusions! (If you want to be serious about chemically analysing/characterising minerals)
https://en.wikipedia.org/wiki/Silicate_minerals
| Quote: | | The silicate minerals are rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified
based on the structure of their silicate group which contain different ratios of silicon and oxygen. They make up the largest and most important class
of rock-forming minerals. |
Trust me, these bastards do not respond to either dilute or concentrated acids! 
[Edited on 26-7-2015 by blogfast25]
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diddi
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its very well intentioned aga, but I have to agree with bf25. rocks are very difficult to dissolve. even piranha or AR take ages with heating. and
Fe, Al, Si, O, Na are exceedingly pervasive as contaminants.
many analyses are carried out using xray diffraction etc.
that said you can get good samples of many minerals which are somewhat unadulterated and if the analysis is not too fine then there could be some
suitable results. a simple example is cinnabar
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diddi
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sample C looks like Kaolin?
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hyfalcon
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Quote: Originally posted by blogfast25  | Be prepared for lots of alkali fusions! (If you want to be serious about chemically analysing/characterising minerals)
https://en.wikipedia.org/wiki/Silicate_minerals
| Quote: | | The silicate minerals are rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified
based on the structure of their silicate group which contain different ratios of silicon and oxygen. They make up the largest and most important class
of rock-forming minerals. |
Trust me, these bastards do not respond to either dilute or concentrated acids!
[Edited on 26-7-2015 by blogfast25] |
I bet ammonium biflouride will put a hurt on them.
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blogfast25
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Ammonium bifluoride would still would still have to be used in the molten form to get fast digestion. Fancy it?
Anhydrous HF is also used to digest silicates (SiF<sub>4</sub> escapes). Fancy a bit of that, do you? 
Seriously, digestion to fluorides isn't for hobbyists.
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blogfast25
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| Quote: Originally posted by diddi | its very well intentioned aga, but I have to agree with bf25. rocks are very difficult to dissolve. even piranha or AR take ages with heating. and
Fe, Al, Si, O, Na are exceedingly pervasive as contaminants.
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'Piranha' and Aqua Regia are largely irrelevant here. With nothing to oxidise, oxidisers don't work here.
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deltaH
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As a second year undergraduate a long time ago, I used to do vacation work at the analytical laboratory performing mineral assays for the mineral's
processing group. The commonly used method that is absolutely not amateur friendly is sequentially boiling the sample in a series of acids including
HF, perchloric and nitric, in a specially designed fume cupboard with a water scrubber on the exhaust.
Amazingly, this did not always work  Some samples high in chromites required
something special. The 'special' was fusing the ore in cherry red hot sodium peroxide in a little zirconium crucible.
The latter simply required a blowtorch, a pair of tongs, a small Zr crucible (a bit pricey) and affordable Na2O2. The nice thing about the method is
that it doesn't produce noxious fumes and is not ridiculously toxic like HF and works for nearly everything.
aga, please take a look at https://en.wikipedia.org/wiki/Peroxide_fusion for an introduction.
Here for a method:
http://www.perkinelmer.com/CMSResources/Images/44-130831APP_...
on page 3.
| Quote: | | In a zirconium crucible, precisely 0.2 g of finely ground sample was fused with 3 g of sodium peroxide (Na2O2) and 0.5 g of sodium carbonate (Na2CO3).
The fused mixture was poured into a beaker containing 250 mL of a 20% acid mixture of 1:1 hydrochloric (HCl) and nitric (HNO3) acids.
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Use face shield at all times and good PPE! Long-sleeved welder's/forgers thick leather gloves are also a must.
The following links to a more detailed description of the procedure, though it has the glaring mistake of talking about a ?1:1 HCl?, which is probably
1:1 HCl:HNO3 (conc. not mentioned). I'd use the quantities and concentrations mentioned in my quote above and just refer to this document for a
description of the procedure's execution.
http://www.personal.psu.edu/hxg3/MCL/naperoxide.pdf
Zr crucibles come up on ebay from time to time for around $75-100 AFAIK. Important: you want 25-35ml (preferably 30ml) zirconium metal crucibles, NOT
zirconium oxide crucibles!
[Edited on 26-7-2015 by deltaH]
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aga
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Thanks for all the references, especially the scary ones.
I never imagined that people had resorted to such extreme measures to get rocks to fully dissolve !
With 90% of the crust being silicates, i suppose that leaves a rather large 10% to more easily investigate before moving on to the silicates.
It could well be.
It is now obvious that samples should be accompanied by a photo of them in their natural environment (noob error: my bad).
Hmm. I think things go sticky after a while when they contain useful reference material !
As well as identifying rocks and minerals, it would be wonderful to know what they are composed of, and how to process them into useful chemicals.
A mini-dream is to have the ability to wander cross the face of this astounding planet with some depth of knowledge of what is underfoot.
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j_sum1
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| Quote: Originally posted by aga |
Hmm. I think things go sticky after a while when they contain useful reference material ! |
Of course you are right. and I did not mean that it should be stickied straight away. I just see potential for this thread to be a repository of a
certain flavour of knowledge -- in particular, tests for ions in solution.
Of course the aluminosilicates are going to be the bane of the whole thing. Getting them to dissolve is going to be trouble. But that need not be a
bad thing.
1. Acid selection can be part of the mineral identification process. (For example, dissolves in HCl with difficulty, AR easily, Sulfuric and nitric
can't touch it. There fore it must be...) I would love to see that kind of information in this thread.
2. Following on from blogfast25's comment on the limitations of HF and AR on minerals... Oxidation is not the only possible process to employ. Given
that silicon can be extracted from silica using a thermite reaction with Al (plus some sulfur), that opens up another possible route for analysis.
3. Part of the analysis involves what can be leached out. The fact that some undissolved material remains is neither here nor there.
Let me kick things off with some things I learned in the last 24 hours -- Credit to kecskesajt and gdflp
Testing for Al3+ in solution
Al3+ will precipitate Al(OH)3 as a gelatinous precipitate on the addition of NaOH. Further addition of NaOH will redissolve the precipitate as
aluminate [Al(OH)4]-. Alizarin 0.1% solution added to the basic solution turns it purple. Addition of dilute acetic acid will cause the purple to disappear. If the
aluminate ion is present then there will be a reddish precipitate remain -- a complex of the aluminate and the alizarin that does not dissolve in
acetic acid.
Testing for Chromium 3+
Cr3+ in solution is a vivid green with perhaps a sense of grey-blue.
Addition of a base will cause a gel-like precipitate of Cr(OH)3.
Addition of a sufficiently strong oxidant (H2O2) will dissolve the hydroxide and oxidise the chromium to chromate (yellow). Basic conditions are
required. Acidifying this will of course produce a nice orange dichromate.
Now, I feel as though I am attempting to tell people basics that they already know. ("Teaching Grandma to suck eggs" as they say back in the home
country.) But, Hey, gotta start somewhere.
Now for some more useful ion tests. Or some tips on identifying minerals through acid digestion.
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aga
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I like the Cr<sup>3+</sup> test - i have those reagents !
The fact that we may not be able to process the silicates 'properly' doesn't really detract from the idea of wandering the countryside with a small
pick axe, then subjecting the collected rock samples to a variety of chemical tests.
It's sort of 'chemistry with a purpose' plus an active and healthy recreational activity (with a small pick axe).
Edit:
Photos of the places i got the samples from. A and C are from basically the same place, A being streak of hard white rock inbetween layers of C.
[Edited on 26-7-2015 by aga]
 
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blogfast25
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| Quote: Originally posted by j_sum1 | Addition of dilute acetic acid will cause the purple to disappear. If the aluminate ion is present then there will be a reddish precipitate remain --
a complex of the aluminate and the alizarin that does not dissolve in acetic acid.
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This is not entirely correct. I've done this test myself and it's quite finnicky.
The solution of Al (as Al<sup>3+</sup> or aluminate) is carefully neutralised in the presence of Alizarin test reagent, Al(OH)3 then drops
out. Al(OH)3 absorbs Alizarin, forming a pink so-called Alizarin Lake complex. The supernatant solution should become clear. Alizarin also forms a
'Lake' complex with Be(OH)2 and with Fe(OH)3, so the sample has to be iron-free for this Al identification method.
As for chromium, after oxidation to chromate/dichromate, add a Pb(+2) salt and yellow PbCrO4 precipitates.
[Edited on 26-7-2015 by blogfast25]
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blogfast25
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Unfortunately, chromium bearing streaks are rare. If you find one, you're probably trespassing on a mining company's property! 
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aga
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Small pick axe and Running Shoes, say Nike for example ...
Do you mean places like these ?
[Edited on 26-7-2015 by aga]
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aga
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Sample B was tested a few minutes ago with ~10w% HCl in a test tube with a cork/tube fitted so the gas could be led through another test tube
containing a dilute calcium hydroxide solution (limewater).
The limewater went milky, so i guess that means it's CO2
The reaction is fairly vigorous.
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blogfast25
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| Quote: Originally posted by aga |
Sample B was tested a few minutes ago with ~10w% HCl in a test tube with a cork/tube fitted so the gas could be led through another test tube
containing a dilute calcium hydroxide solution (limewater).
The limewater went milky, so i guess that means it's CO2
The reaction is fairly vigorous. |
Did it dissolve more or less completely?
These so called carbonaceous rocks are about the only ones significantly attacked by mineral acids.
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aga
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I would have to say that it dissolved significantly.
There is not much left of the solid rock structure, just some dark grey mush with grains of presumably silicates in it.
Is this normal or did i just get lucky ?
Edit:
Easy way to find out : everyone go outside, grab a small rock and see if it dissolves in HCl.
Given the spread of where everyone lives, that should be a good test.
[Edited on 26-7-2015 by aga]
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j_sum1
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| Quote: Originally posted by blogfast25 | | Quote: Originally posted by j_sum1 | Addition of dilute acetic acid will cause the purple to disappear. If the aluminate ion is present then there will be a reddish precipitate remain --
a complex of the aluminate and the alizarin that does not dissolve in acetic acid.
|
This is not entirely correct. I've done this test myself and it's quite finnicky.
The solution of Al (as Al<sup>3+</sup> or aluminate) is carefully neutralised in the presence of Alizarin test reagent, Al(OH)3 then drops
out. Al(OH)3 absorbs Alizarin, forming a pink so-called Alizarin Lake complex. The supernatant solution should become clear. Alizarin also forms a
'Lake' complex with Be(OH)2 and with Fe(OH)3, so the sample has to be iron-free for this Al identification method.
As for chromium, after oxidation to chromate/dichromate, add a Pb(+2) salt and yellow PbCrO4 precipitates.
[Edited on 26-7-2015 by blogfast25] |
I have no doubts that it is finickity . In the little reading that I have done it seems that it forms a wide range of complexes with different
cations -- most in the purple to puce to crimson range of the spectrum with a few mustard yellows thrown in. It is also pH sensitive. This all leads
to it not playing nice in mixtures: which is not what you want to hear if you are making rock soup.
Incidentally, one of the first hits I got while researching was your chemical supply company and its test for fluorides. Again, I don't suppose it
will play nice if there are stray cations in the mix.
This video shows the Al test as well as Fe. The two precipitates do appear different in both texture and colour and so, I guess, could be
differentiated in a careful test if only one was present. It also shows the purple complex with Mg.
I note that the most common scientific application seems to be the analysis of Ca in biological specimens. Good for analysing bone growth.
There is enough interesting here to elevate alizarin from something I have never heard of before to something that I should invest a couple of bucks
on in my next chemical order. The first project would be to make a bunch of complexes with different metal ions and determine their colour. Then it
could be used for some qualitative analysis.
[edit]
Quite right on the chromium and testing with a lead salt. A Ba salt can also be used with a similar result. I don't know how specific these tests
are though. It seems that there is quite a lot out there that can make Pb and Ba precipitate and a yellowish hue would not be that unusual.
[Edited on 26-7-2015 by j_sum1]
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blogfast25
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Here's some stuff I did with Gadolinite:
http://www.sciencemadness.org/talk/viewthread.php?tid=28071#...
| Quote: Originally posted by j_sum1 |
This video shows the Al test as well as Fe. The two precipitates do appear different in both texture and colour and so, I guess, could be
differentiated in a careful test if only one was present. |
Yes, I've done the test with both Fe and Al and the Fe Lake Alizarin complex is distinguishable from the Al complex. But if both are present, Fe would
mask the Al. Fe and Al are easy to separate though (Al's amphoterism).
[Edited on 26-7-2015 by blogfast25]
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blogfast25
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| Quote: Originally posted by aga | Small pick axe and Running Shoes, say Nike for example ...
Do you mean places like these ?
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No. Methinks I see lots of limestone and sandstone there.
[Edited on 26-7-2015 by blogfast25]
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diddi
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whose got a mass spec or AAS handy for all this?
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