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Feldspar staining methods
  
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Bill Carson




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PostPosted: Sep 15, 2010 11:13    Post subject: Feldspar staining methods  

I want to attempt to identify any adularia that may be present in quartz vein material and have read that the usual method is to stain the adularia. But, most of the methods I have found require the use of hydrofluoric acid which is a bit hazardous. So, I am wondering is there a substitute for hydrofluoric acid or another less toxic method to stain feldspars?

If I am stuck with using hydrofluoric acid, is it possible for a private citizen to obtain it?
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alfredo
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PostPosted: Sep 15, 2010 11:22    Post subject: Re: Feldspar staining methods  

You should be able to use ammonium bifluoride instead of HF - same effects, but (a bit) slower and safer.

Also, have you checked fluorescence under shortwave ultraviolet light? Lots of feldspars (although not all) are weakly fluorescent crimson under SW UV light, which enables you to distinguish them without the use of any chemicals at all.
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Bill Carson




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PostPosted: Sep 15, 2010 11:43    Post subject: Re: Feldspar staining methods  

Thanks. I had run across ammonium bifluoride as a possible substitute for hydrofluoric acid, but have yet to find a description of its use for staining feldspars. I will look into a bit deeper.

I have not tried UV detection because I don't own a light, but I wondered about it. I haven't run across any descriptions of the use of UV for feldspars.

One other method I read about, that some people are using for buddingtonite, is to use shortwave infrared reflectance spectroscopy. But I believe it is only sensing the ammonia in buddingtonite.
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alfredo
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PostPosted: Sep 15, 2010 11:55    Post subject: Re: Feldspar staining methods  

Traces of some rare earth elements cause feldspars to fluoresce red, and those REEs seems to be so widely distributed in nature that the non-fluorescing feldspars are more rare than fluorescent ones. A good strong SW UV light is a bit expensive (perhaps around $400), but that's a lot cheaper than the medical bills for a hydrofluoric acid burn! And the light should last you for years. If you can't get one, you may be able to borrow one from members of a local mineral collectors club.

The procedure is simply to shine the light on the rock in a completely dark room, enabling you to estimate the % of feldspar on the rock surface because it will fluoresce crimson, whereas the quartz won't. You can record it photographically if you need a record. Goggles are advisable if you do this a lot, as any UV reflected into your eyes isn't good for them.
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Bill Carson




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PostPosted: Sep 15, 2010 12:04    Post subject: Re: Feldspar staining methods  

Don't laugh, but I did put a sample under one of those cheap black light bulbs you can buy at a hardware store! Of course it did nothing because the wavelengths are wrong.

I may look into UV light, but I would like to see it work first before I invest in a light.
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GneissWare




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PostPosted: Sep 15, 2010 13:25    Post subject: Re: Feldspar staining methods  

Staining using HF with sodium cobaltinitrite (yellow - Kspar) and BaCl2 with amaranth (red - plagioclase) is the only method I know of, but all Kspar will stain yellow. So you can't distinguish microcline from adularia from any other Kspar.

Thin section is possible, but requires a polarizing microscope. Generally, you can determine if the mineral is a feldspar though typical field methods.
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Bill Carson




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PostPosted: Sep 15, 2010 13:42    Post subject: Re: Feldspar staining methods  

Since the vein material I am looking at is low temperature (epithermal), I am hoping that the only Kspar present would be adularia or buddingtonite. While the use of UV intrigues me, I worry that some other mineral(s) might fluoresce and it would not be as definitive as the staining method.

I would like to try and get the ammonium bifluoride and try the staining first, but I am not sure where to get it and the sodium cobaltinitrite. Are there any mineralogy supply places that you would recommend to try?
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GneissWare




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PostPosted: Sep 15, 2010 14:05    Post subject: Re: Feldspar staining methods  

Bill Carson wrote:
Since the vein material I am looking at is low temperature (epithermal), I am hoping that the only Kspar present would be adularia or buddingtonite. While the use of UV intrigues me, I worry that some other mineral(s) might fluoresce and it would not be as definitive as the staining method.

The issue with staining is that all the potassium feldspars will stain the same.
Try googling for the chemicals. They should be available from chemical supply houses.
Alternatively, you might call the local university geology department and talk to the petrologist--you might find a grad student who would thin section and tell you what you have. Probably cheaper than buying all the chemicals and labware.
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Bill Carson




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PostPosted: Sep 15, 2010 14:55    Post subject: Re: Feldspar staining methods  

While all the Kspars would stain the same, I am more interested in just testing for the presence of any adularia, which as far as I know is the only feldspar to form under epithermal conditions.

I had extensive experience with a petrologic microscope when I got my geology degree, but have not used one since. It would be fun to get back into it.

As for fluorescence of adularia, I found one article that suggests it may be possible to enhance it's weak fluorescence by chilling it first.
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Kathryn




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PostPosted: Jul 14, 2011 14:33    Post subject: Re: Feldspar staining methods  

Following on this discussion, I have not been able to locate in the literature whether buddingtonite will stain yellow using sodium cobaltinitrite (the same as all the K-spars). Could anyone help me out with this? Thanks.
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GneissWare




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PostPosted: Jul 14, 2011 15:03    Post subject: Re: Feldspar staining methods  

Kathryn wrote:
Following on this discussion, I have not been able to locate in the literature whether buddingtonite will stain yellow using sodium cobaltinitrite (the same as all the K-spars). Could anyone help me out with this? Thanks.


Buddingtonite is an ammonium feldspar with formula NH4AlSi3O8.

R. C. ERD, D. E. WIIITE, T. J. FAHEY AND D. E. LEE, 1964, BUDDINGTONITE, AN AMMONIUM FELDSPAR WITH ZEOLITIC WATER, Amer. Mineral., v. 49, p. 832, states in part: "Selective staining of the altered rocks for K-feldspar and chemical analysis, proved that potassium was indeed a minor component."

The inference is that the buddingtonite was detected as it had "recognized feldspar-like properties", but did not accept the staining as would be expected by k-spar.

Hope this helps.
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Kathryn




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PostPosted: Jul 14, 2011 15:41    Post subject: Re: Feldspar staining methods  

Hi - Thanks for the reply. What bothers me is the following extract from Wiki in particular that "is used as a qualitative test for potassium and ammonium ions". This is why I'm wondering if buddingtonite might stain yellow.

Sodium cobaltinitrite is a coordination compound with the formula Na3Co(NO2)6. The anion of this yellow-coloured salt consists of a cobalt(III) center bound to six nitrito ligands. It is used as a qualitative test for potassium and ammonium ions (provided that certain other cations are absent).[1] Although the sodium salt is soluble in water, those of potassium and ammonium are insoluble. The potassium and ammonium salts are precipitated as yellow solids.

Sodium cobaltinitrite also forms the basis of a quantitative determination of potassium. However under the recommended reaction conditions the insoluble double salt, K2Na[Co(NO2)6].H2O is precipitated and weighed. [2] Also thallium(I) can be determined as the precipitate is Tl3[Co(NO2)6].[2]
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GneissWare




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PostPosted: Jul 14, 2011 16:08    Post subject: Re: Feldspar staining methods  

Kathryn wrote:
...It is used as a qualitative test for potassium and ammonium ions (provided that certain other cations are absent).


perhaps certain other cations are absent ;))
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mmauthner




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PostPosted: Jul 14, 2011 23:46    Post subject: Re: Feldspar staining methods  

I used both staining methods and UV to detect the extent of albitization (and general petrology) on pegmatite samples for my thesis. Both worked wonderfully. I did use HF to etch the samples, but the SWUV did just as well for inspection purposes...it just was not "permanent" or easy to record as the tonality of the fluorescence of the two species (deep, moderate red fluorescence vs bright, strong whitish fluorescence) was too different for film to accurately capture. However, today's HDR digital capturing techniques would obviate those concerns.
Try borrowing a light. Definitely worth the attempt.

Mark
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