I am trying to clean quartz grains from crushed granite ( about 1 mm grain size).
HF would dissolve any silicate gangue mineral but would also dissolve the quartz grains.
The recommendations I found in the present literature is to use H2SiF6 ( fluorosilicic acid) instead, with or without HCl, and at room temperature or 60°C.
I have however found an old "recipe" where instead, it is advised to use cold acid, and keep in an ice bath the beaker with the acid and the grains.
The recipes do not give many details as to how or why the fluorosilicic acid preferentially dissolves the silicate mineral other than quartz, or how the dissolution takes place. The same with HF.
I wondered if anyone could help me figure out why that would be better, and give me some lead as to how those acids actually dissolve the silicates leaving the quartz relatively untouched.
Thank you!
I can't tell you about H2SiF6. But solvents in the chemical enrichment of quartz are selected based on the final task. If you need to get pure quartz, you can not do without hydrofluoric acid, as it allows you to get rid of mineral inclusions inside quartz grains. https://link.springer.com/article/10.1007/s12633-022-01946-7
Therefore, most often a mixture of hydrofluoric and hydrochloric acids is used to purify quartz.
Thanks! The H2SiF6 dies decompose partially to HF, so you do lose some quartz, but the kids is muck lower than with HF. But, I will download and read the paper you suggested very carefully! Thank you for that suggestion.
The paper is very interesting but the purpose of cleaning the grains of quartz is to measure the content of He 4 dissolved in the liquid inclusions in those grains. High heat could cause diffusion out of the grains, so that there is not hot treatment of those samples. The cooling of the H2SiF6 seems to be reducing the decomposition to HF, and hence the dissolution of quartz leaving the H2SiF6 dissolving other silicates like feldspars. I find it difficult to see how that dissolution takes place, and have found papers that favour heating the solution. I am hoping to understand the process a bit better. HF as you mentioned remains the best cleaner but does dissolve the quartz!
Thank you for the paper which is interesting on the effect of heat and concentration of NaOH on the surface of the grains. That is surprising!
All the best,
Danielle
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