Vanadinite fiasco
A couple of months ago I started to make experiments how to remove different kind of stains from different mineral associations. I had nice results for most of the specimens I tried to clean so I gained not only experience but confidence too. Too much too soon maybe? I start the Preparation blog section with this article as I’d like to raise awareness to You that you shouldn’t overestimate your knowledge (if you haven’t done similar preparation experiments you want to do on a specimen) and always do your homework. In this case, eventhough it would’ve been just a couple of minutes to look into a table, I neglected it and ultimately paid the price.
What I wanted to achieve was to remove the brown iron stain from the Baryte blades of the above Moroccoan Vanadinite specimen. It was my only Vanadinite in my collection - nice hexagonal red crystals which aren’t big but were very lustrous and they had a decent constrast with the Baryte. I’ve had pretty good results removing stains with dilute sodium thiosulfate and hydrochloric acid up until then. (NOTE: I always wear protective equipment when I experiment with chemicals and that’s what you should do, too!)
As you might’ve read already in my bio I am a chemical engineer by education. This does not mean however that I know every detail by heart. Far from that. Unfortunately I’ve put little effort into learning chemistry at the university... (Actually becoming a chem.eng. led me back to minerals but that only came years later.) However what we’ve been taught extensively at the university is how to study and look up information you don’t know. Well, I certainly did not do my homework in this case!
So, I have made a dilute solution ready to immerse my Vanadinite specimen into. I had second thoughts but I’ve done it. Blup; down to the liquid.. Vanadinite is a Lead(2+) Vanadate Chloride. If you are somewhat familiar with acids and their salts you might know that they are similar to any other natural creatures - the tougher you are the more you achieve. What this means in this case is that Vanadic acid is weaker than Hydrochloric acid therefore the latter will try to replace the Vanadate-ions in the salt and take over it’s place. Now, all this is a very sketchy explanation but I just want to paint a picture about the process roughly so you get an idea what happens more or less. Step ahead, all you chemical experts, I’m happy to edit the article with more precise details! :)
My initial thought was that Vanadinite will not really react with the Sodium thiosulfate - Hydrochloric acid solution. I was wrong - as you can see below. The Vanadinite crystals were etched and they lost every bit of lustre whatsoever...
Well, to be 100% fair this experiment was every bit successful! I've managed to remove ALL the iron staining from the Baryte perfectly! :) At first I was furious I made such an avoidable mistake but at least it was a cheap lesson from which now you can learn too. Keep rockin'!
