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Techniques for CollectorsWhich Minerals Get Damaged by Hydrochloric Acid

26th May 2018 19:16 UTCSean

Hey guys,

I'm just curious to know which minerals get damaged (or infected) by hydrochloric acid.

I know the acid can severely damage Calcites, but can it do the same thing to Spinels? I'm just curious to know which minerals get damaged by hydrochloric acid and which ones are immune to it before I use the acid myself.

26th May 2018 19:55 UTCAlfredo Petrov Manager

Sean, you can't draw a hard line between minerals that are damaged or not by HCl, because there is a huge range of solubilities between something completely insoluble and unaffected, like a diamond, and something that quickly disappears completely, like aragonite and calcite. Sometimes a mineral won't disappear but there will be a subtle dulling of the luster or some etching of the crystal surfaces. And of course much depends on the concentration of the acid (how much you dilute it), the temperature you use (the acid is more aggressive when hot), how long you leave it in the acid (minutes? days? months?), and how porous or cracked the mineral is. So using HCl for cleaning might be safe under one set of conditions and not safe under a different combination of conditions.

26th May 2018 20:08 UTCThomas Lühr Expert

Sean,

About one half of the Minerals is stable and the other half is not. So it would be impossible to mention all sensible minerals.

At best you ask for the particular minerals you want to remove or to clean. For sure you will get practicable advices.

Thomas

26th May 2018 20:25 UTCBob Harman

SEAN,

Carbonates are as mentioned; all are affected, either rapidly or slowly, by acids of all concentrations.

Years ago I had several low end Illinois fluorite specimens with accompanying Galena (PbS) and adjacent sphalerite (ZnS). Using dilute muriatic acid, the galena was dulled from a shiny silvery luster to dull battleship gray while the sphalerite luster was noticeably improved.

I no longer use acid or vinegar when galena is present. CHEERS......BOB

26th May 2018 20:45 UTCTed Hadley

What Alfredo said.

Also: one of the problems with many silicates is that the chlorine atom is small and it will get into the structure and slowly etch it. As a _general_ rule, using acids with larger ions will do less harm. Consider using:

Nitric acid

Oxalic acid

Phosphoric acid

Acetic acid

Citric acid

etc.

26th May 2018 21:02 UTCSean

Thanks guys, but what about Spinels? Which acid should I use to find Spinels in Calcites?

26th May 2018 21:10 UTCThomas Lühr Expert

10% hydrochloric acid will remove the calcite with no harm to the spinel

26th May 2018 21:18 UTCKyle Bayliff

Ted, there are a few acids on your list that require some warning before being used as cleaning agents. Nitric acid, for instance, is a strong acid. The other acids you listed are weak acids, which means they do not fully dissociate in solution like nitric acid. For this reason, they are in general much gentler, even at higher concentrations. In addition nitric acid is also a strong oxidizing agent. Depending on the concentration, it is more likely to cause significant damage to samples that are susceptible to oxidation (in fact it's that oxidizing power that allows it to dissolve noble metals like gold and platinum when mixed with HCl as aqua regia). It's also much more of a health hazard as it releases toxic fumes at high concentrations.

Oxalic acid should also be used with caution, too, as it forms water-soluble complexes with some metal ions, such as iron. It's good for getting unwanted iron stains out of quartz, but it will ruin some iron containing minerals.

Additionally, I don't think you need to worry about chloride ions etching your minerals away over time. Chloride ions are very stable, chemically speaking, and not likely to react with most minerals. Chloride ions are good electrolytes however, and can accelerate oxidation in the presence of moisture, but silicates, typically being fully oxidized already, are not in any danger from this.

26th May 2018 21:32 UTCAlfredo Petrov Manager

We use nitric acid for getting calcite off of dioptase, as nitric acid seems to be gentler on silicates than hydrochloric acid is. Hydrochloric acid will gelatinize many silicates, which is why it's used for dissolving natrolite off of benitoite specimens. And I ruined a nice prehnite specimen once in quite dilute hydrochloric acid ;((

Nitric acid should always be used dilute, and then there's no problem with fumes. It's actually more effective at dissolving carbonates when dilute than it is when concentrated. But, as Kyle pointed out, nitric acid (even dilute) can oxidize iron minerals (and that includes even only slightly iron-bearing carbonates) and leave ugly rust stains on your rocks.

If the mineral I'm removing is just calcite and I'm not in a hurry, I use vinegar.

26th May 2018 21:40 UTCWayne Corwin

Alfredo

Is one kind of vinegar better than another?

26th May 2018 22:01 UTCBob Harman

In my supermarket, a Kroger store, here is what is available.

Heinz brand CLEANING VINEGAR, 6% acetic acid. Approx $6.00/gallon.

Heinz brand WHITE VINEGAR for cooking or cleaning. 5% acetic acid. Approx $5.00/gallon.

Kroger store brand WHITE VINEGAR for cooking or cleaning. 5% acetic acid. Approx $4.00/gallon.

Off brand VINEGAR 4% acetic acid. Approx $2.50/gallon.

Then one from both Heinz and Kroger with a lavender fragrance added. 5% acetic acid. Primarily used for cleaning.

About the same price as the unscented vinegar.

All are dilute acetic acid, but a specific dilution need not be stated on the label to qualify as "vinegar".

If used for cooking, I suppose the dilution is with potable water.

As to the original acetic acid source, when diluted to about 5%, and reliably called "vinegar", does it really matter?

Just as does the original source for HCl matter when diluted to 28% and then reliably called muriatic acid?

If the added fragrance has any effects who knows, but this type probably is used only for cleaning????

Same thing available in hardware and building supply stores. CHEERS.....BOB

26th May 2018 23:09 UTCAlfredo Petrov Manager

Any kind of "white vinegar" should do and, as Bob pointed out, they are cheap. Avoid the more "natural" unbleached colored (brownish) food-grade vinegars, because they might perhaps stain some minerals, and are more expensive anyway.

27th May 2018 01:31 UTCPaul Brandes 🌟 Manager

Sean,

Back in 2009, longtime member Jesse Crawford put together a spreadsheet of solubility for over 600 minerals that the collector can use to clean their specimens. Below is the link to that page:

Solubility Data on 646 Common and Not So Common Minerals

28th May 2018 14:04 UTCAndrew Debnam 🌟

Sean, you can also check out this web page. I cannot be certain it is 100 percent accurate. Always best to test a "junk" piece first

marulla.com/mineral-solubility/

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