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Techniques for CollectorsHelvite from Jinlong Hill, China

17th Jul 2012 20:24 UTCJohn Sobolewski 🌟 Expert

In Tucson, I acquired some of the black Helvite crystals on Quartz from Jinlong Hill, China. I wanted to to get rid of some of the iron staining on the Quartz, so I put them overnight in a weak solution of Oxalic acid after checking the book by Sinkankas that Helvite was soluble in Hydrochloric acid with no mention of problems with any other acid. To my amazement, these "Helvites" dissolved completely. The first photo shows the triangular cavity left where a Helvite crystal completely dissolved, while the second photo shows the black renmants of a larger partially dissolved crystal.

The funny thing is that I also cleaned some yellowish Helvites with Quartz and Spessartine from the Wushan Spessartine Mine in the same solution at the same time and these were cleaned and came out intact. The conclusion is that the black Jinlong Hill Helvites behave compleletely different from the yellowish Helvites from Wushan in a solution of weak Oxalic acid. Does anybody have an explanation for this difference?

This experience also demonstrates that you should always first experiment with a small specimen before cleaning a whole batch, no matter what Sinkankas or anybody else says. John S.

17th Jul 2012 20:50 UTCAlfredo Petrov Manager

They were perhaps iron-manganese-oxide pseudomorphs after helvite, and not helvite itself?

17th Jul 2012 20:57 UTCPeter Haas

After a quick look at the photos for this locality, I think that Alfredo is almost certainly right. Many specimens do not look as if they still were helvite. Example:

http://www.mindat.org/photo-359958.html

Same with many "hematites" by the way. Will have to send lots of "inquiries".

17th Jul 2012 22:48 UTCJohn Sobolewski 🌟 Expert

Thank you, Alfredo and Peter. That may explain what happened but I am still surprised at how quickly the 0.5 inch crystals of "Helvite" dissolved even if they were iron-manganese- oxide pseudomorphs after Helvite. It also raises the question if many of the photos of "Helvite" from Jinlong hill are also really these pseudomorphs and should be renamed. With one or two exceptions, they all look my black ones that just vanished in dilute oxalic acid. John S.

17th Jul 2012 23:04 UTCAlfredo Petrov Manager

John, I suspect that the speed of dissolution is due to the fact that pseudomorphs are often fine-grained aggregates with considerable porosity, rather than single crystals as the original helvite was, even when the grains and porosity isn't visible. Porous, granular materials will dissolve much faster than single crystals as so much more surface area is exposed to the acid. Anyway, I feel for you - I've had stuff disappear on me while cleaning, and I hate it when that happens :-(

18th Jul 2012 02:12 UTCJohn Sobolewski 🌟 Expert

Thanks, Alfredo. That may be part of the reason but I still find it hard to understand why half inch crystals dissolved completely before the very thin surface layer of iron oxides on the Quartz had a chance dissolve. John S. .

18th Jul 2012 02:27 UTCAlfredo Petrov Manager

I'm just taking a wild guess here, John, but perhaps the iron oxide film on the quartz is not an iron oxide film? Collectors do tend to assume that any ugly brown film is iron oxides, but several other things are possible too, including iron phosphates, arsenates, silicates (eg: hisingerite). Hisingerite is more common than folk think and isn't dissolved by oxalic acid or "Iron Out".

Incidentally, was any insoluble "sludge" left over on the bottom of the container, or did the "helvites" dissolve completely?

18th Jul 2012 11:40 UTCPeter Haas

Citric acid is a weak acid, a fairly good complexant (chelant) for several cations (most notably Ca²⁺) and a mild reducing agent, and these properties can play together in certain conditions to make it a powerful cleaning agent. As an example, consider an element that exists in different oxidation states. A complexing agent will remove one of the species (oxidation states) preferentially (Note1: the oxdiation state makes them different and they do not form equally stable complexes with a given complexant; therefore, addition of a complexant will always alter their concentration ratio; Note 2: technically, the cation is not removed, but only incoporated in a complex; chemically, however, the complexed and the un-complexed cation are different species). This has an impact on the redox potentials, which all strongly depend on concentrations (Note: tabulated E₀ values only apply to the rather hypothetical case that the activities (roughly approximate to molar concentrations) of all species involved in the reaction are equal to one; only then, the logarithmic term in Nernst's law disappears (because log 1 = 0) and E becomes equal to E₀)

Also, manganese oxides are chemical waste bins. Manganese can combine with a large variety of cations to form a large variety of "manganese oxides" whose chemical properties vary within a large range. If you also keep in mind that their cristallinity is variable and that what is referred to as "manganese oxide crusts" may contain certain amounts of other manganese compounds (phosphates, silicates) and of unrelated material ("limonite", clay), you will have to admit that a comparison of their reactivities will lead nowhere if you don't know their composition.

18th Jul 2012 17:06 UTCWoodrow Thompson

Some of the helvites from Wushan Spessartine Mine are very fresh, while others have at least a coating of (or maybe deep alteration to) a sooty black mineral. Has anyone tried to clean these black crystals from Wushan? I wonder if they, too, might disappear! :-S

18th Jul 2012 19:46 UTCJohn Sobolewski 🌟 Expert

Alferdo,

I did not see any sludge at all. The "Helvites" seem to have dissolved completely. My second photo shows a larger crystal that did not have the time to dissolve completely. As you can see, there is no sludge or anything in the cavity, just the remnants that I am sure would have disolved if left a bit longer in the solution.

As I said before, that leads to the question whether the many photos of the black "Helvite" on Quartz from Jinlong hill should be relabelled that they may be iron-manganese oxide pseudomorphs after Helvite. They need to be analysed more closely to see what they really are. John S.

18th Jul 2012 20:30 UTCPeter Haas

John,

I have already sent inquiries and asked the uploaders to share their thoughts on that matter in this thread.

18th Jul 2012 22:31 UTCRob Woodside 🌟 Manager

Helvite is a beautiful canary yellow. As the Mn oxidises it becomes caramel and finally black. Marc Wilson at the Carnegie analised his large black tetrahedron and found it was Romanèchite. I'm surprised it wasn't a mixture. There is no black Helvite

19th Jul 2012 00:45 UTCJohn Sobolewski 🌟 Expert

Rob,

It is interesting that Marc Wilson found that his black "Helvite" was a Romanechite. As I said in my first post, the yellow Helvites from Wushan came out OK but the black "Helvites" on Quartz from Jinlong hill dissolved completely. There are 15 or so photos of the latter on mindat.org. They should be studied more closely. John S.

19th Jul 2012 13:02 UTCMartin Slama Expert

I have some of these "Helvites" in my collection and I have a thesis. Some of the crystals are rough with a matt finish. Sometimes these xls have a bright core. It seems that the Helvites dissolve and only the Manganoxides is there. But some crystals have an exact form and are very bright. (such as http://www.mindat.org/photo-154540.html)

It seems to be that these xls are the primaryHelvite. My Qustion is, when it is really Helvite and the xls are rotting, where is the Be gone? There`s no other Beryllium mineral listed!

Martin Slama

19th Jul 2012 15:13 UTCWilliam C. van Laer Expert

Rob: PURE helvite is canary-yellow (as found in massive rhodonite in Butte, Montana), but what I have collected from the miarolitic cavities of the Sawtooth Batholith is clearly reddish-brown:

Note that the crystals in this picture were found lying on a boulder ABOVE the pocket from which they came; some one had cleaned out the pocket and left several of these crystals behind! They appear to be somewhat weathered as a result.

I have cleaned many of these in oxalic acid without any deleterious effect.

19th Jul 2012 19:29 UTCPeter Megaw 🌟 Manager

Not at all clear why manganese oxides would dissolve so quickly or thorouoghly, especially romanecheite. Any efferevescence in the process?

On a side note...I believe in his preamble to the "mineral cleaning" section of almost everything Sinkankis wrote he recommended always trying things out on a small inconspicuous area first,

19th Jul 2012 20:12 UTCPeter Haas

"On a side note...I believe in his preamble to the "mineral cleaning" section of almost everything Sinkankis wrote he recommended always trying things out on a small inconspicuous area first."

Well true !

On similar occasions, when things have gone horribly wrong, I use to ask people too why they did not take the time to read the manual before they started to do what they better shouldn't have done - very much to their disliking ... :-D

19th Jul 2012 23:02 UTCPeter Haas

"It seems to be that these xls are the primaryHelvite. My Qustion is, when it is really Helvite and the xls are rotting, where is the Be gone? There`s no other Beryllium mineral listed!"

Most beryllium compounds are readily soluble. Although an alkaline earth element, its chemistry is closer to that of aluminium than to that of magnesium (same with boron: its chemistry is closer to that of silicium than that of aluminium). Be2+ and Al3+ have both very high ion potentials (net charge over diameter ratios) and are easily adsorbed onto materials with a high specific surface area. I wouldn't be surprised if you find the "missing" beryllium in a bulk analysis of the manganese oxides.

20th Jul 2012 05:23 UTCJohn Sobolewski 🌟 Expert

Peter,

No there was no effervescence at all and yes, I still have no idea why manganese oxides, or iron oxides for that matter, would dissappear so quickly. This episode made me realize once again that we all need to test first before we clean several specimens, no matter what others may say.

John S.

18th Jul 2019 05:44 UTCBerthold Ottens Expert

working again on Chinese minerals I have to add information that Jinlong produce also fine crystallized phenakite crystals together with quartz and so called hematite. Hematite seems to be magnetite pseudomorphs after hematite. Check your specimens . Bert O.

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