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[nurbo]

hi all,

I am etching a piece of san benito mine rough and when the Natrolite was beginning to disappear in the acid, I found numerous tabular black/blue crystals sitting on the Natrolite which at first I couldnt really identify. After melting the protective wax from the matrix and alkalising the specimen I rinsed it with cold water and noticed a green powder had formed around these crystals and other larger ones were appearing, a quick search for Cu bearing minerals from the San Benito mine unearthed Djurleite (Cu13S16) I reasoned that the sulfur was bonding with the oxygen in the water to release hydrogen and sulfur dioxide ond the copper was oxidising to form the green powder, copper oxide. I know if Im wrong this is the place where I will be corrected, so heres the point of this posting,

Am I right?

Its quite a piece, very subtle, it has a 10 mm Neptunite, 5 or so Joachinite's, 2 tiny Benitoite's and I think 5 Djurleite's very well spread out across a 65 x 35 mm matrix. I'll post a photo when its finished.


Thanks in advance
db

[GneissWare]

It does occur at the locality. There are a couple of good photos on Mindat that you might want to check.

[lluis]

Hi, Nurbo

I agree with GneissWare that probably what you find is djurleite.
At least I always thought that it is in the pieces I atched when I saw that green..

For the reactions involved

Djurleite is a cuprous (more or less...) sulfide

Sulfhidric acid is a very weak one. Salts of it in presence of strong acids, as is the hydrochloric acid used to etch the natrolite, protonize and give sulfhidric acid, that is a gas more or less soluble in water.
Cuprous cation is oxidized to cupric cation by the oxygen of air.
That leads to complex cupric chlorides that bear that green.
Cupric oxide is black (tenorite) and cuprous oxide is red (cuprite). Very finely divided could be seen as yellow (as prepared in labo in some conditions)

Hope that it serves.

With best wishes

Lluís

[nurbo]

Thanks for that, my chemistry is basic to say the least, Ive got a maths degree but I never studied chemistry. I need to go and research bonding because I'm confused about what bonds with what and what determines it.

So its reacting with the HCl to form cupric chloride not as I assumed with the water to form an oxide?

One observation was that the small flat tabular crystals were completely encased in Natrolite, whereas the larger crystals are growing from the matrix, Im wondering if they form within the Natrolite or if they are washed through the cavities from further up the system and that is how come they are not growing on the matrix?
Cheers
db