Iodargyrite, Broken Hill, New South Wales
Mineral Matters #418
I have made my way through to the silver iodide mineral, iodargyrite, for the upcoming Monthly Mineral Chronicles.
Broken Hill has produced a number of silver halides, minerals that contain silver and one of chlorine, bromine or iodine. The range includes chlorargyrite (AgCl), originally termed “embolite”, aka bromian chlorargyrite; bromargyrite (AgBr), iodargyrite (AgI), boleite (KAg9Pb26Cu24Cl62(OH)48), capgarronite (AgHgClS), perroudite (Ag4Hg5S5(I,Br)2Cl2), miersite ((Ag,Cu)I), and tocornalite ((Ag,Hg)I).
Of these, chlorargyrite is the most common, followed by bromargyrite, iodargyrite and miersite, which are all scarce, and the remainder are much rarer.
Looking through specimens that I have, I came across one that I collected myself. That would have been in the early 1990s. It has been sitting in a box all that time, waiting for the day when it could show off all of its glory!
The iodargyrite looks a bit like a “snail trail”, but there is some semblance of hexagonal form, and some tiny hexagonal tablet crystals dotted around. The bright yellow-green crystals are particularly sharp bromian chlorargyrite. Most chlorargyrite from Broken Hill is just blobs and smears. And usually much darker in colour, often a purplish-green, as the mineral is prone to being affected by light. I knew there was a reason that I had left it in a box all those years! 😆
Below: Iodargyrite and chlorargyrite, Kintore Open Cut, Broken Hill, New South Wales. Width of view 9mm. Click on the image below for a higher resolution version.
Hi Steve. I wonder how easy it is to try and identify silver halide minerals by their colour.
I dimly remember in the distant days of wet analysis, examining precipitates of silver halides.
Silver chloride, when freshly prepared would be a white curdy precipitate. In daylight it would soon turn pink, then purple grey, finally black, presumably by then largely metallic silver. The silver chloride would be easily soluble in dilute ammonia solution ( ‘ ammonium hydroxide’).
Silver bromide would be a pale yellow curdy precipitate. This would be sparingly soluble in dilute ammonia solution, readily soluble in concentrated ammonia solution.
Silver iodide would be a darker yellow precipitate, insoluble in concentrated ammonia solution.
I can’t remember how quickly the silver bromide or iodides were altered by daylight. However I think silver bromide was used early on in photography.
Looking at your recent photo you have done well to protect the snail trail of chlorargyrite from darkening. Most of my samples of silver halide minerals seem to be various waxy shades of greyish yellow and orange. I would be hard pushed to differentiate between chlorargyrite, bromargyrite, iodargyrite, or those in between despite the confident labels provided with the specimens!
As I understand it, most samples are of chlorargyrite with about 40% bromide ‘bromian chlorargyrite ‘. Few are pure end members.
Could your green bromargyrite have a little copper?
On a different note to trying to identify silver mineral halides by colour alone, I wonder what might have been the source of the bromide and iodide ions?
Bill.