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Posted by Reiner Mielke  
Reiner Mielke December 17, 2009 01:24AM
I have two specimens of "mohawkite" from Michigan (photos attached). #1 is extremely tough to break and breaks with a rough granular to somewhat hacky surface texture like a piece of metal, and tarnishes in a matter of hours to brownish then eventually to black. Specimen #1 when freshly broken was silvery white but is now tarnished brownish. Specimen#2 is very brittle and breaks with a concoidal fracture and does not tarnish ( at least not in the 6 months I have had it). The photo is of a freshly broken surface. The color of the fresh broken surface reminds me of nickeline but is less reddish. The old surfaces on specimen #1 are black, the old surfaces on #2 are covered in green annabergite and there are no black surfaces. I thought at first that #2 was nickeline but unlike nickeline it dissolves rapidly in nitric acid with lots of fizzing and turns the solution blue.
The difference in toughness is obviously that one has abundant native copper ( arsenic rich) and the other doesn't. Is this significant in any way in terms of the mineralogy of the CuAs minerals in the samples? Is there any correlation between the presence of native copper and the abundance of algodonite vs. domeykite or any other species for that matter? Could #2 be something other than mohawkite, the annabergite and lack of black tarnish have me puzzled ?

Edited 3 time(s). Last edit at 12/17/2009 02:03AM by Reiner Mielke.
open | download - Mohawkite1.jpg (293.4 KB)
open | download - Mohawkite2.jpg (671.5 KB)
Lloyd Van Duzen December 17, 2009 12:15PM
Reiner, could you do a streak test?

you mention tarnishing, this seems to be a characteristic of some copper sulphides. I believe some pyrite shows a silvery surface on fresh fractures then tarnishes in time. pyrites tend to leave a dark streak.

My guess would be some form of pyite. Not all pyrite form cubic crystals. Chalcopyrite is similar but usually has a irridesent colour accosiated with it. Pyrite, chalcopyrite and pyrrohtite are all forms of copper sulphide.

maybe research copper sulphides.

best of luck.
Reiner Mielke December 17, 2009 02:42PM
Hello NR,

Thanks for your suggestions but the color and tarnishing are wrong for pyrite, cpy or po. Also the reaction to nitric acid is wrong for all of those. The specimens were purchased from dealers labelled as Domeykite and Mohawkite respectively. I am sure that is what they are, just not what the dominant components are. In the case of #1 I am sure copper and domeykite are dominant but #2 has me puzzled.
Lloyd Van Duzen December 17, 2009 10:54PM
Reiner maybe if you know where exactly in michigan it came from you might find the answer to your question by researching its location or by known locations that have confirmed discoveries.

here's a link from the web, don't know if you have seen it or if it will help you but here it is anyway.

hope this helps.
Reiner Mielke December 18, 2009 02:54PM
Found the answers I was looking for at this link:
Paul Brandes December 18, 2009 07:11PM

From the looks of your photos, they both appear to be the same mineral. If the dealers had them listed as Domeykite and Mohawkite, they were selling the same mineral as Mohawkite is the local, Keweenaw name for Domeykite since it was predominately found at the Mohawk Mine; however, it was also noted at the Ahmeek, Gratiot, and Seneca Mines. Domeykite in the Keweenaw is a mixture of copper, cobalt, arsenic, and nickel. It can have varying amounts of each element which can give any specimen a unique colouration as well as unique tarnishing effects.

Unfortunately, your photos are a little too close to get a good idea of the rest of the rock/matrix, in my opinion. I would actually like to see the matrix it's in to get a better idea of its identity and origin. As I stated earlier though, they do look like domeykites and definitely not pyrite, chalcopyrite, or any other of the copper sulphides. While pyrite does exist in the Keweenaw, it is a very rare occurrance. The reference you provided from the Mineralogical Society is a great source of information into domeykite and its associated minerals.

Take care, and good luck!! B)
Ed Drown December 18, 2009 11:25PM
Hi Reiner,

An interesting description of the copper arsenides found in the Keweenaw can be found in "The Copper Deposits of Michigan" by Butler & Burbank; U S Geological Survey Professional Paper 144 (1929).

The Mineralogical Society of America has the text of the paper hosted on their website.

The table of contents can be found here

The section on mineralogy containing the discussion of the copper arsenides is here


Reiner Mielke December 19, 2009 03:14PM
Thanks Ed,

The section on mineralogy was a great help, just what I needed!
Donald Vaughn December 19, 2009 05:43PM
a very enlightening discussion as I have been pondering an Algodonite and Whitneyite specimen recently and was trying to find out more specifically whether Whitneyite was a discrete arsenic compound as some references state or as mindat state a variety of arsenic rich copper as in an alloy/mixture
Bill Baker Barr December 19, 2009 06:31PM
The copper-arsenic relationship at the Mohawk Mine, where most of the material called mohawkite was found, is quite variable. Whitneyite (copper-arsenic alloy with some algodonite) has a Cu:As ratio of 8:1 or greater. In algodonite, the ratio is > 3:1, and in domeykite, it is 3:1. It's even lower in two other copper arsenides, koutekite (5:2) and paxite (1:2). In my observation, the higher the arsenic level, the faster the tarnishing and the blacker the end result. Fresh colors of yellow, pink or orange indicate the presence of free copper; this type of material tarnishes fairly slowly to an iridescent film and eventually a bronzy patina. High copper levels also affect fracture; such material is tougher (very hard on diamond saw blades!), less brittle, and breaks with a hackly surface. Cobalt and nickel are wild cards, of which I know little.

Incidentally, Japanese metalworkers use an alloy called kuromi-do (99% Cu, 1% As) in the mokume-gane (wood-grain metal) technique because it works like copper but acquires a black patina that contrasts well against copper, silver and gold. It could be considered an artificial whitneyite.

Cheers, Bill
Scott Sadlocha December 20, 2009 05:00AM
I have a small bunch of "mohawkite" that I acquired from someone living in Mohawk, Michigan, and was looking it over tonight after reading this post. I am not sure of the associations or relationship of all of the minerals involved, and hope to understand it a bit better after educating myself a bit.

I am in the process of rebuilding my primary PC at home, and I am getting ready to do so tonight or tomorrow morning. Barring any issues, I should be back up and running soon, and I would like to post some pictures of the pieces I have, to get some opinions. I do have a picture of the largest of the pieces, but it is not that good. Hopefully it is okay for starters. Any opinions of the composition would be appreciated.

The color is very silver, and the luster is metallic of course (some of the other pieces are more gold colored, and a few exhibit some iridescence). The matrix is a very small part of the piece, and there is a thin band of light green between the two, I am guessing this to be chrysocolla. This piece is about 7cm x 5cm x 5cm and very heavy for its size.
open | download - Mohawkite (Algodonite-Domeykite) 1A.JPG (101.5 KB)
Reiner Mielke December 20, 2009 04:14PM
My understanding of this, after reading a bunch of papers and doing some testing, is that there are basically three types of "Mohawkite". A tough variety with arsenic rich native copper that tarnishes easily called "Whitneyite", and less tough variety that is predominantly Algodonite with lesser arsenic rich native copper that also tarnishes relatively easily and a brittle kind which does not tarnish easily and is predominantly Domeykite, and then there is everything inbetween. Toughness depends on the amount of native copper in the sample. The brittle kind is often intergrown with rammelsbergite and/or nickeline +/- pararammelsbergite which can result in alteration to Annabergite. The dominant components of "Whitneyite" are Algodonite and arsenic rich native Cu ( approx.5% As). The other mixes have varying ratios of Domeykite/ Algdonite/Copper. There seems to be a correlation between native copper and Algodonite, in other words as the native copper content increases so does the Algodonite/Domeykite ratio. The samples with the highest % Domeykite have the least amount of Native copper and are the most brittle.
Rob Woodside December 20, 2009 07:25PM
It's been a while since I looked at these, so I kept quiet, but Reiner's last post is what I recall.
Paul Brandes December 20, 2009 08:03PM
Remember Reiner, "Mohawkite" is the local, collective term in the Keweenaw Peninsula of Upper Michigan for a silvery metallic mineral that is not silver, so Domeykite, Algodonite and Whitneyite could potentially fall under the umbrella of "Mohawkite" since most of it was found at the Mohawk Mine in Mohawk. While most local rock shops still use the term Mohawkite, it should be called Domeykite.

Your explanation of the differences between the three Reiner is spot-on!! (tu)
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