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Identity HelpFormation Question Regarding Prehnite with Copper

25th Jul 2010 05:21 UTCScott Sadlocha

Hello All,

This is not so much an identity help question, but more of a formation help question. I wasn't sure whether it should be posted here or not, so feel free to move it if needed. Anyways, a while back I was able to purchase a specimen of prehnite, listed from Copper Falls Mine, in Michigan's Upper Peninsula. The specimen is about 5 x 4 x 3 cm in size. The prehnite has a great deal of included copper, as well as a few exposed crystals of copper.

What really has me curious is the form of the prehnite. It basically looks like a prehnite sponge, with many of the holes passing completely through the specimen. There are holes throughout the entire piece, especially on what I would consider the "back" of it. A lot of these holes have geometric shape to them, with the holes being larger toward the back, and smaller toward the front. I figure that this is some type of cast, and that the original mineral dissolved. I thought that maybe the prehnite and copper were on calcite, and the calcite was somehow removed. I was just looking for opinions on it, and if I am right, a little background on how this might have formed.

There is a bit of matrix on the bottom, and it looks to be the maroonish basalt of the Keweenaw peninsula. There are also a few other small contact points throughout.

25th Jul 2010 05:49 UTCScott Sadlocha

Here are a few more shots of the same piece.

25th Jul 2010 06:48 UTCTravis Olds Expert

Scott,

Like you said, it definitely looks like someone dissolved the calcite away. As for how it formed, you can read Tom Rosemeyer's article, "The Kearsarge" in Volume 82 of Rocks and Minerals. "Prehnite...was present in the Kearsarge lode as a vesicle filling and was also crystallized on earlier formed minerals." pg 294.

Travis

25th Jul 2010 06:52 UTCRock Currier Expert

Prehnite grows on many things and makes spectacular caste after anhydrite and glauberite in the pillow basalts of the Watchung range in New Jersey. More recently wonderful clusters of long casts of pale green prehnite after laumontite have come from the quarry at Malad, India, near Bombay and even more recently there have been some large prehnite casts after anhydrite? up to 20 cm from the recent find of prehnite and epidote near Bendoukou village, Kays region, Mali. What your prehnite is after?? Calcite? Quartz? possibly something more exotic? Anhydrite is a possible suspect and many anhydrite casts are rectangular in nature.

25th Jul 2010 13:16 UTCPaul Brandes 🌟 Manager

I have seen specimens like this many times from the Keweenaw; I even have a few myself. What it looks like is that someone has etched out the calcite (most likely with HCL) to expose whatever other minerals were in the rock. Copper Falls has quite a bit of silver and I bet whoever you purchased this from was looking for silver or a copper/silver halfbreed and was disappointed to find out they only had prehnite, crystalline datolite, and a few copper blebs.

Neat little specimen though!! (tu)

30th Jul 2010 02:14 UTCScott Sadlocha

Thanks for the information everyone. I like this specimen a lot, and I appreciate information on its background. I was curious whether the previous mineral was etched out naturally or not, and you answered that for me. I love prehnite, so I am quite happy with this specimen. Also, thanks of the heads up on that issue of Rocks and Minerals--I was not aware of it, but I am very interested. I put in a request for the back issue, and I am waiting for them to contact me. I just hope I can get it before I head up that way later this summer.

Also, I just got my newest issue of R&M today, and it has another article regarding Copper Country, so I need to get to reading.

30th Jul 2010 02:43 UTCScott Sadlocha

Wow, that was quick. I just got a call from Marie Huizing at R&M and the issue should ship out tomorrow. Can't wait to read it, especially because it has an article on the Huron River Uranium prospect as well. Wonderful!

31st Jul 2010 03:58 UTCKeith A. Peregrine

Good bet that the calcite has been dissolved out. Done it plenty of times myself leaving some of the Prehnite specimens looking like swiss cheese.

One more week and I'll be back up in the UP. What a beautiful and satisfying place it is! Copper Country, here I come!!!

1st Aug 2010 00:41 UTCPaul Brandes 🌟 Manager

With any luck, we'll be in the Keweenaw around the 11th or so; that is, if we ever get off Newfoundland. :D

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