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Charlotte · Posted: Apr 18, 2008 11:57 Post subject: Rhodochrosite twinning

As a collector of Rhodochrosite, I have noticed twinning frequently, but, interestingly, only from one locale - Mont St.-Hilaire. I have not encountered twinning from any of the many other localities. Even from Mont St.-Hilaire, I have seen it only in specimens which show the dark brown color, which I have always attributed to Birnessite inclusion.

I wonder if anyone can explain (1) why twinning is so rare in Rhodochrosite, especially considering the extensive twinning in Calcite, similar chemically and in the same crystallization class and (2) what is it about Mont St.-Hilaire that leads to this twinning occuring "only" here?

I would apprecaite any opinions or corrections.

John S. White · Posted: Apr 18, 2008 14:15 Post subject: Re: Rhodochrosite twinning

I have a twinned rhodochrosite from Mont St. Hilaire and it is a typically pink color, not dark brown, but it is zoned with a darker core. I also just obtained an almost identical type of twin but in this case it is dolomite and is from Hormuz Island, Iran. This one is very dark brown, nearly black.

I am unable to explain why such twins are seldom, if ever, seen elsewhere.

Charlotte · Posted: Apr 18, 2008 14:44 Post subject: Re: Rhodochrosite twinning

Thanks, John. It is good to hear someone with your experience and expertise echo my observation. You refer to "such twins". Are you referring to twinning of this particular type (I would love to be able to describe it with a term for this particular twinning plane, but I can't, hence the photo)?

I don't believe I have seen a true Rhodochrosite twin of ANY type from any other locale. Would you agree with that statement?

If it is true, there must be an explanation somewhere in that crystal lattice.

John S. White · Posted: Apr 19, 2008 07:02 Post subject: Re: Rhodochrosite twinning

Photo of my twinned rhodochrosite. It is 3 cm top to bottom.

lluis · Joined: 17 Nov 2006 Posts: 712

Very, very nice!
If anytime you want to part with it, I could gladly offer it a new home! :-)

Thanks for sharing

With best wishes

Lluís

Charlotte · Posted: Apr 19, 2008 09:53 Post subject: Re: Rhodochrosite twinning

That's a beauty! Great example of trigonal symmetry and the first twin of a PINK Rhodochrosite from Mont St.-Hilaire that I have seen. I wonder if the dark core is significant. I am also happy to offer it a new home, should you wish to part with it.

John S. White · Posted: Apr 20, 2008 05:30 Post subject: Re: Rhodochrosite twinning

I acquired this piece from Tyson's Fine Minerals in 2004. Rod may have more of them.

lluis · Joined: 17 Nov 2006 Posts: 712

Thanks a lot, Mr. White

I will try and see how lucky-unlucky I would be :-)

With best wishes

Lluís

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