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Rhodonite {MnSiO3} - Pink
Franklinite {(Zn,Mn,Fe)O(Fe,Mn)2O3} - Black

Most every mineral book has a photograph of rhodonite, one of the classic Franklin minerals.   The books typically show the more spectactular forms:  bladed rhodonite or big, blocky crystals of rhodonite.  It is not very likely you'll find such a piece in the field.   Maybe they should be showing you a more representative specimen, like the one pictured above.  It's still one of the better examples of dump-collected rhodonite, at least with regard to overall color and richness. 

Rhodonite occurs in quite a few other places besides Sussex County, New Jersey -- there are too many localities to name, in fact.  I can think of a few offhand, though -- California, Arizona, Nevada, Washington, Oregon, B.C. (Canada), Russia, Sweden, Japan, Australia.  Rhodonite isn't exactly rare, but it isn't all that common either.  Specimens having good, powder-pink color are even less common.  Actual transparent rhodonite specimens have come from Broken Hill, New South Wales (Australia).  These would probably rank as the rarest of rhodonites.

Rhodonite from New Jersey is almost always opaque.   Some of the bladed rhodonites from Franklin are translucent;  these are rare and sought-after.

Rhodonite's pink color comes from the +2 valence state of manganese.  The Mn2+ ion may not be all that fragile per se, but in rhodonite it oxidizes with suprising ease in the presence of moisture.   (Pink tourmaline also gets its color from Mn2+;  I don't know what would happen to one of these if you left it out in the rain, but I wouldn't like to sacrifice any to find out). 

For all practical purposes, the oxidation of rhodonite is irreversible.  It happens at room temperature or even in the cold, as long as there's moisture.  Once a specimen turns black, you're never going to recapture the original pink color.  It happens fairly quickly on the rock dumps;  rhodonite found there will be pink only on a freshly-broken surface.  These surfaces will lose their vibrant pink color after just a couple days of rain.  After as little as a week they'll be brown.

I once stored a Franklin rhodonite outside but protected from rain and sun;  fog and moisture still ruined it in one summer.  I won't do that again.

The specimen pictured above has the characteristic "rind" of black, weathered material on the back, meaning it must have come from one of the rock dumps at some point.  It looks like something from the Trotter.  Whichever part of the Franklin complex it came from, it is a pretty nice specimen.

Sometimes rhodonite is not pink even when it hasn't sat out in the weather.  It can be more of a brown color or even a dull gray.  Rhodonite can occur together with one or more chemically-related minerals including bustamite, tephroite, and pyroxmangite.

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