Hardystonite, clinohedrite, willemite
Hardystonite: fluorescent blue-violet
Clinohedrite: fluorescent orange
Willemite: fluorescent green
Franklinite: non-fluorescent regions

Hardystonite {Ca2ZnSi2O7} is a classic Franklin mineral that fluoresces blue-violet under shortwave UV. It often fluoresces under longwave, too, but it's not nearly as intense.

Though its formula may not look all that exotic, hardystonite is quite rare worldwide.  In fact, hardystonite is for all practical purposes still unique to Franklin, New Jersey.  I recently checked Mindat and saw hardystonite listed for only one other locality- a slag locality in Germany, where it occurs as micro crystals.  Slags are technically not "natural" deposits, since they would not exist without human activity.  Franklin is still the only place where hardystonite occurs naturally and in any appreciable amount.

Within Franklin, hardystonite is found only in small amounts on the Buckwheat Dump.  It has also been found at the Trotter.  Hardystonite was once thought to be abundant from Franklin, though I suspect most of it is now beneath pavement or buildings-- or buried down in some flooded stope.  

There were legends of 300-lb hardystonite boulders, but nowadays these are like Sasquatch;  I've never seen one or even a photograph of one.  20 to 30-lb rocks containing significant hardystonite have indeed been found over the past decade, but only rarely.  Hence, even at the one or two existing surface dumps, hardystonite is not at all common.  It isn't that hard to find specks here and there, but a fist-sized or half-a-fist-sized specimen with good coverage is a prize to the field-collector.  

I recall a time between 2000 and 2002 when one of the school trips turned up a rock that the late John Cianciulli called "mine-fresh hardystonite".  This is the glassy, grayish stuff that looks almost electric when the UV hits it.  It might have had a "rind" from 60 to 100 years of sitting in the rain, but inside it was solid-- untouched by the creeping scourge we call "weather fractures".   It was a good find.

The specimen shown in the photo above is about 2.5 inches across in largest dimension. It may have been collected at one of the now-closed sites in Franklin;  I bought it at the Franklin mineral show a few years back.

Clinohedrite {CaZnSiO4.H2O}, another rare mineral from Franklin, is an alteration product of hardystonite.  However, this alteration occurs in the ground at some point in the formation of the hydrothermal deposit-- not on the earth's surface as the typical weathering "alteration" we think of.

Clinohedrite sometimes does occur without hardystonite; I've seen specimens where clinohedrite formed a coating on a willemite / andradite matrix. I'm not sure if hardystonite had accompanied it at one time in the geologic history or not.

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