White fluorescent light
Incandescent light
Above (left): The cabochon in white fluorescent light.  The glaucochroite is gray here, with a faint bluish cast.

This cab was cut from an unusual rock found on the Buckwheat Dump.  The cab is now in the collection of Fred L., who unearthed the rough rock while digging with Mark B. and me.  (By the way, I removed the last names because I'm getting tired of these "people locator" sites gathering data on everyone.)

The cab was about 40 mm tall, I think.


Above (right): The same cab shown in incandescent light.  The glaucochroite assumes a lilac color.

The mineral occurs mixed closely with willemite (white to pale grey).  The original assemblage also contained some hardystonite and NF bustamite (not present in the cabochon). 

The color change wasn't noticeable until the rock was slabbed.  Some rare-earth minerals, including cerite and monazite-(La), often exhibit a similar color change.


Short-wave UV
Pictured at left is the cabochon in short-wave UV light.  Now you can see which is the willemite (fl. green) and which is not. The calcite fluoresces red.  The mineral that's lilac in incandescent and gray in normal fluorescent light does not respond to short-wave. 

It's quite likely that the color-change mineral is glaucochroite.  Dunn (1995) states that glaucochroite does sometimes have the "alexandrite effect", a type of color-change effect that depends on the light source.  It also fits Dunn's description of the assemblage and the blue-gray daylight color.

This cab has at least five minerals in one specimen:  willemite, calcite, andradite, franklinite, and [what's tentatively identified as] glaucochroite.



Sources: 

Dunn, Pete J.  Franklin and Sterling Hill, New Jersey:  The World's Most Magnificent Mineral Deposits.  1995.


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