Fluorescent Xonotlite - Franklin, NJ

Xonotlite: FL Blue to violet-blue SW
Clinohedrite: FL Orange SW
Calcite (or possibly roeblingite?): FL Red to pinkish-red SW
Willemite: FL Green SW
Manganaxinite: Non-FL; pale amber to honey-yellow in daylight
Andradite: Non-FL; brown in daylight
Native Copper: Non-FL; tiny specks visible with a loupe

No, the fluorescent violet-blue material in the photo above is not hardystonite. It is xonotlite. Sometimes the difference in fluorescent colors between the two minerals is minor, despite claims from some collectors that "mineral A always fluoresces a particular shade of purple, mineral B always fluoresces a distinct shade of blue". Xonotlite's SW response does tend more toward blue, but it isn't free of purple. An experienced collector can still tell it from hardystonite in most cases. The associations and habit are also important clues. (The presence of clinohedrite, by the way, does not distinguish xonotlite from hardystonite; clinohedrite can accompany either mineral.)

Xonotlite sometimes occurs as seam coatings in manganaxinite, hyalophane, celsian, or andradite. Such coatings may be only fractions of a millimeter thick. Under a microscope these seam coatings may reveal themselves as layers of tiny, thin crystals.

Left: the same specimen in normal light. The xonotlite is a thin coating covering most of a fracture face.

Matrix is mostly manganaxinite, which in this case has no noticeable fluorescence. The axinite does have some specks of native copper in it, however.

Even the weakest acids, including vinegar, will dissolve thin layers of xonotlite; therefore, don't use vinegar or other acids to clean specimens having thin seam coatings or fine crystals of this mineral. It seems to be every bit as soluble as clinohedrite, maybe more.

Xonotlite can also occur as massive, dull-white to gray material, usually with hendricksite mica and sometimes with hancockite. This massive xonotlite may contain small specks or larger spots of roeblingite.

Some Franklin xonotlite resembles cracked plaster in the daylight. This is a classic habit for the Franklin occurence. One such specimen I have that fits this description is in a matrix of andradite garnet, though there are no other fluorescent minerals (except willemite) associated with it.

Finally, xonotlite may occur as white crystals, usually tiny and occurring within a porous matrix of Parker shaft minerals (altered hancockite, ganophyllite, etc.). Dr. Dunn's monograph (1995) describes this assemblage.

The specimen pictured above contains some red- to pinkish-red-fluorescing areas that may be either calcite or roeblingite. Under the microscope these areas do not have the typical appearance of calcite. They consist of tiny crystals, but they have some curvature and are not well-formed. They are subtranslucent and colorless to pale gray. They have the brief intense phosphorescence (BIP) or "flash" in short wave UV, meaning they're probably not axinite. I need to test these for lead and perhaps for sulfate ions. Unfortunately the tests require some sacrifice of material, and I don't have much.

Calcite has the confusing habit of infiltrating other minerals and making them appear to be something else; I've seen calcite-infiltrated minerals that pretended to be fluorescent roeblingite, bustamite, and even manganaxinite. The BIP gives away the impostor in the latter two cases, if BIP is even a consistently reliable indicator. (Side note: I'm still not clear on whether roeblingite gets its fluorescence from calcite anyway. I have seen non-fluorescent roeblingite at least once.)

Pictured specimen is about 2 3/4 inches across maximum dimension.