Forsterite from Sar-e-Sang, Badakhshan, Afghanistan
I replied to a question [in the FB FLM group] asked by Robert Jones about a red phosphorescent mineral in a specimen containing Lazurite from Afghanistan. My reply contained a couple of pictures of an Afghan specimen in my collection that contains red phosphorescent forsterite. This post adds more information and pictures of the forsterite specimen. The forsterite in this specimen has been confirmed with XRD analysis. The specimen is 11cm by 9cm and is from Sar-e-Sang, Badakhshan, Afghanistan.
The first picture shows the fluorescence in “Full Wave” UV light. The specimen was illuminated with LW, MW and SW light. Sodalite in this picture is yellow-orange. There are variations in this sodalite mass. Some of the sodalite is phosphorescent, some has no afterglow. Trace amounts of titanium activate the afterglow in the sodalite. There are several spots in the sodalite that have been altered by radiation by small imbedded grains of uraninite. The spots have a brighter yellow color in the LW picture, and a redder color in SW light. The spots have no afterglow and they are tenebrescent. The other fluorescent minerals in this picture are calcite (red), forsterite (yellow-brown) and diopside (blue-white).
The second picture shows the longwave UV response of the specimen. Sodalite glows yellow-orange in LW light. Di-sulfide ions activate the yellow-orange fluorescence of the sodalite. Diopside fluoresces blue-white. The fluorescence is likely caused by trace amount of titanium. Forsterite in this specimen glows red in longwave light. This is not typical for forsterite from Sar-e-Sang. Typically, Afghan forsterite fluoresces with a weak to moderate intensity that is white to pinkish-white in LW light. Crystals with a coating of diopside and a core of forsterite is a common habit found at Sar-e-Sang.
The third picture shows spectra measured on the sodalite, forsterite and diopside in this specimen. 365nm filtered LED flashlights were used to illuminate the specimen to measure the LW fluorescent response. The spectra of sodalite show a broad peak from 500nm to 750nm with several smaller peaks at 604nm, 622nm, 646nm, 668nm, 692nm. These smaller peaks are characteristic of di-sulfide activation. The forsterite has a broad peak centered at 630nm. This indicates that manganese activates the red fluorescence in forsterite. The diopside fluorescent spectra shows peaks at 419nm, 445nm, and 470nm. Titanium is a common activator of diopside fluorescence. Not sure if these spectra indicate titanium and some other activator.
There is a short video at the bottom showing the red flash or BIP (Brief Intense Phosphorescence) of the forsterite. This red flash is a common characteristic of Afghan forsterite. Scanning the intense light from a Convoy type flashlight across the forsterite causes the red flash.
The fourth picture shows the specimen illuminated in mid-wave (MW) UV light. Afghan calcite fluoresces brightest in MW light.
The fifth picture shows the specimen illuminated by shortwave (SW) light. The sodalite fluorescence is considerably dimmer in shortwave light. Calcite fluorescence is also dimmer in SW light. Diopside fluorescence changes to a bluer color and forsterite has a yellow-brown color. Yellow to yellow-brown SW fluorescence is typical for Afghan forsterite in both massive and crystalline form.
The sixth picture shows the afterglow after exposure to SW UV light. The white sodalite afterglow is very bright and long-lasting. The sodalite afterglow happens after brief exposure to shortwave UV light. The red afterglow of forsterite is less intense, it is long lasting, but not as long as the sodalite afterglow. The forsterite afterglow usually requires at least 30 to 60 second exposure to intense shortwave UV light. I usually hold a 35w or stronger light against the specimen to activate the red afterglow.
The seventh picture shows the white light image of the specimen and the nineth picture shows the tenebrescence of the radiation altered sodalite. The tenebrescence has a dark purple color.
The remaining pictures show the back side of the specimen. Same minerals as the front side. The back side has a couple of large forsterite crystals coated with diopside. The largest crystal in 5cm long. The crystals rise above the calcite matrix. It is common practice with the Pakistani dealers or the Afghan miners to etch specimens in acid to remove calcite to search for pristine crystals of lazurite, afghanite, sodalite, etc. Fortunately, they left enough calcite in this specimen to hold the crystals in place and add to the fluorescent aesthetics. That is not always the case.
The remaining pictures are full wave, longwave, mid-wave, shortwave, afterglow, and white light.