In late September 2022 Michael Farmer posted that he was at the Munich show, Rantila (aubrite) was for sale and that it was fluorescent. With several quick emails I laid claim to a few samples.
Aubrites, enstatite achondrites, are natural candidates for fluorescence because they are low in Fe. Iron tends to quench UV fluorescence. In Rantila the enstatite is Mg# 99.7, En98.4Wo1.0, diopside Mg# 99.8, En54.5Wo45.1 and olivine Mg# 99.7. In other words, each is a nearly iron-free magnesian end member. All that is needed is a fluorescent activator. Fluomin.org, lists possible candidates: Enstatite – Ti, Mn, O Diopside – Ti, Cr, Fe, Mn, Nd Forsterite (Magnesian olivine) – Mn, Cr, Fe, O
Compared to values of twelve early aubrites, Rantila enstatite has elevated Mn and Rantila diopside has elevated Cr and highly elevated Mn. Emission spectra of individual grains could confirm their role.
Rantila with light brown fusion crust for sale in Tucson January 2024.
My largest piece weighs 41.3 grams.
In longwave ultraviolet light.
Rantila is a breccia.
In longwave UV. The enstatite fluoresces pale blue, but unevenly. This is similar to Norton County aubrite. As in Norton County, the yellowish is likely forsterite. Red fluorescence would be diopside.
Midwave UV.
Here a mineral responds in shortwave UV that responds very little in the longer wavelengths. For example, see the light blue grain toward the right end of the specimen.
After exposure to longwave light some minerals continue to glow for a short time. This afterglow is sometimes called phosphorescence. In Rantila this luminescence is quite short.
The yellow glowing mineral fades before the red does.
A closer view of that SW responsive mineral and its surroundings.
The same area in LW.
This 2.55 gram fragment displays tan secondary fusion crust. Ruler marks are 1 mm apart.
The fusion crust blocks the LW UV from the underlying minerals.
The breccia has a broad range of grain size. Note the small grains below the large crystal.
This red 2.3 mm crystal is surrounded by fusion crust and stands out under UV light.
Thin section in transmitted white light. Sample is 15 mm wide.
In LW UV one grain fluoresces brighter than others.
Thin section in cross polarized light, XPL.
In XPL that feature appears glassy – remaining mostly dark as the polarizing filters are rotated.
Thin section in plane polarized light, PPL.
In PPL and in LW UV it appears to be to be partly glassy and partly crystalline.
It is the feathery quench texture of a fragment of a melt produced elsewhere some time before this breccia was assembled. Rantila aubrite thin section. FOV = 1.1 mm wide. PPL.
