Al6(BO3)5(F,OH)3 hexagonal mono borate
After the Namibian location jeremejevite had been found also in volcanic Eifel region mostly Emmelberg and Wannenköpfe area. Especially for Wannenköpfe Fred Kruijen has to be honored for his excavation work with friends beginning of the year 2000 times, see his photos everywhere in social media with different forms of crystals. The area at Wannenköpfe is detroyed since 2014, lot’s of micromounts had been collected from 2000-2009 there by friends of Fred Kruijen who made also great photos of these very rare and fascinating gemstones. I have to say „Thank you very much“ to Fred for lots of discussions in the past.
Jeremejevite has hexagonal bluish to white color, needle form crystals and also balls in sanidine inclusions mostly.
Yellow or greenish jeremejevite - see also my MinDat picture:
with this photo we will clarify some mysteries about items who are sold as "yellow jeremejevite". In my collection I have more than 10 items especially from Emmelberg with sanidin crystals where cavities are filled with this yellow to brownish crystals. All of them I wrote as "yellow jeremejevite". And lot of eifel literature wrote about yellow jeremejevite so collectors think, everything what is yellow has to be jeremejevite.
But the truth came after analyzation:
EDX 1: 1.3% Na, 78.2% Si, 14.8% Si, 5.7% Fe
EDX 2: 1.7% Na, 75.5% Al, 18.5% Si, 4.3% Fe
It's clearly mullite. And the volcanic mullite has often contaminations of Ti, Fe, Na and K. See the publication of Y20 in Mineralienwelt "news from Eifel" by G.Blaß who explained this topic much more in detail. Perhaps we have jeremejevite in yellow also in Eifel volcanic complex - but it's very rare and please ask for an existing analyzation.