Crystal chemistry of dorrite from the Eifel volcanic region, Germany, and chemical variations in the khesinite-dorrite-rhönite-kuratite solid-solution system.

Shchipalkina, Nadezhda V.; Pekov, Igor V.; Chukanov, Nikita V.; Koshlyakova, Natalya N.; Ternes, Bernd; Schüller, Willi

Dorrite, khesinite and rhönite from metamorphosed calcic xenoliths of the Bellerberg paleovolcano (Eifel, Germany) were studied, including first determination of the crystal structure of natural dorrite (R = 0.0636). Dorrite is triclinic, P-1, unit-cell parameters are: a = 10.4316(7), b = 10.8236(9), c = 8.9488(7) Å, α = 105.972(6), β = 96.003(9), γ = 124.67(10)° and V = 754.10(11) Å3. Its crystal-chemical formula is (Z = 1): M8Ca2M9Ca2M1Fe3  M2Fe3 M3(Fe3 0.8Mg0.2)2M4(Fe3 0.8Mg0.2)2M5(Mg0.9Fe3 0.1)2M6(Mg0.5Fe3 0.5)2M7(Fe3 0.9Al0.1)2[T1(Al0.75Si0.20Fe3 0.05)2T2(Al0.77Si0.20Fe3 0.03)2T3(Al0.9Fe3 0.1)2T4Si2T5(Fe3 0.6Al0.4)2T6(Fe3 0.6Al0.4)2]O40. New and earlier published data show that khesinite, dorrite, rhönite and kuratite form a solid-solution system without significant gaps. The chemical variation and isomorphous substitutions in this system are discussed and the following simplified formulae are suggested: dorrite, Ca2(Fe3 ,Mg)5Mg[(Al,Fe3 ,Si)5SiO20], khesinite, Ca2(Fe3 ,Mg)5Mg[(Fe3 ,Al,Si)5SiO20], and rhönite, Ca2(Mg,Fe3 )5Ti[(Si,Al)6O20].

GERMANY; CHEMISTRY; CRYSTAL structure; SOLID solutions

Mineralogy & Petrology, 2019, Vol 113, Issue 2, p249

0930-0708

Academic Journal

10.1007/s00710-018-0645-0