TY - JOUR

T1 - Crystal chemistry of nephelines from ijolites and nepheline-rich pegmatites

T2 - influence of composition and genesis on the crystal structure investigated by X-ray diffraction

AU - Vulić, Predrag

AU - Balic Zunic, Tonci

AU - Belmonte, Louise Josefine

AU - Kahlenberg, Volker

PY - 2011

Y1 - 2011

N2 - Ten nepheline single crystals from five different localities representing rocks from nepheline-syenite pegmatites to urtite, ijolite and cancrinite-ijolite were investigated chemically and structurally. The chemical compositions were determined by electron microprobe, whereas the crystal structures were refined against X-ray diffraction data to R values of 0.022–0.031. In all cases the crystal structures conformed to space group P63, but at the same time all crystals produced some satellite reflections testifying to the modulation effects in their structure. This work confirms the presence of satellite reflections as a universal property of nepheline and resolves some controversy about this issue present in older works. The samples show a range of compositions with variable excess of Si over Al combined with vacancies in the alkaline sites. Vacancies are present exclusively in the symmetric ditrigonal channels occupied preferentially by K atoms in natural nepheline. The influence of the variable amounts of K, Na and vacancies to the increase of volume at this site is shown to be in the order ¿>K>Na. It has been confirmed that natural nepheline can partly depart from Loewenstein’s rule and show some degree of disorder in the Si/Al distribution. Most of the samples revealed a practically full Si-Alordering, but the crystals from one of the nepheline-syenite pegmatites in Langesundsfjord, Norway and from the type locality of ijolite, Iivaara, Finland, showed a disorder of 10–20% suggesting a different cooling history for these rocks.

AB - Ten nepheline single crystals from five different localities representing rocks from nepheline-syenite pegmatites to urtite, ijolite and cancrinite-ijolite were investigated chemically and structurally. The chemical compositions were determined by electron microprobe, whereas the crystal structures were refined against X-ray diffraction data to R values of 0.022–0.031. In all cases the crystal structures conformed to space group P63, but at the same time all crystals produced some satellite reflections testifying to the modulation effects in their structure. This work confirms the presence of satellite reflections as a universal property of nepheline and resolves some controversy about this issue present in older works. The samples show a range of compositions with variable excess of Si over Al combined with vacancies in the alkaline sites. Vacancies are present exclusively in the symmetric ditrigonal channels occupied preferentially by K atoms in natural nepheline. The influence of the variable amounts of K, Na and vacancies to the increase of volume at this site is shown to be in the order ¿>K>Na. It has been confirmed that natural nepheline can partly depart from Loewenstein’s rule and show some degree of disorder in the Si/Al distribution. Most of the samples revealed a practically full Si-Alordering, but the crystals from one of the nepheline-syenite pegmatites in Langesundsfjord, Norway and from the type locality of ijolite, Iivaara, Finland, showed a disorder of 10–20% suggesting a different cooling history for these rocks.

U2 - 10.1007/s00710-010-0143-5

DO - 10.1007/s00710-010-0143-5

M3 - Journal article

VL - 101

SP - 185

EP - 194

JO - Mineralogy and Petrology

JF - Mineralogy and Petrology

SN - 0930-0708

IS - 3

ER -