Julgoldite from Tafjord, Sunnmøre. Contribution to the Mineralogy of Norway, No. 67

Julgoldite occurs in open fissures in a Precambrian granitic gneiss situated between Tafjord and Fjøra. Mineral growth in the fissures started under medium or low grade conditions and was completed, after a break, under very low grade conditions. The julgoldite-bearing association consists of calcite + quartz ±
chlorite ± laumontite ± babingtonite ± julgoldite ± hematite ± titanite. Julgoldite crystals are very scarce and are found as small inclusions in calcite crystals or in equilibrium with calcite + laumontite ± titanite.
The composition of this julgoldite (s.s.) is intermediate, ranging from 62% to 86% of the julgoldite component, and with an average composition corresponding to the formula (Ca7.88Na0.17) (Fe2+,Fe3+)3.62 Mn0.05Mg0.27) (Fe3+5.75Al2.25) (Si11.76Al0.24O43.21-39.59) (OH)12.80-16.41· The large cell size of
this mineral from Tafjord is indicative of extensive substitution of Fe2+ for Fe3+ and hence a low oxidation state.