Ferrisepiolite: a new mineral from Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China
Ferrisepiolite was discovered in the Saishitang copper skarn deposit in Xinghai County, Qinghai Province, China, as late- stage veinlets in copper-sulphide ores hosted in layered hedenbergite-andradite-actinolite skarn related to Indo-Sinian quartz diorite and Lower Permian metamorphosed clastic and carbonate rocks. Ferrisepiolite was formed in a highly oxidizing environment from low-temperature Fe-rich fluids and crystallized in cavities and fractures within the skarn-ore deposit. The mineral occurs in brown earthy and fibrous aggregates and shows brown to red-brown colour with strong pleochroism and 2nd order interference colours in a petrographic microscope. The measured refraction indices in white light for fibrous ferrisepiolite are: g 0 ¼ 1.628(8), a 0 ¼ 1.592-1.620. The thermal analysis of ferrisepiolite reveals a lower dehydration temperature of structural hydroxyl than sepiolite and a small weight loss (0.1-0.9%) in the range 500-700 � C. The average chemical composition from wet chemistry, X-ray fluorescence spectrometry (XRF) and electron probe microanalysis (EPMA) is (Fe 3þ