New data on New Zealand vivianite and metavivianite

Vivianite occurs in nonmarine Quaternary sediments of New Zealand as authigenic crystalline nodules and concretions, earthy patches, and encrustations closely associated with organic debris. New localities include Hikurangi swamp, Happy Valley, Meremere, and Panmure in the Auckland Province, Mangatu Forest in Hawke's Bay, and Ohura in Taranaki. X-ray diffraction studies indicated that both monoclinic vivianite and triclinic metavivianite were present in crystalline samples, while earthy (cryptocrystalline) varieties consisted ofvivianite only. It is not possible to distinguish vivianite, metavivianite, or baricite by infrared absorption spectra, but variations that do occur reflect differences in the positions of water molecules in the structure and the consequential degree of hydrogen bonding. Thermal analysis shows a variable pattern of water loss in the region of 67-317°C; a major exotherm at 657-662°C is attributed to a structural transformation with the production of a high Fe(P04#x0029; modification...