Petrography and Geochemistry of Pyrite and Marcasite in DSDP Leg 40 Sediments

Authigenic iron sulfide, normally reported as pyrite, is a ubiquitous component of deep-sea sediments. Leg 40 sediments are no exception; iron sulfide occurs throughout the cores. Pyrite also occurs in onshore sedimentary sequences. Love and Amstutz (1966) cite more than 100 papers describing the widespread geographic and stratigraphic occurrence of this mineral. Interest in Leg 40 iron sulfides was heightened by the shipboard suspicion, later confirmed, that some grains were marcasite rather than pyrite. Marcasite has rarely been reported in other DSDP cores. Either it is seldom present at other sites, or has not been distinguished from pyrite. It is difficult to distinguish between pyrite and marcasite in smear slides, and X-ray diffraction may not detect small amounts of marcasite. The relatively good crystal forms of the Leg 40 grains, first recognized in the coarse sediment fraction, initially suggested that some grains were marcasite. The purpose of this reconnaissance study is (1) to describe the occurrence of pyrite and marcasite, in particular noting the different morphological forms the minerals assume, and the relative abundance of each, (2) to contrast the major and some minor element contents of grains in different morphological classes, as well as from different drilling sites, and (3) to document the unusual association of pyrite and gypsum found in some of the cores.