19. CHERTS FROM THE WESTERN PACIFIC, LEG 7, DEEP SEA DRILLING PROJECT

Nodular and bedded cherts are present at all sites drilled on Leg 7. Nodules are usually associated with carbonates, but also occur in siliceous sequences. Bedded cherts, usually porcelaneous, are restricted to noncarbonate deposits. Tex-tural and mineralogical characteristics suggest that deep-sea cherts form in two stages. In the first stage, biogenous opal is dissolved and reprecipitated as finely crystalline cristobalite to produce porous porcelanites. The cristobalite is either deposited as interstitial matrix, or replaces pre-existing calcite or montmorillonite. In the second stage, the cristo-balite inverts to quartz, and the remaining porosity is lost. The end product is a classic dense vitreous chert. The second stage inversion may be primarily a solid-solid zero-order reaction of the type described by Ernst and Calvert. Neither the mineralogy nor texture of cherts is related to the age of enclosing sediments in a simple way. However, the occurrence of quartz-rich cherts seems to be favored by higher-than-average temperatures in the sediment