Laumontite and heulandite-clinoptilolite pseudomorphous after Jurassic gastropods from Ponganui, New Zealand
At two rich fossil localities in Ponganui (eastern Port Waikato), zeolite-facies, mid-Jurassic metasandstones and metasiltstones of Murihiku Supergroup contain high-spired gastropods whose tests have been pseudomorphed by laumontite. Upper whorls of the spires are infilled by heulandite-clinoptilolite and laumontite, accompanied by minor potash-feldspar, quartz and pyrite. Body whorls are infilled by zeolitised rock matrix. Laumontite has low alkali and is closer to the ideal formula than other laumontites reported from metamorphic rocks of New Zealand. Dehydration of laumontite to leonhardite is indicated by x-ray diffraction: a = 14.75, b = 13.13, c = 7.57Angstrom, beta = 111.84°. Thermal analysis of heulandite-clinoptilolite gives ambiguous results consistent with the transitional composition these crystals possess within this mineral series. The crystal chemistry and associations of both zeolites is in accord with genetic models proposed by Boles and Coombs for similar Murihiku metasedimentary zeolites.