Abstract: The unstable glassy component of the Oligocene tuffaceous sediments of the Metaxades area, Thrace, Greece, has undergone extensive zeolitic diagenetic alteration. The authigenic minerals occur as crypto- to micro-crystalline aggregates making up most of the matrix in the altered tuffs and as precipitates in cavities produced by dissolved glass fragments. Glass-shard pseudomorphs are ubiquitous and most of them are partly filled by one or more of the minerals smectite, heulandite, mordenite and silica.

The heulandite group zeolites are mostly high-silica calcium-rich heulandites showing intermediate thermal behaviour between most heulandites and clinoptilolites. Their Si/Al ratios are similar to clinoptilolite (4.74–5.19) but their divalent/monovalent cation ratios (1.5–3.24) are partly superposed onto the ratios of heulandite group 1 and 2 and differ considerably from the values of the relevant ratio in clinoptilolite. Rare K-rich clinoptilolite crystals have been identified in one sample only.

Based on field observations, compositions and paragenetic relationships of coexisting authigenic minerals and the absence of critical phases such as laumontite, analcime or authigenic albite, it is suggested that the Metaxades pyroclastic rocks underwent burial diagenesis intermediate between Iijima's (1978) zones II and III commonly developed in silica saturated environments.