Occurrence and origin of analcime in a Neogene volcano-sedimentary lacustrine environment, Beypazari-Cayirhan basin, Ankara, Turkey

摘要

Analcime is a common mineral in the Middle-Upper Miocene sedimentary and volcano-sedimentary units of the Bey-pazan-f ayirhan basin that comprise claystone, sandstone, limestone, dolomite, tuff and altered tuff. Besides analcime, scarce heulandite and mordenite-type zeolite, smectite, palygorskite, scarce sepiolite and loughlinite-type clay minerals, dolomite, cal-cite and, locally, magnesite minerals, gypsum, feldspar, illite, quartz and opal-CT, are the main minerals in the analcime-bearing units. Cryptocrystalline analcime occurs in euhedral and subhedral forms. Euhedral analcime is present as 5-7cm thick, white, smooth, film- or paper-like forms, which precipitated during sedimentation. This analcime shows well-developed trapezohedral crystals, and no precursors of clays, other zeolites or relicts of volcanic glass were detected. Whereas fine-grained subhedral analcime crystals dominate in altered tuffaceous units, analcime appears as subrounded grains and grain masses in dissolution voids and on partly dissolved volcanic glass. Na, Al and Si may have been derived from hydrolysis of volcanic glass. Release of Na increased the alkalinity of the environment favouring the precipitation of analcime in the presence of Al and Si. Our field observations and mineralogical investigations of analcime in the basin indicate that euhedral analcime formed by precipitation from Na-rich very shallow alkaline lake water following a decrease of fresh water input and increase of evaporation, whereas the subhedral analcime formed by dissolution-precipitation from volcanic materials, such as glass shards.