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The mechanism of the replacement of feldspars by hydrothermal laumontite was examined in the hydrothermally altered Strzelin granite, SW Poland. The granite was altered along fractures and hydrothermal quartz-prehnite veins to mineral assemblages typical of low-grade burial metamorphism including laumontite, prehnite, chlorite and clinozoisite. The occurrence of abundant secondary laumontite in the granite is limited to the depth of 70 m below ground level. The present-day 70 meter boundary is apparently related to the temperature limit of the laumontite stability field (up to 250°C) and pressure up to 2.2 kbar. Laumontite occurs within the granite as (1) a primary hydrothermal mineral which crystallized in fractures, microcaverns, and in intersticies or voids between cleavage planes in feldspars directly from the hydrothermal fluid enriched in Ca2+; and (2) a secondary hydrothermal mineral which partially or entirely replaced primary magmatic minerals mainly feldspars.
Observations by optical microscopy and BSE imaging revealed that the replacement of plagioclase (oligoclase-andesine) by laumontite was preceded by the removal of Ca-rich (anorthitic) portions from the plagioclase. That process may have been a major source of Ca2+ ions in hydrothermal fluid. Preferential removal of Ca from plagioclases resulted in the formation of secondary microporosity within plagioclase due to the volume difference between Ca-rich and Na-rich portions of the plagioclase. The microporosity may have enabled fluid penetration through the whole plagioclase grain. Moreover the preferential replacement of albite by laumontite over K-feldspar was observed, which may be explained by the similarity of ionic radii of Ca2+ and Na+ (0.124 Å, 0.120 Å, respectively).
The topotactic replacement of feldspar by laumontite is suggested by identical optical orientation of both minerals. We propose a mechanism of the topotactic replacement based on the similarity of the framework topology between feldspars and laumontite.
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