Characterization of 2M pyrophyllite associated with argillic alteration in steam-heated environment, Solo, Mabini, Philippines

作者: Edwin. M. MOJARES , Katsutoshi TOMITA , Motoharu KAWANO

DOI: 10.2465/JMPS.96.109

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摘要: Localized deposit of Fe-rich pyrophyllite was encountered in the kaolinite-dominated alteration halo Solo, Mabini, Philippines. The environment formation this dioctahedral mineral is related to low-temperature advanced argillic found shallow volcanic region. crystallizes as a two-layer monoclinic (2M) structure with following unit cells, a=5.16A, b=8.95A, c=18.70A, β=99.87° while its dehydroxylate form yields slightly different cell parameters a=5.17A, b=9.02A, c=18.76A, β=99.80°. expansion b on dehydroxylation arises largely from rearrangement Al ions octahedral layer and insertions oxygen planes. obtained tetrahedral rotation (twisting SiO4 groups) 11°52' closely comparable 9°40' angle twist dehydroxylate. This strongly suggests that networks original are almost similarly twisted phase. X-ray powder data demonstrate process accompanied by significant enlargement basal spacing an observed about 1.99%. Thermal decomposition shows single dehydration event, which takes place over temperature range 500-700°C peak at 604°C. occurrence can be regarded consequence replacement Fe presence some impurities. Chemical EDX analysis give average structural formula Ca0.01K0.10(Al1.81Fe3+0.16Ti0.01)(Si3.88Al0.12)O10(OH)2 Si/Al3++Fe3+ ratio 1.86. Analyses representative samples indicate site filled 1.76 1.84 cations, relatively lower than 1.90 1.95 cations exhibited ordinary pyrophyllite. deficiency believed compensated considerably high Fe3+ occupancy site. source correlated previous minor amount pyrite associated alteration. SEM observations show common disoriented stacks plates, but most visible component loosely compacted thin elongated blade-like particles randomly dispersed porous aggregates.    The coexistence kaolinite sluggish crystallization quartz caused local variation silica activity, explains instead stable low (<300°C).

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