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

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