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A deseription is given of a microperthite myrmekite-rich lencogranite from a place 15 km esst of Puerto Aysen, Chile. The origin of the perthites which constitute films, strings aud pateles in the miecrocline is considered. No final conclusion is reached, but exsolntion is believed to be the preponderant process. This may have been aceentuated Ly slight cutaciasis which might also have permitted the introduction of replaciog solutions. The grains and semi-vermicular growths of quertz in the perthites — us well as the margins darkened by very fine argillaceons-ferrogineons material — may be the consecuente of exsolation. The first effect would have resulted from liberation of an éexcess of silica arising originally fron eryptocristalline dispersión of silica or from lack of ad jastment of the silica-alumina relatishbip in the structure of the origival alkaline feldspar due to the incorporation of calciom. The myrmekites are classified in accordance with their distribution in the fabrie, and in accordance with the presenee or absence of plagioeclase nueclei: A) Myrmekites interposed between plagioclase and microperthite, as thin margios, or as penetrating lobes ln tle latter. B) Interstitial myrmekites between larger grains of mieropesrihite, and without plagioclase nuelei. C) Myrmekitio plagioclase grains generally anhedral, independent of plagioclase erystals on which they might have grown. D) Albitic borders of plagioclase in microperthite. Ej) Myrmekitic plagioclase without nuelei, included in microperthite. F) Myrmekitie perthite with quartz grains and semi-vermicalar growths. Other types are transitional amobg the foregoing, or are of doubifa interpretation. The vermicoalar development of quartz are more characteristic of types A and C,
4. The myrmekites of types A aud D are interpreted as constructional forms, by secondary growth of the more basic plagioclase which ácts as nucleous of precipitation of the solution or difterentiated magma evriched in sodiom. The final growth
is inmediately anterior to the termination of crystallization of potassium feldspar. The enrichment of sodiam in the erystallizing solution may be accentnated by the lesser sodinm conteut of the potassinm feldspar during de final stage of grwth of
this lutter. The myrmekite corresspouds to the field of formation of two alkaline feldspars. The apparent penetration of myrmekite in microcline is due to lobate growth to which the microcline finally adapts itself, 5. The quartz worms are distributed iu correspondence with ihe lobes. They generally are sapported ou the inver border. In the zonal myrmekites, with the two zones separated by a turbid line, there are Jikewise a donble formation of quartz worms, in correspondence with the form of the myrmekite. The plagioclase twins that erosses the quartz worms are seen by transparente when are thioner than the thin section, The quartz has separated by segregation as the myrmekite grow becauso the solotion was rich in silica, or by liberation of excessea to the Si/Al ratio of the anorthite, while this mineral erystallized in the myrmekite in the contitions
of its own great structoral ordering. Where microcline corrodes plagioclase myrmekite is not generally found,
6. Interstitial myrmekites are formed by direct erystallization of the magmatic derivatives, accompanied by a marginal replacement of mierocline. They represent the last episode of the process of consolidation of the rock, Types C and E can be
refered by comparation with the foregoing types; those of type € being larger ineratitial formations.
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