Cristallogenesi e cristallolisi dell'ematite del basamento sedimentario etneo

Baldanza, Bartolo and Rondelli, F. (1974) Cristallogenesi e cristallolisi dell'ematite del basamento sedimentario etneo. Accademia Peloritana dei Pericolanti, Classe di Scienze FF. MM. NN., LVI. pp. 71-104.

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Abstract

Very thin hematite crystals, platy on {0001} and quite large (max, 60 mm), were recovered from marine clays of the sedimentary basement supporting the pyroclastic tuffs and the lava flows of the southern slopes of Mt. Etna. lnterpherometric investigations on both the growth and the etching figures shown by the large basal pinacoid, led to argue about local environmental evolution, that accompanied the geological process related to the formation of the hematite crystals. They formed in a fine to very fine clastic deposit of shallow marine waters, more or less severely disturbed by the volcanic activity of the bearing young Etna, whose first ashes and tuffs emissions, together with lava flows, were accompanied by repeated local uplifting, that gave origin to fissures and cracks. At least in some cases olistosthromic structures formed, that added further disturbances to the already complicated local tectonic situation. Circulation of hot waters from depth, along the minute fracture systems, alternatively operated as active mineralizing or as dissolution agent, depending upon local evolution of environmental conditions, primarily variations of concentration, pressure and temperature. Measurements of the step heights, made by precision multiple beam interference, gave values of 6,87 amstrong in the tongue-like terraces and of 6,56 amstrong in the archimedean spyral growth layers respectively, that is clearly a half of the height of a unit cell. Interpretation of positive (relief) end negative (etch pits) features of microtopography on the {0001} faces led to recognize that the crystal growth of the hematite may be divided in four stages. In the first one, characterized by stern oversaturation and little concentration gradiente, hematite crystals appeared and grew as initial aggregates of parallel superimposed multiple flattened blades. Crystals expanded laterally by multiple formation of local microdomains of high growth velocity, forming regular triangular tongues, directed forward all along the edges of each blade, or blade forming layers. The tongues elongate no more than two or three times the width of the base. Thereafter, by advancing of traits comprehended between tongues, a new continuous and complete edge is formed. In the second stage, primarily characterized by a sudden slowing down of the growth of the crystals by lateral expansion, first appear very few dislocation with high value of the Buerger vector. Thereafter crystal growth proceedes by large Frank spirals, initially of the Archimedean type, that later on, through poligonization, convert into truncated, flat triangular pyramids. In the whole the second stage growth indicates a quite severe reduction of overesturation. In the third stage, characterized by a further reduction of overesaturation, crystal growth slows down more and more, while dlslocations, with a continually decreasing value of the Buerger vector, cover the whole surface of the basal pinacoid. Poligonized spyrale, much reduced in size, with steps about the lenght of the unit cell of the hematite, become numerous. Poligonization attains the highest value afforded by the simmetry of hematite, finally yelding flat, but perfectly shaped trigonal pyramids. In the fourth stage overesturation attains the lowest value and is followed by undersaturation, that initiates the dissolution of the hematite crystals. These are slowly and orderly solubilized, following lines perfectly in accordance, but in inverse sequence, with the crystal growth. So all along the borders of the crystal-forming multiple-blades appear typical solubilization tongues, that now are oriented inwardly, and often branch in peculiar patterns. Thereafter by further reduction of the concentration, regular trilobate etch pits appear, followed by flat and more or less wide trigonal hoppers, similar to flight of downward steps contouring triangular depressions. Finally it seems possible to remove the doubte expressed by Sunagawa et al. (1962), who tentatively ascribed the movements of dislocations after growth to environments characterized by severe applied stress, as those produced by volcanic explosions or tectonic activity. The investigated hematite crystals occur exactly in the basement formations of a compound volcano, whose birth and growth were accompanied by explosions and vigorous tectonic activity. Altough no evidence may be furnished to support the existence of precise and direct correlations between the stages of the hematite growth and the volcano-tectonic events, marking the building and up-lifting of the young Etna, it seems quite satisfactory that at a simple biunivocal correspondence may be perceived to link steps of the crystal growth of a mineral species, with sets of initial events, that gave rise to an intermediate actual volcano still in activity.

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