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Experimental alteration of the Nautilus shell by factors involved in diagenesis and in metamorphism. III. Thermal and hydrothermal changes in the organic and mineral components of the mural mother-of-pearl
Grégoire, Ch. (1972). Experimental alteration of the Nautilus shell by factors involved in diagenesis and in metamorphism. III. Thermal and hydrothermal changes in the organic and mineral components of the mural mother-of-pearl. Bull. Kon. Belg. Inst. Natuurwet. Biologie 48(6): 1-85, plates I-XLII (+ erratum)
In: Bulletin van het Koninklijk Belgisch Instituut voor Natuurwetenschappen. Biologie = Bulletin de l'Institut Royal des Sciences Naturelles de Belgique. Biologie. Koninklijk Belgisch Instituut voor Natuurwetenschappen: Bruxelles. ISSN 0374-6429, more
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  • Grégoire, Ch.

Abstract
    1. The pyrolytic changes in the organic and mineral components of mother-of-pearl of the modern Nautilus have been studied with the transmission and scanning electron microscopes in 86 groups of samples. 2. Pyrolysis was performed in the range of 100 °C to 900 °C for periods extending from a few minutes to 53 days, in four sets of experiments : 1. samples boiled in sea water; 2. samples heated dry in open vessels; 3. samples heated dry in tubes sealed under vacuum; 4. samples heated with sea or distilled water in tubes sealed under vacuum. 3. The present observations confirm and extend those of previous studies on the thermal changes in the ultrastructure of the interlamellar conchiolin matrices: a. Conchiolin is thermoresistant. It does not disappear and is still biuret-positive in samples pyrolysed at 900 °C for 5 hours. b. The pyrolytic changes in the conchiolin ultrastructure in experiments of a few minutes to 21 days duration, consist of loosening of the lace- like networks, flattening, widening and coalescence of the trabeculae into continuous or fenestrate membranes (predominant in sealed tubes), fragmentation into corpuscles (predominant in the more altered samples heated in open vessels), and, in experiments of longer duration (e.g. 53 days), of disintegration into substantial tangled fibrils. The factors possibly involved in these pyrolytic changes in conchiolin (compression by volatile constituents, increase in volume during recrystallization of the surrounding crystals) have been examined in the discussion. The thermal conchiolin changes were similar or identical to those recorded till now in the organic remnants of nacreous layers in more than 250 species of fossil cephalopods (nautiloids and ammonoids). c. The nature of the pyrolytic alterations in the conchiolin matrices did not distinctly differ in samples heated dry or wet in sealed tubes. d. The hydrothermal changes developed in still aragonitic samples and, in agreement with former biochemical studies, seemed to be stabilized, except for subsidiary modifications in structure, at temperatures over 275 °C -300 °C. At high temperatures (e.g. 600 °C), this stabilization was already established after a few minutes and the changes in structure of the conchiolin did not differ greatly frorn those recorded after 5 hours and 21 days. e. In samples heated under 275 °C -300 °C, protracted heating at a lower temperature (e.g. 225 °C -21 days), produced changes identical to those obtained by heating at a higher temperature (e.g. 300 °C) for shorter periods of time (5 hours). " 4. Except for scattered modifications ( dissociation of the lamellae into individual crystals, inflation, fragmentation and coalescence of adjacent crystals), temperatures in the range of 150 °C -300 °C did not distinctly modify the texture of mother-of pearl. Over 275 °C- 300 °C, the changes consisted of coalescence of single colurnnar stacks of crystals or of parallel groups of these structures characteristic of the Nautilus nacre. Coalescence of groups of crystals induced formation of well delimited, variously oriented foliate aggregates in which the original lamination of the nacre was temporarily preserved. In the range of 300 °C, the mineral components of the nacre were differently affected by pyrolysis in adjacent regions of a same sample; foliate aggregates were frequently contiguous to less altered parts of the samples in which the original colurnnar texture still subsisted. In further stages ( 600 °C -900 °C, the foliate aggregates were transformed progressively into mosaics of interlocking, polyhedral, anhedral, subhedral or euhedral crystals. Inflation and coalescence of the micro- crystallites constituting the inner configuration of the original crystals of aragonite play possibly a part in this transformation. 5. Onder the experimental conditions realized in this study, recrystallization of aragonite into calcite took place by a process of solid-solid reaction. 6. During pyrolysis, needle-like crystals resembling whiskers appeared on the surfaces of spontaneous cleavage into mineral sheets (sublayers) of the samples along the interlamellar planes. Decalcification of these crystals left organic ghosts in the form of perforated tubular sheaths of complex structure. 7. Several of these pyrolytic changes in the microtexture of mother- of-pearl are similar or identical to those recorded in the mineral components of aragonitic and recrystallized nacreous layers of fossil cephalopods of different ages (Permian, Jurassic, Cretaceous). 8. The present results suggest that simulation by pyrolysis of modern shells of diagenetic changes in fossil mother-of-pearl can be realized, not only in the conchiolin matrices, as shown in previous studies, but also in the microtexture of this substance. Experiments of longer duration than those performed till now, and in which samples of Nautilus nacre are subjected to heat and pressure are in progress.

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