| Resumo: |
1. This paper is an electron microscope study of the alterations produced by temperatures in range of 100°C to 900°C in mother-of-pearl of the modern <i>Nautilus</i> shell. 2. Fragments of nacreous layers of the shell wall in the living chamber were exposed for 5 minutes to 21 days to dry heat in open vessels and in quartz tubes sealed under vacuum or argon. Other fragments were boiled in sea mud and sea water. 3. In open vessels and in sealed tubes, conversion of aragonite into calcite took place in the samples heated at 400 °C and above. In the 600 °C-900 °C samples, calcite was mixed with varying amounts of calcium oxide. With elevation of temperature, the original hard, compact and stratified mother-of-pearl became progressively brittle, porous and cleavable into booklets of mineral sheets, each of them composed of groups of nacreous lamella.In the 200 °C-300 °C samples, iridescence increased in intensity with glowing metallic tints. Iridescence disapeared above 400 °C in the open air, and persisted at higher temperatures in sealed tubes. 4. Demineralization of the pyrolysed fragments including the 800 °C (in open vessel) and 900 °C (in sealed tubes) samples, left biuretpositive conchiolin shreds. The present results give evidence of the considerable resistance to themal stress of conchiolin, sheltered between mineral lamellae. 5. Heated at the same temperature, the conchiolin structures were differently and distincly less altered in the fragments pyrolysed in the absence of oxygen than in those heated in open vessels, in which the alterations were enhanced by oxidation.Progressive fragmentation of the trabeculae of the matrix into branched rods and into spheroidal pebble-shaped corpuscles characterized the alterations in the conchiolin residues of the samples heated in open vessels. Fusion and flattening of the interlamellar matrix into membranes, followed by dislocatiun of these membranes, were the predominant modifications in the fragments pyrolysed in the absence of oxygen. In both groups of preparations, fragmentartion of the matrix residues occured, either spontaneously in the suspension of the decalcified samples, or was slightely enhanced by mechanical dissociation (shaking, ultrasonic irradiation). The latter procedures were used in order to obtain structures permeable to the electron beam from condensed or firmly agglutinated organic shreds. 6. Protracted ebullition of mother-of-pearl in sea mud and in sea water for 17, 42, 53 and 58 hours did not destroy the nautiloid pattern of structure and caused only diffuse alterations in the trabeculae of the matrix. 7. The various kinds of alteration in ultrastructure of conchiolin matrix produced in moter-of-pearl of modern <i>Nautilus</i> by heat and oxidation (in open vessels), by heat similar or identical to those recorded previously in residues of nacreous conchiolin matrix of Paleozoic and Mesozoic nautiloids and ammonoids. In the samples heated to 800 °C and 900 °C, new structures developped, which has not yet been detected in the organic debris of fossil mother-of-pearl. 8. Pyrolysis of shells of modern molluscs can furnish information for identification of fossil remnants of conchiolin matrix in which the original structure pattern is no more recognizable and could be confused with foreign organic contaminants.
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