Giant organic-gypsum intergrowths from the Miocene evaporites of Carpathian Foredeep Basin
Creators: name: Bąbel, Maciej
Publisher: name: figshare
Year: 2023
Subjects: classificationCode: 370509
subject: Sedimentology
classificationCode: 370505
subject: Mineralogy and crystallography
classificationCode: 370501
subject: Biomineralisation

description: Peculiar asymmetric intergrowths of gypsum, interpreted as mineral-organic in nature, occur within the Middle Miocene (Badenian) evaporites of the Carpathian Foredeep Basin. The intergrowths are very similar to the type present in contact (<sup>-</sup>101) gypsum twins and are composed of vertically elongated crystals decimetric to 3.5 m in size. These giant crystals, arranged into a palisade-like bed, form a single layer up to several m thick seen in nearly the whole northern area of the Carpathian Foredeep Basin/ The intergrowths crystallized directly on the floor of the evaporite basin under a permanent cover of Ca-sulphate saturated brine, at a depth of no more than several metres. Similariy to true twins, the intergrowths show (Babel 1990): i) two component crystals connected along the composition surface (CS), ii) constant crystallographically oriented contact along CS, iii) the re-entrant angle near CS at apices of intergrowths (indicating common growth of component crystals upward from the sub- strate exactly as in swallow-tail type of gypsum twins), iv) habit flatted along CS typical of contact twins. On the other hand, the intergrowths show features never or rarely recorded in twins: i) lack of any symmetry between component crystals, ii) lack of a strictly defined rule of crystaliographic orientation (orientation of crystals in every particular intergrowth is always slightly different compared to other ones, although sta- tistically the crystals fali into a certain range of orientations), iii) lack of cohesion on CS which is a distinct joint and displays a relief characteristic of induction or compromise boundaries, iv) faces meeting on CS are real crystal faces in environment of growth (because the studied deposits contain single crystals terminating in such faces, some intergrowths show one component crystal larger and terminating in such faces above the ‘abandoned’ re-entrant angle, and some intergrowths show primary intercrystalline pores along CS framed by such faces, v) constant oriented contact is preserved only along CS but is lost in rare penetration intergrowths in the places where other crystal surfaces are in contact.Lack of symmetry is not the only unusual feature of the intergrowths. More striking are their disymmetry <i>(=sensu </i>Pasteur, a feature of some objects which makes it impossible to coincide them with their mirror reflections) and asymmetrv in the freauency distribution of their morphological features. Disymmetry is visible between forms of intergrowths representing the mutual mirror reflections.Taking into account all these features, growth structures of the intergrowths, depositional environment, and the experimentally proven fact that gypsum faces (such as those present <i>on CS) are </i>commonly created by selective adsorption of organic compounds, the author concluded (Babel 1990) that some bioiogically produced organic compounds took part in chemical bonding along CS and that they participated in the ‘nucleation’ of the intergrowths. The nucleation was supposedly by the <i>bridging mechanism </i>in which large organic molecules adsorbed onto two gypsum microcrystals and initiated their oriented aggregation. The disymmetry and asymmetry of the intergrowths resulted from the fact that these compounds were homochiral, like almost every biologically produced organic substance, or (more probably) showed predominantly one (left or right) chirality. Such compounds influenced the orientation of nucleating intergrowths leading to their asymmetry. The variability in crystallographic orientation of the intergrowths and their disymmetry is related to the epitaxial nature of the gypsum-organic substance-gypsum contacts along CS and to the complex structure of organic<sup> </sup>compounds, which could be a mixture of various substances showing both left and right chirality. The intergrowths are thus not growth twins but complicated mineral-organic intergrowths. A film of organic compounds originally present on CS was supposedly destroyed during diagenesis and now it is difficult to be detected. It is to add that the studied crystals are full of included organic matter (mainly algal remnants) derived from the brine of the evaporite basin.The previously suggested mineral-organic nature of the intergrowths has been recently strongly supported by new data supplied by experiments which proved the undoubtful role of organic compounds in determining the (<sup>-</sup>101) type of gypsum twinning (close similarity of the intergrowths to (<sup>-</sup>101) twins was already mentioned). It was demonstrated that one specific compound in solution (a-amylase - an enzyme produced by bacteria, algae and plant roots) was able to promote crystallization of these relatively rare (<sup>-</sup>101) gypsum twins (in comparison with very common (100) twins). Among the many compounds used in the laboratory, it was the only one which produced such a twinning as well as lenticular habit of gypsum, and was found in sufficient abundance in laguno-lacustrine environments. Moreover, in other experiments it was proved that 28 homochiral organic substances, and a-amylase among them, promoted asymmetry of equivalent gypsum crystal faces through stereospecific selective adsorption.The growth of component crystals along CS to some extent might be similar to that recognized in lamellar mixed crystals of octacalcium phosphate and apatite in tooth enamel. In the enamel, an extremely thin lamella or membrane of octacalcium phosphate is a ‘leader’ of growth and the apatite crystallizes epitaxially on both sides of it. In the case of the intergrowths, it seems possible that some organic substances could form a leading film or template for epitaxial-like crystallization of gypsum on its both sides, as it was previously suggested (Babel 1990). Such a growth along CS seems to be amazingly similar to that observed not only in teeth but also in many other biomineral structures: bones, kidney stones, shells, spicules, etc. It is generalty recognized that in many of these structures an organic matrix is formed first and it appears to guide the crystal orientation of the later-deposited mineral phase.The origin of the intergrowths still remains a mystery and requires further investigation since the component organic substances are unrecognized and stereochemical nature of supposed oriented organic-gypsum contact is not reconstructed.