Deformation understanding in the Upper Paleozoic of Ventana Ranges at Southwest Gondwana Boundary.

At the east of the Ventana Ranges, Buenos Aires, Argentina, outcrops the Carboniferous-Permian Pillahuincó Group (Sauce Grande, Piedra Azul, Bonete and Tunas Formation). We carried out an Anisotropy of Magnetic Susceptibility (AMS) study on Sauce Grande, Piedra Azul and Bonete Formation that displays ellipsoids with constant Kmax axes trending NW-SE, parallel to the fold axes. The Kmin axes are orientated in the NE-SW quadrants, oscillating from horizontal (base of the sequence-western) to vertical (top of the sequence-eastern) positions, showing a change from tectonic to almost sedimentary fabric. This is in concordance with the type and direction of foliation measured in petrographic thin sections which is continuous and penetrative to the base and spaced and less developed to the top. We integrated this study with previous Tunas Formation results (Permian). Similar changes in the AMS pattern (tectonic to sedimentary fabric), as well as other characteristics such as the paleo-environmental and sharp curvature in the apparent polar wander path of Gondwana, marks a new threshold in the evolution of the basin. Those changes along the Pillahuincó deposition indicate two different spasm in the tectonic deformation that according to the ages of the rocks are 300-290 Ma (Sauce Grande to Bonete Formation deposition) and 290-276 Ma (Tunas Formation deposition). This Carboniferous-Permian deformation is locally assigned to the San Rafael (Hercinian) orogenic phase, interpreted as the result of rearrangements of the microplates that collided previously with Gondwana, and latitudinal movements of Gondwana toward north and Laurentia toward south to reach the Triassic Pangea.

Biosilicification (chalcedony) in human cerebral cortex, hippocampus and cerebellum from aged patients.

Aluminum and silicon have been observed to be present in the human degenerated brain and normal elderly brains by using a combination of scanning electron microscopy and X-ray spectrometry (EDS-SEM). Al and Si of electric organs were also reported--in electrocytes and cholinergic nerves--from living electric fish (family Rajidae). A biogenically produced crystalline mineral phase (i.e., chalcedony) has also been observed in electric organs by using a mineralogical microscope. Based on this evidence we decided to explore the presence of chalcedony (SiO2) in the human central nervous system (CNS). Sections from aged patients (mean, 81 years) were collected after autopsy and observed using a Leica DMLP mineralogical microscope. Chalcedony was detected in cerebral cortex and cerebellum. In plane-polarized light, chalcedony is rounded in shape, 12-20 microm in size, translucent, with a low refraction index. The crossed-polarizer image shows first order birefringence color (grey-white) and radial extinction. Chalcedony was also detected in the hippocampus in large amounts and sizes (50-60 microm). Chalcedony is a microcrystalline fibrous form of silica. It consists of nanoscale intergrowths of quartz and the optically length-slow fibrous silica polymorph moganite. Chalcedony precipitation occurs at a specific pH (7-8) and oxidation potential (Eh; 0.0 to -0.2) in geological environments. This observation supports the important role played by pH and Eh conditions in silica precipitation in elderly brains, as has also been reported in peripheral cholinergic nerves in electric organ from living electric fish. Carbonic anhydrases (CAs) (silicase) are involved in physiological pH regulation and may also be participating in the polymerization-depolymerization of chalcedony in the human brain. This is the first time a biogenically produced crystalline mineral phase (i.e., chalcedony) has been observed in the human CNS from aged patients.

Chalcedony (a crystalline variety of silica): biogenic origin in electric organs from living Psammobatis extenta (family Rajidae).

The electric organs of electric fish have been used extensively for the study of peripheral cholinergic synapses. Aluminum and silicon have been observed in the electrocytes of Psammobatis extenta, a fish belonging to the family Rajidae, using a combination of scanning electron microscopy and X-ray spectrometry. Based on this evidence, the presence of silica minerals has been documented by means of mineralogical techniques. Electric organ cryostat sections and subcellular fractions were observed using a Leica DMLP mineralogical microscope. The shape, size and color, among other properties, were analyzed in plane-polarized light, while birefringence and the extinction angle, which allow for mineral identification, were observed through crossed-polarized illumination. The distribution of chalcedony, an oxide silicon mineral, in the sections and all the fractions of the electric organ was recorded. X-ray diffraction analysis of the electric organ segments showed a similar result, with a low-quartz variety. Chalcedony precipitation occurred at a specific pH (7-8) and oxidation potential (Eh; 0.0 to -0.2). This observation supports the important role played by pH and Eh conditions in silica precipitation in electrocytes, as has been reported in geological environments. It is possible that silica formation and silica degradation in electric organs are also related to the enzymes, silicatein and silicase, that direct the polymerization and depolymerization of amorphous silica in sponges. Carbonic anhydrases (silicase) are involved in physiological pH regulation. Crystallization of chalcedony via spiral growth from a partially polymerized fluid is consistent with processes known to occur in organic systems. This is the first time that a biogenically produced crystalline mineral phase (i.e., chalcedony) has been observed in the electrocytes and cholinergic nerves from living electric fish.