Silicification

In the present work, a mineralogical and chemical cartographic study of shelly limestones from the Lower Oligocene of Côte d'Ivoire Ivorian emerged basin was carried out. The analyzes were realized under a scanning microscope which is equipped with an energy diffusion spectrometer (SEM-EDS). The objective is to understand the post-diagenetic processes which occurred in shelly limestones during Lower Oligocene Transgression. The results showed that shelly limestones are composed of more than 80% of calcium which are accompanied in order of abundance of silicon (6.73), sulfur (1.94%), iron (1.62%), aluminum (1.57%) and magnesium (1.44%). The results also reveal during the Lower Oligocene Transgression, mineralogical inclusions were formed in chronological order: glauconite and pyrite. During this transgression, silicification (quartzification) of some bioclasts occurred between depths 3000 m and calcite compensation depth (CCD) explaining the presence of silica in these limestones by chemical micro-mapping. This new marine deposit (deeper sea) has caused the formation of hard-ground on shelly limestones surface.

Silica is deposited in and around the spore coat layer of Bacillus cereus, and enhances the spore's acid resistance. Several peptides and proteins, including diatom silaffin and silacidin peptides, are involved in eukaryotic silica biomineralization (biosilicifica-tion). Homologous sequence search revealed a silacidin-like sequence in the C-terminal region of CotB1, a spore coat protein of B. cereus. The negatively charged silacidin-like sequence is followed by a positively charged arginine-rich sequence of 14 amino acids, which is remarkably similar to the silaffins. These sequences impart a zwitterionic character to the C terminus of CotB1. Interestingly, the cotB1 gene appears to form a bicistronic operon with its paralog, cotB2, the product of which, however, lacks the C-terminal zwitterionic sequence. A cotB1B2 mutant strain grew as fast and formed spores at the same rate as wild-type bacteria but did not show biosilicification. Complementation analysis showed that CotB1, but neither CotB2 nor C-terminally truncated mutants of CotB1, could restore the biosilicification activity in the cotB1B2 mutant, suggesting that the C-terminal zwitterionic sequence of CotB1 is essential for the process. We found that the kinetics of CotB1 expression, as well as its localiza-tion, correlated well with the time course of biosilicification and the location of the deposited silica. To our knowledge, this is the first report of a protein directly involved in prokaryotic biosilicification. IMPORTANCE Biosilicification is the process by which organisms incorporate soluble silicate in the form of insoluble silica. Although the mechanisms underlying eukaryotic biosilicification have been intensively investigated, prokaryotic biosilicification was not studied until recently. We previously demonstrated that biosilicification occurs in Bacillus cereus and its close relatives, and that silica is deposited in and around a spore coat layer as a protective coating against acid. The present study reveals that a B. cereus spore coat protein, CotB1, which carried a C-terminal zwitterionic sequence, is essential for biosilicification. Our results provide the first insight into mechanisms required for biosilicification in prokaryotes.

Petrographic imaging, in combination with qualitative and quantitative instrumental analyses of mineral
mass, allow us to obtain material signatures of silicified plant stems that are relatively common in sediments
of continental basins since the late Palaeozoic. These fossils can be found in their original strata but
commonly have been removed from their environmental and stratigraphic context, redeposited, and
scattered on the recent land surface after erosion of the parent rocks. Analytical data gathered from X-ray
diffraction analysis, hot cathodoluminescence (CL) imaging and spectroscopy, electron microprobe analysis,
Raman spectrometry, and polarised light microscopy serve to characterise material signatures of samples
from basins in Brazil, Germany, the Czech Republic, Sultanate of Oman, Mongolia, Antarctica, France, and the
USA. This collection includes silicified Pennsylvanian and Permian plant taxa (and a few from the Triassic)
found in fluvial environments and sites influenced by volcanism with the purpose to discern fundamental
material characteristics formed under particular environmental circumstances. Late Pennsylvanian and
Permian silicified stems in fluvial rocks include the presence of well-crystalline quartz (α-SiO2), sometimes
with a trace of kaolinite, showing weak CL (mostly blue or dark reddish), occasional mosaic or patchy
preservation of anatomical details, and other signs of pressure distortion of wet trunks in fluvial deposits and
subsequent diagenetic recrystallisation. The presumed silica source for the initial stage of silicification was
weathering of labile minerals, mostly feldspars in the alluvium. In wood from aeolian deposits, moganite in
combination with goethite was detected. Based on our results, we propose that the stems were silicified in
sandy or gravelly fluvial deposits, most frequently in arkoses and arkosic sands, indicators of relatively warm
climate with pronounced seasonal distribution of precipitation. Excluded from this interpretation are stems
silicified primarily by volcanic material; these are distinguished by a higher species diversity, silicification
close to the site of growth, miscellaneous mineralogy, usually with very colourful CL shades, and the
presence of metastable forms of SiO2, opal-CT or moganite. This volcanic influence on silicification mode is
less clearly controlled by seasonality of precipitation or palaeoclimate itself.

Syn-sedimentary post-diagenetic phenomena are remarkable in Moroccan phosphates. Indeed they are reference deposits for geological study, where important interests are given of determination and identification of these geochemical phenomena. Silicification is one of the phenomena that we have studied to determine the diagenetic processes that control the new-formation of silica in phosphate series of Ouled Abdoun. This study was performed on deposit EL Halassa, which belongs to the Ouled Abdoun Basin; they spread out the Maastrichtian to Lutetian. At first, petrographic approach was applied on different terms composing the phosphate series. In a second step, a geochemical approach on the same silica and silicified facies of phosphate series Ouled Abdoun Basin for characterizing silicification deduced the probable links between this diagenetic phenomenon and facies and forms’ affect. Ten major trace elements were measured. According to the phases’ petrographic relationship, the results were presented and discussed: silicate phase, the carbonate phase and clay phase.

Online access for this thesis was created in part with support from the Institute of Museum and Library Services (IMLS) administered by the Nevada State Library, Archives and Public Records through the Library Services and Technology Act (LSTA). To obtain a high quality image or document please contact the DeLaMare Library at https://unr.libanswers.com/ or call: 775-784-6945.The Big Springs Gold Mine is located in the Independence Mountains in northern Elko County. The mine is a sediment hosted disseminated gold deposit within Pennsylvanian to Mississippian age siltstones of the Schoonover formation. Gold mineralization is tightly structurally controlled. Ore structures are predominately east-west striking north dipping normal faults formed during Pre-Basin and Range extension. Alteration consists of dolomitization, sericitization, decalcification, and silicification. Gold is deposited syn or post genetically with decalcification and silicification. Gold occurs as free grains, up to...

This paper outlines the significance of Optical Microscopy (OM) and Scanning Electron Microscopy (SEM-EDS) in understanding the pathology in order to propose a tailor-made conservation plan for petrified wood. Twelve (12) samples from five different locations of the longest excavated fossil trunk at the Petrified Forest at Ban Tak, Thailand (BT-1 fossil tree) were observed through optical microscopy and SEM-EDS to record their pathology and identify deterioration mechanisms. OM observations revealed the presence of microfractures that allow water movement and eventually facilitate microbial growth. OM indicated that the fossilisation process due to the Si-rich water that runs through the body of the fossil is still in progress. Active microorganisms and suface deposits including salts, dried mud, etc. instigate high tensions which together with chemical alterations due to high RH contribute to the detachment of fossilised cells. OM revealed distinct mineral heterogeneity, and SEM-EDS analysis concluded that Si is the main material of the fossil whereas Mg, Fe and Ti participate in its composition as trace elements, Fe also results as an oxidation product, particularly between cells, and Al appears as surface deposit of aluminosilicate soil. The heterogeneity of the material, including various mineral habits, cracks and void areas, leads to differential water absorption, thus creating different micro-environments throughout the body of the fossil, a parameter that should be taken into consideration, especially during consolidation and chemical cleaning. Conclusively, the findings of this research study underline the presence of microfissures, material loss, deterioration, pulverization, erosion, disintegration, oxidation, discolouration and biological colonisation which need to be taken into consideration.