(U)SAXS characterization of porous microstructure of chert: insights into organic matter preservation

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Munoz, Patricio ; Ilavsky, Jan ; Newville, Matthew ; Wetter, Niklaus U. ; Lourenco, Rafael Andre ; de Andrade, Marcelo Barbosa ; Martins, Tereza S. ; Dipold, Jessica ; Freitas, Anderson Z. ; Cides da Silva, Luis Carlos ; Pinto Oliveira, Cristiano Luis

• Tipo de documento: Artigo Científico
• Fonte: JOURNAL OF APPLIED CRYSTALLOGRAPHY; v. 56, p. 15-pg., 2023-12-01.

Resumo

This study characterizes the microstructure and mineralogy of 132 (ODP sample), 1000 and 1880 million-year-old chert samples. By using ultra-small-angle X-ray scattering (USAXS), wide-angle X-ray scattering and other techniques, the preservation of organic matter (OM) in these samples is studied. The scarce microstructural data reported on chert contrast with many studies addressing porosity evolution in other sedimentary rocks. The aim of this work is to solve the distribution of OM and silica in chert by characterizing samples before and after combustion to pinpoint the OM distribution inside the porous silica matrix. The samples are predominantly composed of alpha quartz and show increasing crystallite sizes up to 33 +/- 5 nm (1 sigma standard deviation or SD). In older samples, low water abundances (similar to 0.03%) suggest progressive dehydration. (U)SAXS data reveal a porous matrix that evolves over geological time, including, from younger to older samples, (1) a decreasing pore volume down to 1%, (2) greater pore sizes hosting OM, (3) decreasing specific surface area values from younger (9.3 +/- 0.1 m(2) g(-1)) to older samples (0.63 +/- 0.07 m(2) g(-1), 1 sigma SD) and (4) a lower background intensity correlated to decreasing hydrogen abundances. The pore-volume distributions (PVDs) show that pores ranging from 4 to 100 nm accumulate the greater volume fraction of OM. Raman data show aromatic organic clusters up to 20 nm in older samples. Raman and PVD data suggest that OM is located mostly in mesopores. Observed structural changes, silica-OM interactions and the hydrophobicity of the OM could explain the OM preservation in chert. (AU)

• Processo FAPESP: 13/03487-8 - Centro de caracterização de novas espécies minerais: espectroscopia Raman, difração por raios X e nêutrons, e microssonda eletrônica
• Beneficiário: Marcelo Barbosa de Andrade
• Modalidade de apoio: Auxílio à Pesquisa - Jovens Pesquisadores
• Processo FAPESP: 19/08582-5 - EMU concedido no processo 17/17844-8:Analisador térmico - TG/DTA modelo sta 2500 TG/DTA TEMP. ambiente a 1100 °c
• Beneficiário: Tereza da Silva Martins
• Modalidade de apoio: Auxílio à Pesquisa - Programa Equipamentos Multiusuários
• Processo FAPESP: 18/19240-5 - Equipamento multiusuário (EMU) concedido no processo 2017/50332-0: sistema de análise STM-AFM Raman SNOM
• Beneficiário: Anderson Zanardi de Freitas
• Modalidade de apoio: Auxílio à Pesquisa - Programa Equipamentos Multiusuários
• Processo FAPESP: 17/50332-0 - Capacitação científica, tecnológica e em infraestrutura em radiofármacos, radiações e empreendedorismo a serviço da saúde (PDIp)
• Beneficiário: Marcelo Linardi
• Modalidade de apoio: Auxílio à Pesquisa - Programa Modernização de Institutos Estaduais de Pesquisa