(U)SAXS characterization of porous microstructure of chert: insights into organic matter preservation
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 (1σ standard deviation or SD). In older samples, low water abundances (∼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 , 1σ 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.
Citação
@online{patricio2023,
author = {Patricio , Munoz and Jan , Ilavsky and Matthew , Newville
and Niklaus U. , Wetter and Rafael André , Lourenço and De Andrade,
Marcelo, Barbosa and Tereza S. , Martins and Jessica , Dipold and
Anderson Z. , Freitas and Da Silva, Luis Carlos, Cides and Cristiano
Luis Pinto , Oliveira},
title = {(U)SAXS characterization of porous microstructure of chert:
insights into organic matter preservation},
volume = {56},
number = {6},
date = {2023-12-01},
doi = {10.1107/S1600576723008889},
langid = {pt-BR},
abstract = {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 (1σ standard deviation or SD). In
older samples, low water abundances (∼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 , 1σ 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.}
}