TY - JOUR
T1 - Cation redistribution in the octahedral sheet during diagenesis of illite-smectites from Jurassic and Cambrian oil source rock shales
AU - Dainyak, Lidia G.
AU - Drits, Victor A.
AU - Zviagina, Bella B.
AU - Lindgreen, Holger
PY - 2006/4
Y1 - 2006/4
N2 - During diagenesis of Jurassic and Cambrian oil source rock shales illite-smectite(-vermiculite) [I-S(-V)] is transformed to illite-tobelite-smectite(-vermiculite) [I-T-S(-V)]. This transformation of S layers to T layers takes place by an increase in tetrahedral charge through Al for Si substitution and subsequent fixation of interlayer NH
4, accompanied by an increase in Al and a decrease in Fe and Mg in the octahedral sheet. In the present investigation, the distribution of isomorphous cations in octahedral sheets of trans-vacant I-S(-V) and I-T-S(-V) was studied by Mössbauer and Infrared (IR) spectroscopies. Mössbauer spectra have been modeled using numerical values of the Fe
3+ and Fe
2+ quadrupole doublets corresponding to local cation arrangements around Fe
3+ and Fe
2+ in octahedral sheets of micaceous minerals. To interpret IR spectra in the OH-stretching region, frequencies for each pair of cations bonded to OH groups determined for micas and I-S are used. Combination of Mössbauer and IR data by computer simulation provides two-dimensional cation distributions of octahedral cations. The Jurassic and Cambrian I-S(-V) and I-T-S(-V) have clustered octahedral sheets. Ordered clusters of mixed cation composition (Mg, Al, Fe
3+, and Fe
2+) with regular alternation of di- and trivalent cations and Fe
3+-clusters dispersed over an Al-matrix are found in detrital samples. In diagenetically transformed samples, ordered clusters persist while Fe
3+-clusters degenerate to either short chains consisting of two Fe-Fe pairs or to isolated Fe-Fe pairs oriented along the b, b
1, and b
2 directions. The release of Fe and Mg during diagenesis occurs from Fe
3+ clusters and through partial destruction of ordered clusters and of b
1, b
2-oriented Mg-Mg pairs. However, as the cation composition and the short-range cation order within the clusters are preserved and the Al for Fe and Mg substitution occurs at cluster edges, the diagenetic transformation of S (and V) to T layers in both the Jurassic and Cambrian I-S(-V) proceeds through a solid-phase transformation and not through dissolution-reprecipitation.
AB - During diagenesis of Jurassic and Cambrian oil source rock shales illite-smectite(-vermiculite) [I-S(-V)] is transformed to illite-tobelite-smectite(-vermiculite) [I-T-S(-V)]. This transformation of S layers to T layers takes place by an increase in tetrahedral charge through Al for Si substitution and subsequent fixation of interlayer NH
4, accompanied by an increase in Al and a decrease in Fe and Mg in the octahedral sheet. In the present investigation, the distribution of isomorphous cations in octahedral sheets of trans-vacant I-S(-V) and I-T-S(-V) was studied by Mössbauer and Infrared (IR) spectroscopies. Mössbauer spectra have been modeled using numerical values of the Fe
3+ and Fe
2+ quadrupole doublets corresponding to local cation arrangements around Fe
3+ and Fe
2+ in octahedral sheets of micaceous minerals. To interpret IR spectra in the OH-stretching region, frequencies for each pair of cations bonded to OH groups determined for micas and I-S are used. Combination of Mössbauer and IR data by computer simulation provides two-dimensional cation distributions of octahedral cations. The Jurassic and Cambrian I-S(-V) and I-T-S(-V) have clustered octahedral sheets. Ordered clusters of mixed cation composition (Mg, Al, Fe
3+, and Fe
2+) with regular alternation of di- and trivalent cations and Fe
3+-clusters dispersed over an Al-matrix are found in detrital samples. In diagenetically transformed samples, ordered clusters persist while Fe
3+-clusters degenerate to either short chains consisting of two Fe-Fe pairs or to isolated Fe-Fe pairs oriented along the b, b
1, and b
2 directions. The release of Fe and Mg during diagenesis occurs from Fe
3+ clusters and through partial destruction of ordered clusters and of b
1, b
2-oriented Mg-Mg pairs. However, as the cation composition and the short-range cation order within the clusters are preserved and the Al for Fe and Mg substitution occurs at cluster edges, the diagenetic transformation of S (and V) to T layers in both the Jurassic and Cambrian I-S(-V) proceeds through a solid-phase transformation and not through dissolution-reprecipitation.
KW - Cation distribution
KW - Computer simulation
KW - Illite-smectite
UR - http://www.scopus.com/inward/record.url?scp=33646443128&partnerID=8YFLogxK
U2 - 10.2138/am.2006.2047
DO - 10.2138/am.2006.2047
M3 - Article
VL - 91
SP - 589
EP - 603
JO - American Mineralogist
JF - American Mineralogist
SN - 0003-004X
IS - 4
ER -