M. D. Ruiz Cruz

Kaolinite and dickite formation during shale diagenesis: isotopic data

Abstract X-ray diffraction, scanning electron microscopy and O-isotope geochemistry have been used to investigate the origin and possible controls on polymorphic transformation of kaolin minerals filling veins in Cretaceous shales from the Gibraltar Strait area (southern Spain). The mineralogy of the enclosing shales indicates that kaolin minerals formed from smectite dissolution, a process that silmultaneously originated I/S mixed-layers and quartz. Kaolinite and dickite δ18O values suggest that an increase in the water isotopic composition, from Cretaceous sea water values (−1%) to values of about 3%, occurred parallel to smectite dissolution, the intensity of this process being the main factor controlling the isotopic composition of kaolin minerals. The minimum formation temperature ranges from 62°C for kaolinite to 86–96°C for dickite, indicating that the depth of burial was the main control on polymorph formation. This temperature range agrees with that deduced for illite/smectite ordering. The passage from kaolinite- to dickite-rich veins was accompanied, as deduced from SEM examination, by a morphologic evolution characterized by the division of large vermiculae, dominant in kaolinite samples, and the formation of short stacks and platy crystals, which are predominant in dickite. The mechanism of dickite formation, however, remains uncertain.

Factors influencing the intercalation degree (‘reactivity’) of kaolin minerals with potassium acetate, formamide, dimethylsulphoxide and hydrazine

Abstract Factors influencing the degree of intercalation (‘reactivity’) of kaolinite group minerals have been investigated using six kaolin minerals, with variable crystallinity and particle size. Tests were performed to measure the degree of intercalation with potassium acetate, formamide, dimethylsulphoxide and hydrazine. These experiments indicate that intercalation degree depends on a number of factors, including particle size, degree of ordering, type of guest molecules, method of intercalation, and presence and types of impurities. Anew method for determination of the degree of intercalation is proposed, which is not dependent on the intensity of complex reflections. The results indicate that formamide is the most sensitive reagent to distinguish differently ordered kaolin minerals whereas hydrazine appears as the most appropriate reagent to differentiate kaolin minerals from other 7 Å phases.

Genesis and transformation of dickite in Permo-Triassic sediments (Betic Cordilleras, Spain)

Abstract In the Maláguide Complex (Betic Cordilleras, Spain), dickite is widely developed in clastic Permo-Triassic sequences. The lateral extent stretches at least 300 km along the Betic range, whilst vertical extent is variable and appears limited to the lowest 20-150m of these sequences. Textural, chemical and crystallochemical characteristics of the dickite, illite/mica and chlorite in dickite-bearing rocks and in the overlying (Permo-Triassic) and underlying (Carboniferous) rocks have been investigated to determine the approximate conditions in which dickite has developed. Using the chlorite geothermometer (based on AlIV contents), temperatures of 146±28°C and 169±12°C have been deduced for two Permo-Triassic members, and 305±12°C for Carboniferous. The Si content in illites has been used as a geobarometer, and pressures of 4.8±2 kbar have been estimated in Carboniferous rocks and tentative pressures of 2.7±3 and 2.1±2 kbar in Permo- Triassic members. Chemical evolution of phyllosilicates is accompanied by increasing illite crystallinity and % 2M1 polytype.

Textural and structural transformations of kaolinites in aqueous solutions at 200°C

Abstract We have studied the processes affecting disordered kaolinites during hydrothermal treatment at 200°C, pH=4 and increasing time of reaction. The starting materials include untreated (natural), poorly ordered Georgia kaolinite (KGa-2), and this sample after intense grinding. Chemical analyses of equilibrium solutions show a higher dissolution rate in the case of the ground kaolinite. In both cases, Si contents are higher than Al contents, indicating a non-stoichiometric dissolution. The solid products of the reactions with natural kaolinite show scarce differences with the starting sample: The XRD patterns reveal a slight sharpening of the basal reflections and a moderate increase of the Hinckley index. The solid products of the reactions with ground kaolinite show, on the contrary, remarkable textural and structural differences with the starting material: Most of the observed particles are spherical and the granulometric analyses indicate a notable increase of the mean particle size. The XRD patterns show an increase in both intensity and resolution of the non-basal reflections, which reflect the decrease of translation defects at the same time that the 131 reflection indicates an increase of the WC-type defects. The evolution observed on the IR spectra, at increasing run times, also indicate a notable increase in crystallinity. The formation of spherical aggregates of kaolinite crystals and the increase in structural order indicate that the hydrothermal reactions caused the recrystallization of the ground kaolinite. These results suggest a dissolution–precipitation process, which is notably less developed in the case of natural kaolinite.

Silicate and oxide exsolution in pseudo-spinifex olivine from metaultramafic rocks of the Betic Ophiolitic Association: A TEM study

Abstract Transmission electron microscopy (TEM) has been used to study submicroscopic particles in spinifex- like textured olivine from secondary harzburgites collected from the Cerro del Almirez locality in the Mulhacén Complex (Betic Cordillera, SE Spain). Three main types of submicroscopic oxides have been identified: (1) equidimensional Fe-rich spinel (magnetite), with average grain size in the order of 1-2 μm; (2) elongated Cr-bearing spinels (Fe-chromite to Cr-magnetite) with sizes ranging from 0.2 to 1 μm long and 0.01 to 0.1 μm thick; and (3) equidimensional Ti-rich particles from <0.01 to 0.1 μm and compositions ranging from ilmenite to Fe-Cr-Ti oxide. Chromite and ilmenite particles form parallel, chromite-rich and ilmenite-rich bands, extending along the a-axis of the host olivine. Both phases show a fixed orientation relationship with olivine, with the approximately hexagonal close-packed oxygen planes being parallel in both structures. These textural relationships indicate that both chromite and ilmenite were formed during a common exsolution process. Magnetite particles also are orientated preferentially relative to the olivine, but these particles are homogeneously distributed within the olivine, suggesting either a primary origin or an exsolution process that was not contemporaneous with formation of chromite and ilmenite. Chromite particles commonly are accompanied by lamellae of talc and/or enstatite, both showing a consistent orientation relationship with olivine. Talc lamellae are twice as thick as the associated chromite crystals, whereas enstatite lamellae show a greater thickness and, moreover, form single enstatite particles, which consist of clino- and orthoenstatite intergrowths. Talc formation may be explained by exsolution, together with spinel, from olivine containing OH-groups, probably related to incomplete dehydration of serpentine during olivine formation. On the basis of these results and previously reported petrological data, we have concluded that exsolution of chromite-silicate and ilmenite occurred during the retrograde stage that followed the climax of the eo-Alpine metamorphic event.

Diagenetic Kaolinite/Dickite (Betic Cordilleras, Spain)

The lower Permo-Triassic sediments of the Malaguide Complex contain abundant dickite. Whole rocks were studied by optical microscopy, scanning electron microscopy, and X-ray powder diffraction. The 2–20 µm and < 2 µm size fractions were extracted and analyzed by scanning and transmission electron microscopy, X-ray powder diffraction, infrared spectroscopy, differential thermal analysis, and thermogravimetry.The lower Permo-Triassic sediments of the Maláguide Complex contain abundant dickite. Whole rocks were studied by optical microscopy, scanning electron microscopy, and X-ray powder diffraction. The 2–20 µm and < 2 µm size fractions were extracted and analyzed by scanning and transmission electron microscopy, X-ray powder diffraction, infrared spectroscopy, differential thermal analysis, and thermogravimetry.In the coarse-grained samples, the 2–20 µm size fraction consisted of well-crystallized dickite associated with minor quantities of kaolinite, illite, quartz, and hematite. XRD patterns of the fine-grained samples and the <2 µm fractions showed the existence of well-crystallized minerals in which several reflections of dickite (11l, 02l) were absent and the 132/13

Hydrothermal transformation of kaolinite to illite at 200 and 300°C

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Mixed-layer mica-chlorite in very low-grade metaclastites from the Maláguide Complex (Betic Cordilleras, Spain)

reflections were shifted. These patterns suggest the presence of an intermediate member between well-crystallized dickite and well-crystallized kaolinite. Only locally high-order reflections are present at 10.5 Å and 18–22 Å. DTA and IR data agree with those from XRD.The observed compositional and structural variations are a function of the lithology and the particle size of the sample. The sequence kaolinite → kaolinite/dickite → dickite is proposed for the development of these materials during Alpine metamorphism.

Thermal behaviour of dickite-dimethylsulfoxide intercalation complex

Abstract The hydrothermal reaction of kaolinite in the system Na2O-K2O-(MgO)-Al2O3-SiO2- H2O has been investigated at 200 and 300ºC. The study of the solid products indicates that, at 200ºC, disordered illite was the first phase formed, which evolved at increasing run times towards well ordered illite and analcime. After longer reaction times illite and kaolinite were the sole phases identified by XRD. The b parameter of the illites increased at longer reaction times, from 9.05 to 9.08 Å . Study by TEM/AEM revealed the presence of surface chlorite-like layers, covering the illite packets, however. The trend observed in the products of the reaction at 300ºC was similar, except for the presence of boehmite or pseudoboehmite at the higher temperature. The values of the b parameter of illite, of the order of 9.12 - 9.16 Å , indicate that illites formed at 300ºC have a more trioctahedral character. The results obtained also indicate that Mg determines which product will be formed, Kand Na-zeolites and Al-rich illites being the phases formed in the absence of Mg.

Correlation between crystallochemical parameters of phyllosilicates and mineral facies in very low-grade metasediments of the Betic Cordilleras, Spain: a synthesis

Abstract Mixed-layered phyllosilicates with composition intermediate between mica and chlorite were identified in very low-grade metaclastites from the Malaguide Complex (Betic Cordilleras, Spain), and studied by X-ray diffraction, and transmission and analytical electron microscopy. They occur both as small grains in the rock matrix, and associated with muscovite-chlorite stacks. Transmission electron microscope observations revealed a transition from chlorite to ordered 1:1 interstratifications through complex 1:2 and 1:3 interstratifications. Analytical electron microscopy data indicate a composition slightly different from the sum of discrete trioctahedral chlorite and dioctahedral mica The types of layer transitions suggest that mixed-layer formation included two main processes: (I) the replacement of a brucite sheet by a cation sheet in the chlorite structure; and (2) the precipitation of mica-like layers between the chlorite layers. The strongest diffraction lines in oriented X-ray patterns are: 12.60 Å (002), 7.98 Å (003), 4.82 Å (005) and 3.48 Å (007).