ALBERT

Description: Ultrathin sections of reference 2:1 layer silicates treated with octadecylammonium cations were examined using high-resolution transmission electron microscopy (HRTEM) to establish the layer structure. Hitherto, few HRTEM ultrathin-section data existed on the expansion behavior of smectite-group minerals with different interlayer-charge values. Without such information, the expansion behavior of both low-charge and high-charge smectite minerals cannot be characterized and the structures observed in HRTEM images of clay-mineral mixtures cannot be interpreted reliably. Reference smectite-group minerals (Upton, Wyoming low-charge montmorillonite; Otay, California high-charge montmorillonite; a synthetic fluorohectorite; and a Jeanne d’Arc Basin offshore Newfoundland clay sample) with a range of layer charge values were examined. To prevent possible intrusion of epoxy resin into interlayers during embedding, the clay samples were first embedded in epoxy, sectioned with an ultra microtome, and then treated with octadecylammonium cations before examination using HRTEM. Lattice-fringe images showed that lower-charge (〈0.38 eq/O 10 (OH) 2 ) 2:1 layers had 13–14 Å spacings, whereas higher-charge (〉0.38 eq/O 10 (OH) 2 ) 2:1 layers had 21 and 45 Å spacings. These differently expanded silicate layers can occur within the same crystal and an alternation of these layer types can generate rectorite-like structures. For comparison, clay samples were also treated with octadecylammonium before epoxy embedding and sectioning and then examined with HRTEM. These samples mostly had highly expanded interlayers due to epoxy intrusion in the interlayer space. The reference clay minerals embedded in epoxy resin, sectioned, and treated with octadecylammonium cations were used to characterize smectite-group minerals in a natural clay sample from the Jeanne d’Arc Basin, Eastern Canada. Smectite-group minerals in this sample revealed similar structures in lattice-fringe images to those observed in the pure reference clay samples. Rectorite-like structures observed in lattice-fringe images were in fact smectite crystals with short, alternating sequences of low-charge and high-charge smectite layers rather than illite-smectite (I-S) phases with expanded smectite layers and non-expanded 10 Å illite layers.

Description: The intercalating growth of new silicate layers or metal hydroxide layers in the interlayer space of other clay minerals is known from various mixed-layer clay minerals such as illite-smectite (I-S), chlorite-vermiculite, and mica-vermiculite. In a recent study, the present authors proposed that smectite-group minerals can be synthesized from solution as new 2:1 silicate layers within the low-charge interlayers of rectorite. That study showed how oxalate catalyzes the crystallization of saponite from a silicate gel at low temperatures (60ºC) and ambient pressure. As an extension of this work the aim of the present study was to test the claim that new 2:1 silicate layers can be synthesized as new intercalating layers in the low-charge interlayers of rectorite and whether oxalate could promote such an intercalation synthesis. Two experiments were conducted at 60ºC and atmospheric pressure. First, disodium oxalate solution was added to a suspension of rectorite in order to investigate the effects that oxalate anions have on the structure of rectorite. In a second experiment, silicate gel of saponitic composition (calculated interlayer charge –0.33 eq/O 10 (OH) 2 ) was mixed with a suspension of rectorite and incubated in disodium oxalate solution. The synthesis products were extracted after 3 months and analyzed by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The treatment of ultrathin sections with octadecylammonium ( n C = 18) cations revealed the presence of 2:1 layer silicates with different interlayer charges that grew from the silicate gel. The oxalate-promoted nucleation of saponite and talc crystallites on the rectorite led to the alteration and ultimately to the destruction of the rectorite structure. The change was documented in HRTEM lattice-fringe images. The crystallization of new 2:1 layer silicates also occurred within the expandable interlayers of rectorite but not as new 2:1 silicate layers parallel to the previous 2:1 silicate layers. Instead, they grew independently of any orientation predetermined by the rectorite crystal substrate and their crystallization was responsible for the destruction of the rectorite structure.

Description: Lower Palaeozoic shales and slates in the External Domain of the southern Canadian Appalachians are composed predominantly of illite and chlorite with minor occurrences of I-S mixed-layer minerals (restricted to samples with illite crystallinity, IC 〉 0·62°Δ2Θ) and paragonite (restricted to samples with IC 〈 0·42°Δ2Θ). Inverted diagenesis has occurred in the NW part of the Chaudière Nappe, indicating pre-orogenic deep burial diagenesis at the original depositional site, whereas to the SE, the diagenetic pattern was affected by synorogenic heating. Within the east-dipping thrust-fold belt and the St Lawrence Lowlands, increasing grade towards the S suggests a gradual southward increase in post-tectonic burial depth. Narrow (3–5 km) thermal haloes around the Cretaceous Monteregian intrusions show limited effects on the country rocks.The percentage of 2M1 mica polytypes and bo increase with decreasing IC. Chlorite crystallinity (CC) increases with increasing IC. Good correlations between IC, %2M1, CC and bo of micas indicate that these parameters are reliable monitors of high-grade diagenesis and low-grade metamorphism in clay-rich sedimentary rocks. IC and CC improve with increasing grain size, illustrating the effect of grain size on IC and CC. Organic material affects IC more strongly in strata with lower permeability than in those with higher permeability. In the diagenetic zone, glycolation does not uniformly produce a narrowing of the 10 Å illite peak, but may also broaden it by up to 15%, probably due to the presence of Kalkberg-type mixed-layers.

Description: The expandability of K-depleted biotite and natural vermiculite was studied using transmission electron microscopy (TEM) and X-ray diffraction (XRD). K-depletion in layer-silicates was achieved by treating ultrathin sections with 0·1 m CaCl2 and BaCl2 solutions. The natural sample of biotite, which by XRD revealed no expandability on ethylene glycol or glycerol solvation, displayed 10–15% expanded layers when viewed by TEM after alkylammonium intercalation. The proportion of expanded layers increased after CaCl2 treatment. XRD of vermiculite samples revealed two sets of expandable interlayers after alkylammonium treatment, corresponding to two types of particles with different layer structures observed by TEM. Identification of vermiculite by TEM based on basal spacings is reliable with alkylammonium treatment. Intercalation of alkylammonium ions into the interlayers of vermiculite improved the degree of stacking order of the 2:1 layers.