Bond strength of orthodontic brackets using different light and self-curing cements

The purpose of the study was to evaluate the shear bond strength of stainless steel orthodontic brackets directly bonded to extracted human premolar teeth. Fifty teeth were randomly divided into ¿ve groups: (1) System One (chemically cured composite resin), (2) Light Bond (light-cured composite resin), (3) Vivaglass Cem (self-curing glass ionomer cement), (4) Fuji Ortho LC (light-cured glass ionomer cement) used after 37% orthophosphoric acid¿etching of enamel (5) Fuji Ortho LC without orthophosphoric acid¿etching. The brackets were placed on the buccal and lingual surfaces of each tooth, and the specimens were stored in distilled water (24 hours) at 378C and thermocycled. Teeth were mounted on acrylic block frames, and brackets were debonded using an Instron machine. Shear bond strength values at fracture (Nw)were recorded. ANOVA and Student-Newman-Keuls multiple comparison tests were performed (P , .05). Bonding failure site was recorded by stereomicroscope and analyzed by Chi-square test, selected specimens of each group were observed by scanning electron microscope. System One attained the highest bond strength. Light Bond and Fuji Ortho LC, when using an acid-etching technique, obtained bond strengths that were within the range of estimated bond strength values for successful clinical bonding. Fuji Ortho LC and Vivaglass Cem left an almost clean enamel surface after debracketing.

Genesis of self-organized zebra textures in burial dolomites: Displacive veins, induced stress, and dolomitization

The dolomite veins making up rhythmites common in burial dolomites are not cement infillings of supposed cavities, as in the prevailing view, but are instead displacive veins, veins that pushed aside the host dolostone as they grew. Evidence that the veins are displacive includes a) small transform-fault-like displacements that could not have taken place if the veins were passive cements, and b) stylolites in host rock that formed as the veins grew in order to compensate for the volume added by the veins. Each zebra vein consists of crystals that grow inward from both sides, and displaces its walls via the local induced stress generated by the crystal growth itself. The petrographic criterion used in recent literature to interpret zebra veins in dolomites as cements - namely, that euhedral crystals can grow only in a prior void - disregards evidence to the contrary. The idea that flat voids did form in dolostones is incompatible with the observed optical continuity between the saddle dolomite euhedra of a vein and the replacive dolomite crystals of the host. The induced stress is also the key to the self-organization of zebra veins: In a set of many incipient, randomly-spaced, parallel veins just starting to grow in a host dolostone, each vein¿s induced stress prevents too-close neighbor veins from nucleating, or redissolves them by pressure-solution. The veins that survive this triage are those just outside their neighbors¿s induced stress haloes, now forming a set of equidistant veins, as observed.

Ophiolite-related ultramafic rocks (Serpentinites) in the Caribbean region: a review of their occurrence, composition, origin, emplacements and Ni-Laterite soil formation

Ultramafic rocks, mainly serpentinized peridotites of mantle origin, are mostly associated with the ophiolites of Mesozoic age that occur in belts along three of the margins of the Caribbean plate. The most extensive exposures are in Cuba. The ultramafic-mafic association (ophiolites) were formed and emplaced in several different tectonic environments. Mineralogical studies of the ultramafic rocks and the chemistry of the associated mafic rocks indicate that most of the ultramafic-mafic associations in both the northern and southern margins of the plate were formed in arc-related environments. There is little mantle peridotite exposed in the ophiolitic associations of the west coast of Central America, in the south Caribbean in Curacao and in the Andean belts in Colombia. In these occurrences the chemistry and age of the mafic rocks indicates that this association is mainly part of the 89 Ma Caribbean plateau province. The age of the mantle peridotites and associated ophiolites is probably mainly late Jurassic or Early Cretaceous. Emplacement of the ophiolites possibly began in the Early Cretaceous in Hispaniola and Puerto Rico, but most emplacement took place in the Late Cretaceous to Eocene (e.g. Cuba). Along the northern South America plate margin, in the Caribbean mountain belt, emplacement was by major thrusting and probably was not completed until the Oligocene or even the early Miocene. Caribbean mantle peridotites, before serpentinization, were mainly harzburgites, but dunites and lherzolites are also present. In detail, the mineralogical and chemical composition varies even within one ultramafic body, reflecting melting processes and peridotite/melt interaction in the upper mantle. At least for the northern Caribbean, uplift (postemplacement tectonics) exposed the ultramafic massifs as a land surface to effective laterization in the beginning of the Miocene. Tectonic factors, determining the uplift, exposing the peridotites to weathering varied. In the northern Caribbean, in Guatemala, Jamaica, and Hispaniola, uplift occurred as a result of transpresional movement along pre-existing major faults. In Cuba, uplift occurred on a regional scale, determined by isostatic adjustment. In the south Caribbean, uplift of the Cordillera de la Costa and Serrania del Interior exposing the peridotites, also appears to be related to strike-slip movement along the El Pilar fault system. In the Caribbean, Ni-laterite deposits are currently being mined in the central Dominican Republic, eastern Cuba, northern Venezuela and northwest Colombia. Although apparently formed over ultramafic rocks of similar composition and under similar climatic conditions, the composition of the lateritic soils varies. Factors that probably determined these differences in laterite composition are geomorphology, topography, drainage and tectonics. According to the mineralogy of principal ore-bearing phases, Dominican Ni-laterite deposits are classified as the hydrous silicate-type. The main Ni-bearing minerals are hydrated Mg-Ni silicates (serpentine and ¿garnierite¿) occurring deeper in the profile (saprolite horizon). In contrast, in the deposits of eastern Cuba, the Ni and Cooccurs mainly in the limonite zone composed of Fe hydroxides and oxides as the dominant mineralogy in the upper part of the profile, and are classified as the oxide-type.

Diagenetic processes in a partially dolomitized carbonate reservoir: Casablanca oil field, Valencia Trough, offshore Spain.

Mesozoic and Neogene carbonates located in the Valencia Trough (offshore Spain, western Mediterranean Sea)are oil reservoirs. This paper investigates the diagenetic evolution of the Upper Jurassic limestones, currently dolomitized, that constitute the main reservoir of the Casablanca oil field. Core samples from Casablanca-1A well have been studied to determine the diagenetic products and their relation with porosity evolution, and to reconstruct the fluid flow history prior to and during oil emplacement. On the basis of petrological observations and geochemical analyses (major, minor and trace element composition and oxygen, carbon and strontium isotope composition), a major dolomitization event is recognized postdating subaerial exposure, erosion and karstification. The dolomitization event originated two replacive dolomites (RD1 and RD2) and two dolomite cements (saddle dolomite cement, SDC, and milky-white dolomite cement, MDC)which are partially cogenetic. RD1, RD2 and SDC precipitated at increasing temperatures (over 60ºC and below 110ºC), probably from meteoric water mixed with marine water. The last dolomite type milky-white dolomite cement) precipitated with increasing burial conditions and by arrival of hydrothermal fluids during the Miocene. The post-dolomitization sequence comprises precipitation of calcite cement and partial calcitization of all previous dolomites. The oxygen, carbon and strontium isotope compositions suggest that this calcite cementation occurred from meteoric waters mixed with Burdigalian - Langhian marine waters trapped in the sediments and expelled by compaction in the moderate to deep burial realm. Normal faults were the conduits for upward migration of these fluids as well as for later oil expulsion from the Burdigalian - Langhian source rocks. Late corrosion associated with organic acid-enriched fluids took place prior or simultaneously to oil migration during the Pliocene, enhancing porosity and increasing eservoir quality.