Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Oxygen in Rocks: A Basis for Petrographic Calculations

Abstract: The standard cell is defined as a rock unit containing 160 oxygens. The sum of the cations (silicon and metals) associated with this unit is very nearly 100 in all ordinary rock types. It is shown that, by using the standard cell as a basis, important petrogenetic relations can be surveyed and quantitatively defined more satisfactorily than by using any other method of petrographic calculation, including the norm method.

Topaz–albite granites and rare-metal mineralization in the Limu District, Guangxi Province, southeast China

Abstract: The topaz–albite granites of the Limu district are ultra-acidic, peraluminous, Li–F–Na-rich and Sn–Ta–Nb-mineralized. A distinct vertical zonation is developed in the granite stocks. There is an upward, systematic transition from leucocratic microcline-albite granite, through albite-microcline granite, topaz–albite granite, pegmatite stockscheider and layered pegmatite–aplite dikes, to K-feldspar–quartz veins and lepidolite–fluorite stringers in the country rocks. Snow-ball textures, homogeneous distribution of rock-forming and accessory minerals, disseminated mineralization, and melt inclusions in quartz, topaz, and albite are typical features indicative of their crystallization from the late stage Li–F–Na-rich and Sn–Ta–Nb-bearing residual granitic melts at a higher intrusion level. A comparison with rare-metal-bearing pegmatite, ongonite, topaz rhyolite and obsidian glass from other regions shows the worldwide existence of these specialized residual melts. Their emplacement and crystallization in a variety of geological environments result in the formation of a series of chemically similar rocks with different petrographic textures and mineral associations. The topaz–albite granites and associated mineralization in the Limu district provide a good example of highly evolved magmatic fractionation in the F-rich granite system and fluid/melt partitioning behavior of rare-metal elements during magmatic-hydrothermal evolution.

Facies analysis and depositional sequences of the upper jurassic mozduran formation, a carbonate reservoir in the kopet dagh basin, ne iran

Abstract: Upper Jurassic carbonates of the Mozduran Formation constitute the principal reservoir intervals at the giant Khangiran and Gonbadli gasfields in the Kopet Dagh Basin, NE Iran. These carbonates were investigated using detailed field studies and petrographic and wireline log analyses in order to clarify their depositional facies and sequence stratigraphy. Facies were interpreted to reflect deep basin, fore-shoal, shelf margin, lagoon, tidal flat and coastal plain depositional systems. The Mozduran Formation is composed of six depositional sequences. Thickness variations were controlled by differential subsidence. Aggradation on the platform margin and reduced carbonate production in the deep basin together with differential subsidence resulted in the creation of a narrow seaway during the late Oxfordian. Petrographic studies suggest that Mozduran Formation carbonates had a low-Mg calcite mineralogy during the Oxfordian, and an aragonite to high-Mg calcite mineralogy during the Kimmeridgian. Reservoir pay zones are located in highstand systems tracts within the lower and middle Kimmeridgian depositional sequences. The rapid lateral thickness variations of these sequences were controlled by tectonic factors, leading to compartmentalization of the Mozduran Formation reservoir with the possible creation of stratigraphic traps, especially at the Khangiran field.

Whiteschist: A High-Pressure Rock and its Geologic Significance

Abstract: Schists containing the high-pressure assemblage talc + kyanite were recently found in a number of localities, partly in regional distribution. It is proposed to introduce these rocks under the name "whiteschists" into petrographic nomenclature. They may be far more common than is presently known. Because they are stable only at water pressures greater than about 10 kbar, they may provide important information about the geologic and metamorphic history of portions of the continental crust.