Microtextures of Igneous and Metamorphic Rocks (Petrology by J.P. Bard

By J.P. Bard

At a time while 'textural' proof is considered being 'obvious' ( . . . ) it turns into increasingly more tough to discover illustrations or maybe descriptions of the preparations of a few of the materials of 'traumatized' rocks. it's useful as a result to propose geology scholars that the learn of skinny sections is not just enthusiastic about the identity in their mineral content material. to take action could suggest they can no longer see the wooden for the bushes. actual id of the indi­ vidual minerals that shape rocks is key of their description however the research in their textures and behavior can be crucial. examine of textural gains enforces constraints upon the inter­ pretation of the foundation and background of a rock. The research of micro­ textures can't and may by no means be an goal in itself, out has to be sup­ ported through qualitative and quantitative correlations with theories of petrogenesis. the purpose this is to assist the reader to bridge the distance among his observations of rocks unqer the microscope and petrogenetic theories. The behavior or architectures of crystals in rocks may possibly resemble these studied by means of metallurgists and glass scientists. research of micro­ textures is present process swap engendered via comparisonS among manu­ factured and accordingly minerals. this is obvious from the elevated variety of courses facing crystal ~rowth or deformation procedures at microscopic scales to which the identify of 'nanotectonics' has been utilized.

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Extra info for Microtextures of Igneous and Metamorphic Rocks (Petrology and Structural Geology)

Example text

It has been shown experimentally that growth can occur either parallel to those surfaces (via punctual or linear defects in the twinned crystal) or at the intersection with the twin plane (u ranging from 1 to 2°). 5. 1. Liquid ~ solid Transformations After having considered the main mechanisms regulating nucleation and crystal growth and before examining problems of crystal form and size, it is appropriate to have a closer look at the way matter is transported in metamorphic and ingneous rocks.

Large minerals (several cm long, usually idomorphic minerals) are known as phenocrysts (or 'phanerocrystals') or porphyroblasts (or 'phaneroblasts') depending on whether they crystallized within a magmatic melt or in a solid. Those large crystals can contain numerous, irregularly scattered, mineral inclusions but still preserve their idiomorphic shape (case of the poikilocrystals and poikiloblasts). In very deformed rocks with prekinematic crystals or in igneous rocks reorganizing relatively older minerals, the large minerals are known as porphyroclasts (phaneroclasts) or, more simply as clasts.

However, they cannot notably lower the free-energy of the mineral, and will never develop a considerable lateral extension: they could even disappear towards the end of the growth process. 10 The common idiomorphic appearance of porphyroblasts in metamorphic rocks has intrigued petrologists for a long time, since those minerals developed in a heterogeneous solid matrix subjected to confining pressures (average pressure (PI + P2 + P3 )/3) or possibly lkb). Those large minerals can very high shearing stress (PI - P3 ~ sprout from a single nucleus or from several nuclei.

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