Lesson 3 metamorphic textures

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Metamorphic Textures  Metamorphism literally means to “change form.”  The degree of metamorphism is reflected in the rocks texture and mineralogy METAMORPHISM CHANGES TEXTURE  Two textures are formed as a result of metamorphism; 1) Foliated Texture 2) Non-Foliated Texture Reference: Tarbuck and Lutgens Pages Metamorphic Textures Depends On; 1) Allignment of minerals  Directional stress causes minerals to align when recrystallizing The orientation of these minerals is usually perpendicular to the compressional forces  This gives the resulting metamorphic rock a layered appearance  This is common during high-grade metamorphism and often produces a foliated texture Example: Gneiss Gneiss Metamorphic Textures Depends On; 2) Recrystallization of minerals  Pressure causes minerals to recrystallize which generally forms larger, more dense crystals  The recrystallization process causes some metamorphic rocks to display visible crystals with no visible evidence of the original mineral grains For example, quartz sandstone changes to form quartzite Quartz Sandstone Recrystallizes Quartzite  This is common during low-grade metamorphism and often produces a non-foliated texture Two types of textures: 1)Foliated (i.e layered or banded) 2)Non-foliated • Note that the texture of a metamorphic rock is usually coarser than the parent rock (i.e previous rock) Foliated Non-foliated Metamorphic Textures Two textures are formed as a result of metamorphism; 1) Foliated Texture  Results when the minerals of a rock are brought into parallel alignment during recrystallization Produces a banded or layered appearance Before Metamorphism After Metamorphism  Granite when metamorphosed forms Gneiss Examples: Slate, Phyllite, Schist & Gniess Metamorphic Textures Two textures are formed as a result of metamorphism; 2) Non - Foliated Texture  Results when the minerals of a rock are recrystallized creating uniform sized crystals with no visible foliation  This occurs when limestone (consist of pure calcite) undergoes metamorphism and the smaller calcite crystals combine to form larger interlocking crystals This process of recrystallization of calcite forms Marble Marble Examples: Quartzite & Marble Pairs Parent Rock Limestone (Sedimentary) Sandstone (Sedimentary) Shale (Sedimentary) Granite (Igneous) Metamorphic Rock Changes to Marble Changes to Quartzite Changes to Slate (Phyllite – Schist - Gniess) Changes to Gniess Sample Problem Using examples, distinguish between foliated and non-foliated metamorphic rocks Answer: Foliated: show layering or banding of different minerals created by the alignment of minerals during directional stress and recrystallization (Ex: schist, gneiss) Non foliated: a mass or network of interlocking crystals consisting of one mineral with no visible layering or banding (Ex: marble, quartzite)
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