(b) What weathering history can you interpret from the following mineral assembl
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Question
(b) What weathering history can you interpret from the following mineral assemblages found in clastic sedimentary rocks? For example: What was the source rock? Was it weathered for a short time or a long time? Think broadly and remember that there may be more than one possible interpretation () All quartz grains (n Nearly equal amounts of quartz, K-feldspar, and Na-plagioclase with a small amount of hematite All fine-grained clay minerals with some hematite and limonite (ii) v) A mixture of quartz grains and clay minerals 2 Rock fragments composed of Ca- plagloclase and pyroxene (continued)Explanation / Answer
Weathering is the breaking down of rocks, soil, and minerals through contact with the Earth's atmosphere, water, and biological organisms. Weathering may involve physical, chemical and breakdown by biological agents. Mechanical (or physical) weathering refers to rock fragmentation by such forces as ice- or salt-wedging and the unloading of pressure on rocks formed far underground and then exposed at the Earth’s surface. Chemical weathering covers processes that weather rock through chemical reactions (when minerals in rocks are dissolved or replaced through exposure to air or water).
i) A clastic sedimentary rock containing all quartz grains will be formed at the surface (at lower temperature conditions; see Bowen's reaction series). A rock containing quartz grains in high proportions will be a very resistant rock (quartz is also generally the end product of weathering). Thus this rock's resistance to weathering is high and will be weathered over very long periods of time. Thus a clastic sedimentary rock with all quartz grains will be weathered over a long period of time as further breakdown is very tough.
ii) A clastic sedimentary rock containing nearly equal quantities of quartz, K-Feldspar, and Na-plagioclase will be susceptible to weathering. As a rock containing minerals like K-feldspar and Na-plagioclase, these types of rocks are formed at slightly higher temperatures (and thus at higher depths; following Bowen's reaction series). These kinds of rocks maybe examples of hypabyssal igneous rocks which were converted to sedimentary rocks when exposed at the surface.
iii) A clastic sedimentary rock will all fine-grained clay minerals and some hematite and limonite inclusions will not experience weathering as clay particles being very small sized particles may not be further susceptible to weathering. This type of rock will be a surface rock.
iv) A clastic sedimentary rock with quartz crystals and clay minerals will be a surface rock. This rock may also be well resistant to erosion and will be weathered over long periods of time (longer compared to (i) but shorter than (iii)).
v) Rock fragments composed of some Ca-plagioclase and pyroxenes are formed at higher temperatures (see Bowen's reaction series). Thus, these rocks will be formed at greater depths, may contain source rocks formed at greater depths (source rocks may be plutonic igneous rocks which may have been converted to sedimentary rocks on exposure at the surface). These rocks are very susceptible to weathering and will be weathered over shorter periods of time.
Thus, sedimentary rocks having source rocks at the surface: (i), (iii) and (iv); sedimentary rocks having source rocks formed at certain depths: (ii), (v).
Rate of weathering of rocks (shortest time to longest time to weather the rock): (v) < (ii) < (i) < (iv) < (iii)
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