EXERCISE 6.1 Name: Section: (c) Rank the following enviconments in the order in
ID: 290019 • Letter: E
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EXERCISE 6.1 Name: Section: (c) Rank the following enviconments in the order in which you would expect sediments to become mineralogicaly mature, from slowest to fastest. (Hinc: Think about the temperature and the amount of rainfall available) desert, tropical rain foeest Explain your reasoning temperate climate (d) Which of the following mineral assemblages (as seen in Exercise 6Tb) are mineralogically mature? Which are mineralogically immature (D All quartz grains C) Nearly equal amounts of quartz,K-feldspar, and Na-plagoclase with a small amount of hematihe (i) All fine-grained clay minerals with some hematite and limonite (v) A mixture of quartz grains and clay minerals, ()Rock fragments composed of Ca-plagjoclase and pyroxene EXERCISE 6. Section: Date: Name: Course During chemical weathering, water, oxygen, and carbon dioside combine with minerals in previcusly existing rocks to destroy some minerals and create new ones (a) How does the mineral assemblage in chemically weathered sediment change with the amount of chemical (b) What weathering history can you interpret from the following mineral assemblages found in clastic sedimentary ? 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. Niearly equal amounts of quartz, K-feldspar, and Na-plagioclase with a small amount of hematite All fine- grained clay minerals with some hematte and limonite (0 All quartz grains (u) All fine-8 (v) A mixture of quartz grains and clay minerals ) Rock fragments composed of Ca plgoclase and pyroxene (continued) HAPTER 6USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORYExplanation / Answer
Exercise:6.1(a) In chemically weathered rocks, the amount and intensity of chemical weathering determine the mineral assemblage of the rock. Chemical weathering is caused by water entrained with oxygen and carbon dioxide making it slightly active and acidic. Elements that dissolve readily in this water are removed by leaching. Some elements that do not easily get dissolved remain as residue and form clay minerals like gibbsite, kaolinite. Ultimately many other elements are removed leaving behind in residue only iron-aluminium oxides I.e; laterite, bauxite. Thus the mineral assemblage changes from silicate minerals to alumino-silicate clay minerals to iron-aluminium oxides.
(B) (i) All quartz grain: parent rock can be quartz rich granite or pegmatitic granite which weathered for a long time and all of its feldspar are removed by leaching ( first converted into clays and then washed away).
(ii) Nearly equal amounts of quartz, k-felfdspar, and plagioclase and little amount of haematite: the source rock would be granite or gneiss. The presence of feldspar in such huge amount indicates very less amount of weathering has occured.
(iii) All fine grained clay with some haematite and limonite: the source rock would be a granitoid because on their weathering feldspar will be converted into clays and haematite and limonite will be accumulated as residue. The source rock maybe anorthosite also. It must have gone through a long term weathering.
(iv) A mixture of quartz grains and clay minerals: the source rock maybe granitic , quartzo-feldspathic or arkosic. It must've gone through a moderately long term weathering.
(v) Rock fragments composed of ca-plagioclase and pyroxenes: the source rock maybe basaltic/gabbroic/doleritic. The duration chemical weathering will be very small because no chemical change has occured.
(C) mineralogical maturity of sediments:
Deserts- in deserts wind erosion is very active and less amount of rainfall causes small chemical weathering. So the mineralogical maturity of the rock would be moderately high.
Tropical rainforest- in these areas the rainfall is very high resulting in intense chemical weathering. This leads to compositional immaturity.
(iii) In temperate climates both physical and chemical weathering is high. Thus the rocks are moderately mature in mineralogy.
(D) I and v are mature ii moderate and iii and iv immature.
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