mature. T he effects of weathering are examined further in Exercise 6.1 weatlern
ID: 295912 • Letter: M
Question
mature. T he effects of weathering are examined further in Exercise 6.1 weatlerng (slable minerals) are known as mineralogicaly EXERCISE 6.1 Looking at Weathering Products Name: Course: Section: Date: During chemical weathering, water, oxygen, and carbon dioxide combine with minerals in previously 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 weathering? (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 ancd remember that there may be more than one possible interpretation O All quartz grains o Nearly oqual amounts of quartz, K-feldspar, and Na- plagioclase with asmall amount of hematite (ii) All fine-grained clay minerals with some hematite and limonite (v) A mixture of quartz grains and clay minerals Rock fragments composed of Ca-plagioclase and pyroxene (continued) CHAPTER 6 USING SEDIMENTARY ROCKS TO INTERPRET EARTH HISTORY 138Explanation / 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.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.