Discussion
Read the following statements and choose the correct answer from the codes given below :
(i) Dune sand is most commonly composed of mineral quartz.
(ii) Mineral quartz is extremely hard and largely immune to chemical decay.
Answer: A
Both (i) and (ii) are true.
(i) Most dune sand is made of quartz grains, often well rounded and frosted by wind abrasion. (ii) Quartz has a hardness of 7 on the Mohs scale and is chemically very stable, so it survives weathering and transport long after feldspars and ferromagnesian minerals have broken down to clay.
In fact (ii) is the reason for (i): because quartz resists both mechanical and chemical breakdown, it is concentrated in mature sediments such as dune and beach sands. Options calling either statement false are therefore wrong. (Gypsum dunes, as at White Sands, and carbonate dunes are exceptions, not the rule.)
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