A sedimentary rock sample after weathering contains 55 % quartz, 30 % feldspar, 10 % mica and 5 % clay by weight. The weathering process removed 10 % of the original feldspar mass while leaving the other minerals unchanged. Determine the original proportion of feldspar in the rock before weathering and, using the WAEC classification, state whether the original rock is a sandstone, an arkose (quartz 40‑60 % and feldspar  25 %) or a graywacke (quartz  60 % with significant clay).

Question

A sedimentary rock sample after weathering contains 55 % quartz, 30 % feldspar, 10 % mica and 5 % clay by weight. The weathering process removed 10 % of the original feldspar mass while leaving the other minerals unchanged. Determine the original proportion of feldspar in the rock before weathering and, using the WAEC classification, state whether the original rock is a sandstone, an arkose (quartz 40‑60 % and feldspar > 25 %) or a graywacke (quartz > 60 % with significant clay).

Accepted Answer

Original feldspar ≈ 33.3 %; the rock is classified as an arkose.

Waec Model Questions Vol1 2018 Geography Question 25

Answer Options

About This Question

Master the Exam!