Question

Which mineral or mineral group occurs exclusively in sedimentary rocks? clay minerals feldspar olivine garnet?

Answer 1

Clay minerals is the mineral group that occurs exclusively in sedimentary rocks.

Sedimentary rocks are formed from low temperature accumulations pf particles, OR by precipitation from solution. They are made up of detritus that has been eroded by the actions of wind, water and ice from pre-existing rocks of all types. Minerals included in the sedimentary rocks are: clay-, silt-, and sand- up to pebble-, cobble-, and boulder-size materials. However, clay is found only in them.

Answer 2

All these minerals listed ie clay minerals, feldspar, garnet and olivine can be constituents of sedimentary rocks but they are not restricted to sedimentary rocks. For example, clay minerals can occur in igneous rocks as alteration of the feldspars especially in orebodies where either hydrothermal fluids have altered them or they feldspars have just been attacked by the elements ie from weathering. Also, feldspar, garnet and olivine can also be constituents of igneous and metamorphic rocks. I am a geologist so I know I am right about the clay minerals that they can occur in igneous rocks just as much as in sedimentary rocks. What the question assumes is that by clay minerals they mean detrital clay from grinding down larger particles as opposed to clay from alteration of feldspars which occurs in  igneous rocks.