Plagioclase feldspar is a very common mineral in igneous and

Plagioclase feldspar is a very common mineral in igneous and metamorphic rocks, but very uncommon in sedimentary rocks. By comparison, quartz (SiO_2) is less abundance than plagioclase in many igneous rocks, by extremely common in many sedimentary rocks. Explain how and why the relative abundance of these two minerals changes with progress through the weathering and sedimentary cycle. [N.B. Remember to mention bonding-types and other physical properties of minerals in your answer].

Solution

Plagioclase feldspar is commonly found mineral in igneous and metamorphic rock but very uncommon in sedimentary rock because Plagioclase is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite, Na(AlSi3O8), to pure anorthite, Ca(Al2Si2O8). Minerals in this series are a homogenous mixture of albite and anorthite.Members of the plagioclase group are the most common rock-forming minerals. They are important to dominant minerals in most igneous rocks of the Earth’s crust. They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, basalt etc. Plagioclase minerals are important constituents of metamorphic rocks, such as gneiss, where they can be inherited from an igneous protolith or formed during the regional metamorphism of sedimentary rocks but are not very common in sedimentary rocks.

Plagioclase is a common clast produced during the weathering of igneous and metamorphic rocks. It can be the most abundant clast in sediments located close to their source area and decreases in abundance downstream. This decrease is partly because quartz is more physically and chemically durable than feldspar and persists in greater relative quantities downstream in eroded sediments.

All feldspar minerals have two directions of perfect cleavage. It is usually easy to distinguish plagioclase feldspars because their two planes of cleavage intersect at 90-degree angles, and their cleavage faces often display striations. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. Plagioclase in granitic rocks is normally white, pink, or red in color. In basaltic rocks, it is normally gray to black.