I could very easily answer this question for you. However I won't as this appears to be a homework question. So, I am going to do you a favor and tell you to read your textbook. I am sure that the answer is in there.

I'll give you a hint. It is built of silica tetrahedra that are sharing all of their oxygen atoms to create a three dimensional network, but it is not quartz and it does not contain much potassium. However, there is another mineral in the same family that does contain a lot of potassium. Taken together, this mineral and its potassium rich relative constitute slightly more than 50% of the crust.

--- EDITED (Introduction and point 5) --------- The most common silicate *rock* is granit. The most common silicate *mineral group* is feldspar. Quarz is the most abundant mineral in the Earth's *continental* crust. 1) "Sial or SiAl is the name for the upper layer of the Earth's crust, which is also known as the continental crust because it is absent in the wide oceanic basins. This layer is made of rocks rich in silicates and aluminium minerals. When the sial comes to the surface it is typically granitic, so sometimes this layer is called the 'granitic layer' of the crust. Geologists often refer to the rocks in this layer as felsic. On the continental plates the sial runs between 5 km and 70 km deep. Because in a real sense the sial floats on the sima, mountains extend down as well as up, much like icebergs on the ocean, hence the great variation in depth. See Isostasy. The name 'sial' was taken from the first two letters of silica and of aluminium. Comparable is the name 'sima' which is the name for the lower layer of the Earth's crust, which is exposed in the ocean basins." Source and further information: http://en.wikipedia.org/wiki/Sial "alkali feldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite." Source and further information: http://en.wikipedia.org/wiki/Granite 2) "Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in silicates and magnesium minerals. Typically the sima when it comes to the surface is basalt, so sometimes this layer is called the 'basalt layer' of the crust. The sima layer is also called the 'basal crust' or 'basal layer' because it is the lowest layer of the crust. Because the ocean floors are mainly sima, it is also sometimes called the 'oceanic crust'. The name 'sima' was taken from the first two letters of silica and of magnesium. Comparable is the name 'Sial' which is the name for the upper layer of the Earth's crust." Source and further information: http://en.wikipedia.org/wiki/Sima_%28geology%29 "Basalt generally has a composition of 45-55 wt% SiO2, 2-6 wt% total alkalis, 0.5-2.0 wt% TiO2, 5-14 wt% FeO and 14 wt% or more Al2O3. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%." Source and further information: http://en.wikipedia.org/wiki/Basalt 3) "Granite is the ultimate silicate rock. As discussed elsewhere in greater detail, on average oxygen and silica account for 75% of the earth's crust. The remaining 25% is split among several other elements, with aluminum and potassium contributing the most to the formation of the continental granitic rocks. Relatively small amounts of iron and magnesium occur, but since they have generally higher densities it's not surprising that there isn't very much in the granite. Due to the process of differentiation, most of the heavier elements are moving towards the core of the earth, allowing the silica and oxygen to accumulate on the surface. And accumulate it has. Enough granitic "scum" has differentiated to the surface to cover 25% to 30% of the earth with the good stuff. We call this purified material felsic because of the relatively high percentage of silica and oxygen." "Basalt is considered a mafic silicate rock. Among other characteristics, mafic minerals and rocks are generally dark in color and high in specific gravity. This is in large part due to the amount of iron, magnesium, and several other relatively heavy elements which "contaminate" the silica and oxygen. But this heavy stuff really isn't happy near the surface, and will take any opportunity it can to head for deeper levels. The trick is to heat the basalt back up again so it can melt and give the iron another shot at the core. It wants to be there, and heat is the key which unlocks the door. As it turns out, most of the ocean floor is basalt, and most of the continents are granite. Basaltic crust is dark and thin and heavy, while granite is light and accumulates into continent-sized rafts which bob about like corks in this "sea of basalt." " Source and further information: http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry27.html 4) "The silicates, owing to their abundance on the Earth, constitute the most important mineral class. Approximately 25 percent of all known minerals and 40 percent of the most common ones are silicates; the igneous rocks that make up more than 90 percent of the Earth's crust are composed of virtually all silicates." "The great majority of the igneous rocks are composed of silicate minerals (meaning that the basic building blocks for the magmas that formed them are made of silicon [Si] and oxygen [O]), but minor occurrences of carbonate-rich igneous rocks are found as well." Source and further information: http://www.britannica.com/eb/topic-544248/silicate-mineral 5) "QUARTZ is undoubtedly the single most common mineral in the Earth's crust [see note], ranging from perhaps 12% of continental crust to as much as 50% of oceanic crust as indicated by the composition of spreading-ridge volcanic lavas. Some estimates place quartz at 21% of the Earth's total lithosphere." Source and further information: http://mineral.galleries.com/Minerals/By_Name.htm (Note: It seems that the *continental* crust was meant here) "Tectosilicates, or "framework silicates", have a three-dimensional framework of silicate tetrahedra with SiO2 or a 1:2 ratio. This group comprises nearly 75% of the crust of the Earth. Tectosilicates with the exception of the quartz group are aluminosilicates." Source and further information: http://en.wikipedia.org/wiki/Silicate_minerals "In geology and astronomy, the term silicate is used to denote types of rock that consist predominantly of silicate minerals. Such rocks include a wide range of igneous, metamorphic and sedimentary types. Most of the Earth's mantle and crust are made up of silicate rocks. " Source and further information: http://en.wikipedia.org/wiki/Silicate "Quartz (from German Quarz) is the most abundant mineral in the Earth's continental crust (although feldspar is more common in the world as a whole). It is made up of a lattice of silica (SiO2) tetrahedra." Source and further information: http://en.wikipedia.org/wiki/Quarz "Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock." "This group of minerals consists of framework or tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers: - K-feldspar endmember KAlSi3O8 - Albite endmember NaAlSi3O8 - Anorthite endmember CaAl2Si2O8 Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar." Source and further information: http://en.wikipedia.org/wiki/Feldspar Further information: http://en.wikipedia.org/wiki/Structure_of_the_Earth#Crust http://en.wikipedia.org/wiki/Crust_%28geology%29