In this book, the authors present current research in the study of the mineral composition, geology and petrogenesis significance of feldspar, a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust. Topics discussed in this compilation include the patterns of phosphorus enrichment in alkali feldspars; crystallisation conditions of feldspars in basement rocks in Bangladesh; industrial evaluation and beneficiation of Jordanian feldspar for application in the ceramic industry; unimpeded growth of K-feldspar megacrysts in granitic liquids; the extent to which simple twinning occurs in igneous K-feldspar; and multiple growth twinning of K-feldspar in hypersolvus granite.