斜长石（英语：Plagioclase）是长石的一种，是一种在地球上很常见且很重要的硅酸盐矿物。斜长石并没有特定的化学成分，而是由钠长石和钙长石按不同比例形成的固溶体系列。“斜长石”这个名称来源于希腊语中的“倾斜的切面”，指其二向完全解理的两个不同夹角。斜长石是两种矿物的固溶体这一性质首先是由德国矿物学家约翰·海赛尔(Johann F. C. Hessel)于1826年发现的。在斜长石中，钠原子和钙原子可以在晶格中相互替代，按此两种原子的比例可将斜长石继续划分成从钠长石到钙长石的不同子类。在地质样品中，斜长石常因其易形成孪晶，以及表面由于解理形成类似唱片表面刻痕的性质被识别出来。

Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more properly known as the plagioclase feldspar series (from the Greek "oblique fracture", in reference to its two cleavage angles). This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi₃O₈ to CaAl₂Si₂O₈), where sodium and calcium atoms can substitute for each other in the mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.