"长英质"（felsic）在地质学中是指主要由轻的元素如硅, 氧, 铝, 钠, 钾组成的硅酸盐矿物, 岩浆, 与岩石。其颜色较浅，比重小于3. 最常见的长英质岩石是花岗岩, 此外还有石英, 白云母, 正长石, 富钠的斜长岩 长石等. 与长英质相对立的是铁镁质（或称基性）的矿物、岩石、岩浆。

酸性岩(acid rock), 往往是长英质的同义概念，指二氧化硅含量高 (大于63%的质量是SiO₂) 火山岩, 如流纹岩. 与酸性岩相对是基性岩(basic rock)。

In geology, felsic refers to igneous rocks that are relatively rich in elements that form feldspar and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. It refers to those rocks rich in silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium.