通式A₂SO₄·B₂(SO₄)₃·24H₂O中，A为+1价的阳离子，如Na、K、Rb、Cs、Tl、NH₄、CH₃NH₃、HONH₃、N₂H₅，B为+3价的阳离子，如Al、Cr、Ti、Mn、V、Fe、Co、Ga、Mo、In、Ru、Rh、Ir。具体的离子组合与离子半径有关。所有矾类都是立方晶系的，一般有生成规则的立方形大晶体的倾向。根据离子大小的不同，矾类可以分为三种晶体结构：α-、β-、和γ-型。α-型是由中等大小的+1价离子生成的，如钾和铷；β-型由较大的+1价离子生成，如铯；γ-型由较小的+1价离子生成，如钠。硒酸盐可以代替硫酸盐生成“硒矾”；其中一个硫酸根离子也可被其他阴离子取代，比如K₂BeF₄·Al₂(SO₄)₃·24H₂O。

Alum /ˈæləm/ is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl(SO
4)₂·12H
2O. More widely, alums are double sulfate salts, with the general formula A
2(SO
4).M
2(SO
4)
3.24H
2O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium or chromium(III). When the trivalent ion is aluminium, the alum is named after the monovalent ion.