| Uakitite | VN | Cubic | — | — |
| Uchucchacuaite | AgMnPb₃Sb₅S₁₂ | Orthorhombic | — | — |
| Udinaite | NaMg₄(VO₄)₃ | Tetragonal | — | 3.26 g/cm³ |
| Uduminelite | Ca₃Al₈(PO₄)₂O₁₂·2H₂O | — | — | — |
| Uedaite-(Ce) | Mn²⁺CeAl₂Fe²⁺(Si₂O₇)(SiO₄)O(OH) | Monoclinic | — | — |
| Uklonskovite | NaMg(SO₄)F·2H₂O | Monoclinic | — | — |
| Ulexite | NaCaB₅O₆(OH)₆·5H₂O | Triclinic | 2.5 | — |
| Ulfanderssonite-(Ce) | (Ce₁₅Ca)∑₁₆Mg₂(SiO₄)₁₀(SiO₃OH)(OH,F)₅Cl₃ | Monoclinic | — | — |
| Ullmannite | NiSbS | Cubic | — | — |
| Ulrichite | CaCu(UO₂)(PO₄)₂·4H₂O | Monoclinic | — | — |
| Ulvöspinel | Fe²⁺₂TiO₄ | Cubic | — | — |
| Umangite | Cu₃Se₂ | Tetragonal | 3.0 | 6.70 g/cm³ |
| Umbite | K₂ZrSi₃O₉·H₂O | Orthorhombic | 4.5 | — |
| Umbozerite | Na₃Sr₄ThSi₈(O,OH)₂₄ | Amorphous | — | — |
| Umbrianite | K₇Na₂Ca₂[Al₃Si₁₀O₂₉]F₂Cl₂ | Orthorhombic | — | — |
| Umohoite | (UO₂)(MoO₄)(H₂O)·H₂O | Triclinic | 2.0 | — |
| Ungavaite | Pd₄Sb₃ | Tetragonal | — | — |
| Ungemachite | K₃Na₈Fe³⁺(SO₄)₆(NO₃)₂·6H₂O | Trigonal | 2.5 | — |
| Upalite | Al(UO₂)₃(PO₄)₂O(OH)·7H₂O | Monoclinic | — | — |
| Uralborite | CaB₂O₂(OH)₄ | Monoclinic | — | — |
| Uralolite | Ca₂Be₄(PO₄)₃(OH)₃·5H₂O | Monoclinic | — | — |
| Uramarsite | (NH₄)(UO₂)(AsO₄)·3H₂O | Tetragonal | — | — |
| Uramphite | NH₄(UO₂)PO₄·3H₂O | Tetragonal | — | — |
| Urancalcarite | Ca(UO₂)₃(CO₃)(OH)₆·3H₂O | Orthorhombic | — | — |
| Uraninite | UO₂ | Cubic | — | 11.06 g/cm³ |
| Uranocircite | Ba(UO₂)₂(PO₄)₂·10H₂O | Tetragonal | — | 3.46 g/cm³ |
| Uranocircite-I | Ba(UO₂)₂(PO₄)₂·12H₂O | — | — | — |
| Uranoclite | (UO₂)₂(OH)₂Cl₂(H₂O)₄ | Monoclinic | — | — |
| Uranophane | Ca(UO₂)₂(SiO₃OH)₂·5H₂O | Monoclinic | — | — |
| Uranopilite | (UO₂)₆SO₄O₂(OH)₆·14H₂O | Triclinic | 2.0 | — |
| Uranopolycrase | (U,Y)(Ti,Nb,Ta)₂(O,OH)₆ | Orthorhombic | — | — |
| Uranosilite | (UO₂)Si₇O₁₅ | Orthorhombic | — | — |
| Uranospathite | (Al,☐)(UO₂)₂F(PO₄)₂·20H₂O | Orthorhombic | — | — |
| Uranosphaerite | Bi(UO₂)O₂(OH) | Monoclinic | — | — |
| Uranospinite | Ca(UO₂)₂(AsO₄)₂·10H₂O | Tetragonal | — | — |
| Uranotungstite | Fe(UO₂)₂(WO₄)(OH)₄·12H₂O | Orthorhombic | — | — |
| Urea | CO(NH₂)₂ | Tetragonal | — | — |
| Uricite | C₅H₄N₄O₃ | Monoclinic | — | — |
| Uroxite | [(UO₂)₂(C₂O₄)(OH)₂(H₂O)₂]·H₂O | Monoclinic | — | — |
| Urusovite | CuAlO(AsO₄) | Monoclinic | — | — |
| Urvantsevite | Pd(Bi,Pb)₂ | Tetragonal | 2.0 | 9.66 g/cm³ |
| Ushkovite | MgFe³⁺₂(PO₄)₂(OH)₂·8H₂O | Triclinic | — | — |
| Usovite | Ba₂CaMgAl₂F₁₄ | Monoclinic | — | 4.05 g/cm³ |
| Ussingite | Na₂AlSi₃O₈(OH) | Triclinic | — | 2.42 g/cm³ |
| Ustarasite | Pb(Bi,Sb)₆S₁₀ (?) | — | — | — |
| Usturite | Ca₃(SbZr)(FeO₄)₃ | Cubic | 7.0 | — |
| Utahite | MgCu²⁺₄Zn₂Te⁶⁺₃O1₄(OH)₄(H₂O)₆ | Triclinic | — | — |
| Uvanite | (UO₂)₂V⁵⁺₆O₁₇·15H₂O (?) | Orthorhombic | — | — |
| Uvarovite | Ca₃Cr₂(SiO₄)₃ | Cubic | — | 3.83 g/cm³ |
| Uvite | — | Trigonal | 7.5 | — |
| Uytenbogaardtite | Ag₃AuS₂ | Trigonal | 2.0 | — |
| Uzonite | As₄S₅ | Monoclinic | — | — |