Carlfriesite is a new species found in relative abundance at the Bambollita (now La Oriental) mine in the Sierra La Huerta near Moctezuma, Sonora. Colour primrose yellow (R.H.S. 4B) with pale yellow streak. H = 3½, Dmeas = 6·3, Dcalc = 5·93. Fair cleavage on {010}. Monoclinic with a 12·585 Å, b 5·658, c 9·985, β 115° 35′. Space group Cc or C2/c. Strongest lines are 3·167 (10), 3·082 (9B), 5·063 (6), 2.832 (6), 3·369 (4), 4·825 (3), 2·698 (3), and 4·o89 (2). Feeble pleochlroism in pale yellow with γ > α = β α = 1·982, β = 2·095, γ 2·19 2Vα, = 79° calc. Occurs with cerussite and chlorargyrite in an oxidized assemblage derived from hessite, galena, and bornite.