Summary: Thin apple-green crusts on several specimens from Hingston Down Consols mine, Calstock, Cornwall, proved to consist of a new mineral, alone or intimately mixed with pharmacosiderite (which it closely resembles), or with an unidentified mineral of the alunite-beudantite group having a 7·04, c 16·6 Å, or with both. The new mineral, for which the name arthurite is proposed, for Sir Arthur Russell and Mr. Arthur W. G. Kingsbury, who independently collected material and suggested that it might be new, gives X-ray powder photographs with their three strongest lines at 4·28, 4·81, and 6·97 Å ; the photographs could be satisfactorily indexed on a monoclinic unit-cell with a 10·09, b 9·62, c 5·55_o Å (all ± 0·01 Å), β 92·2° ± 0·2° and containing [Cu₂Fe₄(AsO₄)₃(OH)₇.6H₂O]. Chemical analysis on 1·1 mg gave: CuO 16·8, Fe₂O₃ 32·4, As₂O₅ 34·3, H₂O 16·5 % (reduced to 100 % after deduction of 27·4 % quartz) ; sp. gr. 3·2 (calc. 3·07). Under the microscope arthurite is pale olive-green and very finely granular; n 1·78, birefringence low to moderate.