Summary: X-ray, infra-red, and optical studies suggest that the bluish-grey crust (previously described as plumbogummite) on arsenian pyromorphite from Roughten Gill, Cumberland, is a carbonate-bearing mix-crystal of plumbogummite, hinsdalite, and hidalgoite with a 7·018 Å, c 16·784 Å; a_rh 6·907, a 61° 3′; ω 1·680, ɛ 1·698.