Welshite, triclinic, Ca₂Mg₄Fe³⁺Sb⁵⁺O₂ [Si₄Be₂O₁₈], a = 10·28 Å, b = 10·69 Å, _c = 8·83 Å, α = 106·1°, β = 96·3°, γ = 124·8°, P¯1 with pseudo-monoclinic cell a_m = 9·68(1) Å, b_m = 14·77(2) Å, c_m = 5·14(1) Å, β_m = 101·5(2)°, c-centred, occurs as lustrous subadaman-tine, thick prismatic reddish-black crystals up to 3 mm in greatest dimension associated with roméite, adelite, swedenborgite, berzeliite, manganoan phlogopite, richterite in crystalline dolomite from Làngban, Sweden. Forms include {itb}{010}, a{100}, k{130}, m{110}, j{021}, p{111}, q{121}, v{131}, r{¯111}, s{¯121}, t{¯131}, and v{¯141} (monoclinic setting). Streak pale-brown, hardness 6, specific gravity 3·77, fracture conchoidal, no cleavage, α = 1·81(1), γ = 1·83(1), 2E(obs.) ≈45°, no noticcable pleochroism.

Welshite is a new member of the aenigmatite-rhönite group and obtains from the substitution Sb⁵⁺→ Ti⁴⁺ and Be²⁺ → Al³⁺ in rhönite.