Barstowite, ideally 3PbCl2.PbCO3.H2O is a new mineral from a lead-antimony vein at the south-western end of Bounds Cliff, St Endellion, Cornwall. It occurs as very small (0.2–0.5 mm) subparallel elongate intergrown crystals which form aggregates (up to 3 mm) containing minor inclusions of galena and jamesonite. Other associated minerals are quartz, dolomite, phosgenite, cerussite, sphalerite, pyrite and chalcopyrite. Barstowite is colourless to white and is transparent with an adamantine lustre and white streak. It is brittle, has an imperfect prismatic cleavage, and an uneven fracture. Low reflecting, barstowite has a moderate bireflectance from grey to dark grey in hue and weak to moderate anisotropy. Internal reflections (colourless) are abundant. Measured reflectance values in air and oil are tabulated. Colour values relative to the CIE illuminant C for R1 and R2 in air respectively are: Y% 12.9, 13.3; λd 477, 476; Pe% 3.5, 3.7. VHN100 111 (range 108–117); calculated Mohs hardness is 3. Infrared spectra of barstowite, phosgenite and cerussite are compared.

X-ray studies show that barstowite is monoclinic with space group P21/m and a 4.218(2), b 9.180(2), c 16.673(4) Å, β 91.49(3)°. It has a cell volume of 645.38 Å3 with Z = 2. Dcalc. is 5.76 g/cm3, Dmeas. is 5.71 g/cm3. The strongest six lines of the X-ray powder pattern are [d in Å (I) (hkl)] 4.02 (10) (022); 2.296(8) (040; ; 106); 2.377(6) (007; 026); 4.16(5) (004); 2.108(4) (200); 3.79(3) (014). The name is for the late Richard William Barstow, the Cornish mineral dealer.

The crystal structure of barstowite (3PbCl2·PbCO3·H2O, P21/m, a = 4.2023(5), b = 9.2020(7), c = 16.6608(6)(Å), β = 91.829(9)°) from Bounds Cliff, St Endellion, Cornwall, has been determined to an R = 0.036. The structure can be described in terms of three layers stacked along [001] in the sequence …CACBCACBC… to give a 16.7 Å repeat. Layer A consists of face-sharing PbCl8 distorted square antiprisms. Layer B is composed of Pb coordinated to 5 Cl and 2 O each with two H to form H2O groups. Layer C is formed by Pb coordinated to 5 Cl and 3 O which are from CO3 groups. The H positions are deduced and are H-bonded to O from carbonates. The structural details were in agreement with previous deductions based on infrared spectra.