• journal_title：The Canadian Mineralogist
• Contributor：Jiří Sejkora ; Frank C. Hawthorne ; Mark A. Cooper ; Joel D. Grice ; Josef Vajdak ; John L. Jambor
• Publisher：Mineralogical Association of Canada
• Date：2009-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.47.1.159
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：47
• issue：1
• firstpage：159
• section：Articles

Burgessite, Co₂ (H₂O)₄ [AsO₃(OH)]₂ (H₂O), is a new mineral species of supergene origin from the Keeley mine, South Lorrain Township, Timiskaming District, Ontario, Canada. Burgessite forms rosettes up to 0.8 mm across consisting of prismatic crystals up to 0.1 mm long, elongate along a, with the following forms: {011} >> {h0l}, and no observable twinning. It is associated with primary skutterudite, cobaltite and bismuth, and supergene arsenolite, bismutoferrite and erythrite. Burgessite is transparent, purplish pink with a slight brown tint, and has a colorless to pale pink streak and a vitreous luster; it does not fluoresce under ultraviolet light. It exhibits two cleavages, {010} good and {001} fair. The Mohs hardness is ~3; burgessite is brittle with a hackly fracture, and has measured and calculated densities of 2.93(2) and 2.94 g/cm³, respectively. Burgessite is biaxial positive with α 1.596, β 1.604, γ 1.628, all ±0.002, with a 2V_obs of 70(2)° and 2V_calc = 61°. It is nonpleochroic, with an optic orientation X = b, Y ∧ c = 29° (in β obtuse), Z ∧ a = 20° (in β acute). Burgessite is monoclinic, space group P2₁/n, a 4.7058(12), b 9.299(3), c 12.738(4) Å, β 98.933(8)°, V 550.6(5) ų, Z = 2, a:b:c = 0.5061:1:1.3698. The strongest seven lines in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 7.446(100)(011), 6.267(44)(002), 3.725(29)(022), 3.260(25)(121̅), 2.998(31)(031), 2.970(21)(014) and 2.596(23)(024). A chemical analysis with an electron microprobe gave As₂O₅ 47.91, P₂O₅ 0.03, CoO 27.31, NiO 3.52, ZnO 0.04, CaO 0.18, SO₃ 0.13, H₂O (calc.) 22.65, for a sum of 101.77 wt.%; the valence states of As and Co, and the amount of H₂O, were determined by crystal-structure analysis. The resulting empirical formula on the basis of 13 (O, OH, H₂O) anions is (Co_1.75 Ni_0.23 Ca_0.02)_∑2.00 (AsO₃OH)_∑2.00 (H₂O)₅. The crystal structure of burgessite consists of chains of the form [Co₂(H₂O)₄(As⁵⁺O₃(OH))₂] that extend in the a direction and are linked by hydrogen bonds, and is closely related to that of erythrite.