• journal_title：Mineralogical Magazine
• Contributor：A. R. Kampf ; A. C. Roberts ; K. E. Venance ; C. Carbone ; D. Belmonte ; G. E. Dunning ; R. E. Walstrom
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2013.077.1.07
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：77
• issue：1
• firstpage：69
• section：Letter

The ideal formula for members of the cerchiaraite group is Ba₄M₄(Si₄O₁₂)O₂(OH)₄Cl₂[Si₂O₃(OH)₄], where M represents Mn³⁺, Fe³⁺ or Al in the octahedral site. A suffix-based naming scheme is used in which the original cerchiaraite is renamed cerchiaraite-(Mn) and two new minerals are named cerchiaraite-(Fe) and cerchiaraite-(Al). The type localities for cerchiaraite-(Fe) are the Cerchiara mine, Liguria, Italy and the Esquire No. 7 and No. 8 claims, Big Creek, Fresno County, California, USA. The type localities for cerchiaraite-(Al) are the Esquire No. 1 claim, Rush Creek, Fresno County, California, USA and the Esquire No. 7 and No. 8 claims noted above. At the Cerchiara mine, cerchiaraite-(Fe) occurs in small fractures and veinlets in a Jurassic ophiolitic sequence. It is of secondary hydrothermal origin and occurs as tan to brown thin prisms and matted fibres. Cerchiaraite-(Fe) and cerchiaraite-(Al) from the Esquire No. 1, No. 7 and No. 8 claims occur in parallel-bedded quartz-sanbornite vein assemblages which formed as a result of fluid interaction along the margin of the vein. At the Esquire No. 1, No. 7 and No. 8 claims, both cerchiaraite-(Fe) and cerchiaraite-(Al) occur as subparallel aggregates of blue to bluish green irregular prisms. Both minerals are transparent with a vitreous lustre, Mohs hardness ~4½, brittle tenacity, irregular fracture and no cleavage. The calculated density of cerchiaraite-(Fe) is 3.710 g cm⁻³; the measured density of cerchiaraite-(Al) is 3.69(3) g cm⁻³ and the calculated density is 3.643 g cm⁻³. Cerchiaraite-(Fe) is uniaxial (+), with ω = 1.741(2) and ε = 1.768(2); it is weakly pleochroic and O is colourless and E is yellow. Cerchiaraite-(Al) is uniaxial (−), with ω = 1.695(2) and ε = 1.677(2); it is strongly pleochroic and O is colourless and E is blue. Electron-microprobe analyses yielded empirical formulae ranging from (Ba_3.82Na_0.02Ca_0.04)_Σ3.88(alt="Formula" src="69/embed/mml-math-1.gif" /> alt="Formula" src="69/embed/mml-math-2.gif" /> alt="Formula" src="69/embed/mml-math-3.gif" /> alt="Formula" src="69/embed/mml-math-4.gif" /> Mg_0.02)_Σ4.00Si_5.62O_15.47(OH)_9.31Cl_2.22 (Cerchiara mine) to Ba_4.00(alt="Formula" src="69/embed/mml-math-5.gif" /> alt="Formula" src="69/embed/mml-math-6.gif" /> Mg_0.15alt="Formula" src="69/embed/mml-math-7.gif" /> alt="Formula" src="69/embed/mml-math-8.gif" /> )_Σ3.85Si_5.78O_15.34(OH)_8.75Cl_2.91 (Esquire No. 1 claim). Cerchiaraite is tetragonal with Z = 2 and crystallizes in space group I4/mmm. The cell parameters for cerchiaraite-(Fe) are a = 14.3554(12), c = 6.0065(5) Å and V = 1237.80(5) ų; those for cerchiaraite-(Al) are a = 14.317(4), c = 6.0037(18) Å and V = 1230.6(6) ų. In the cerchiaraite-(Fe) structure, SiO₄ tetrahedra share corners forming a four-membered Si₄O₁₂ ring. The ring is corner-linked to an edge-sharing chain of Fe³⁺O₆ octahedra running parallel to c. A Cl site alternates along c with the Si₄O₁₂ ring. A large channel in the framework contains Ba atoms around its periphery and statistically distributed Si₂O₇ silicate dimers and Cl atoms. The strong blue pleochroic colour is attributed to Fe²⁺ –Fe³⁺ intervalence charge transfer along the octahedral chain.