Search

Wonesite, an interlayer-deficient trioctahedral sodium mica, has been investigated mainly by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) using a Gandolfi camera. The true structure of wonesite is triclinic with a large layer offset, i.e., displacement between the two tetrahedral sheets across the interlayer region, which is partially occupied by sodium ions. The direction and amount of the layer offset is approximately [110] and 1.25 A respectively, with unit-cell dimensions of a = 5.31, b = 9.18, and c = 9.75 A, [alpha] = 96.2, [beta] = 96.5, [gamma] = 89.9 [degrees]. This interlayer structure, which is similar to that in talc, explains well the exsolution lamellae of talc in wonesite with continuous 2:1 layers. The volume ratio of wonesite and exsolved talc is estimated to be 4:1 to 5:1 from the XRD patterns. Disorder in the directions of the layer offset is occasionally observed in TEM. The interlayer in sodium micas (preiswerkite, paragonite, aspidolite, wonesite, etc.) possesses various amounts of layer offset, depending on the cavity space in the tetrahedral sheet that is primarily determined by the ditrigonal rotation angle.

Coexisting minerals of the hollandite-cryptomelane series, piemontite-group, and abswurmbachite-braunite series occur as a small nodule in a quartzose schist (metachert) from the epidote-amphibolite facies area of the Sanbagawa metamorphic belt, central Shikoku, Japan. Strontium, Ba, K, and Pb contents of the hollandite-cryptomelane minerals are 0.00-0.42, 0.07-0.99, 0.00-0.75 and 0.00-0.05 apfu (atoms per formula unit: O = 16), respectively. The piemontite-group minerals can be divided into common piemontite (Ca [is greater than] Sr in the ten-coordinated A2-site) and strontiopiemontite (Sr [is greater than] Ca). Strontium, Ba and Pb contents of the strontiopiemontite reach 0.71, 0.26 and 0.20 apfu (O = 12.5), respectively, suggesting the existence of two possible new end-members for epidote-group minerals: 'Ba-piemontite [BaCa[([Mn.sup.3+], [Fe.sup.3+], Al).sub.3][Si.sub.3][O.sub.12](OH)]' and 'Pb-piemontite [PbCa[([Mn.sup.3+], [Fe.sup.3+], Al).sub.3][Si.sub.3][O.sub.12](OH)].' The abswurmbachite-braunite series minerals (Cu = 0.30-0.53 apfu: O = 12) contain up to 0.34 apfu Mg, indicating the exchange Mg[([Mn.sup.2+], Cu).sub.-1]. Distribution coefficients for Sr between the hollandite-cryptomelane series and the piemontite-group minerals are up to 20, implying that the hollandite-type structure with its large open tunnel is more suitable for incorporation of Sr than the A2-site of the piemontite-group minerals. A synthetic aluminosilicate analogue of the hollandite-cryptomelane compound is stable under P [is greater than] 8-10 GPa, and suggests the hollandite-type aluminosilicate may be a reservoir of Sr and other large-ion-lithophile elements in the upper mantle.