Datolite

Abstract: abst R act The crystal chemistry of six natural datolites from different localities was investigated by electron microprobe analysis in the wavelength dispersive mode and single-crystal X-ray diffraction. The chemi cal analyses show no significant site substitution. The single-crystal structure refinements confirm the structural model of datolite previously reported (with a ~ 4.83, b ~ 7.61, c ~ 9.63 A, and β ~ 90.15°, space group P21/c). Intra-polyhedral bond distances and angles show common features in all samples at room T: (1) the Si-tetrahedron is strongly deformed, with Si-O distances ranging between ~1.57 and ~1.66 A and O-Si-O angles ranging between ~105.4 and ~115.3°; (2) the B-tetrahedron is almost regular; (3) the Ca-polyhedron is significantly distorted, with bond distances ranging between ~2.28 and ~2.67 A; (4) only one independent H-site occurs and its refined position suggests a bifurcated hydrogen bonding scheme with O5 as donor and O4 and O2 as acceptors [with O5-H ~ 0.8 A and (1) O5···O4 ~ 2.99 A, H···O4 ~ 2.33 A, and O5-H···O4 ~ 140°, and (2) O5···O2 ~ 2.96 A, H···O2 ~ 2.36 A, and O5-H···O2 ~ 131°]. Low-temperature diffraction measurements between 300 and 100 K show that the thermal expansion of datolite is mainly governed by the axial response along [100] and [010], whereas the c-axis length is almost unchanged in this temperature interval. The volume thermal expansion coefficient (αV = V –1 ∂V/∂T) between 100 and 280 K is αV = 1.5(2)·10 –5 K –1 . The higher thermal expansion of the a-axis is due to the layered nature of the structure of datolite: the Ca-O bond distances are the most compressible and expandable, and govern the contraction, upon cooling, along the direction perpendicular to the polyhedral layers. The tetrahedral layer is significantly more rigid and no changes of the tetrahedral tilts are observed from 300 to 100 K.

Abstract: Mereheadite, ideally Pb2O(OH)Cl, is a new mineral related to litharge and which is structurally similar to synthetic bismuth-oxyhalides. With other lead- and lead-copper oxychlorides, it occupies lenses and cavities in veins of manganese and iron oxide minerals which cut through a sequence of dolomitic limestones at Merehead quarry, Cranmore, Somerset (51°12′N, 2°26′W) Mereheadite is pale yellow to reddish-orange, transparent to translucent and has a white streak and a vitreous or resinous lustre. It is not fluorescent. Individual grains, up to a few mm across, cluster together in compact masses of 10–30 mm in size, but discrete crystals have not been observed. Specular reflectance data on randomly orientated grains from 400 to 700 nm are provided, and refractive indices calculated from these at 590 nm range from 2.19 to 2.28. H = 3.5, VHN100 = 171, D (meas) = 7.12(10) g/cm3, Dcalc = 7.31 g/cm3. The mineral is brittle with an uneven, conchoidal to hackly fracture and has a perfect (001) cleavage which is parallel to the sheets of PbO and Cl. It is intimately associated with mendipite, blixite, cerussite, hydrocerussite and calcite in lenses and pods in the veins. Other minerals which occupy cavities in these veins include chloroxiphite, paralaurionite, parkinsonite and the borosilicate datolite. Mereheadite is monoclinic, space group C2/c, and its cell parameters, refined from powder X-ray diffraction are: a = 5.680(2), b = 5.565(3), c = 13.143(9) A, β=90.64(4)°, V = 415.4 (8) A3, Z = 4. The ten strongest reflections in the X-ray powder diffraction pattern are [d in A, (I, hkl)]: 2.930(10,113), 3.785(5,111, –111), 2.825(4,200), 6.581(4,002), 2.182(4,115), 2.780(4,020), 3.267(4,004), 1.980(3,–220), 1.695(3,224,132,117), 1.716(3,026). Its empirical formula is Pb8O4.19(BO3)0.51 (CO3)0.62(OH)0.76Cl4.09. Although it is very similar chemically to blixite, it has notably different cell parameters. There is some uncertainty about the essential nature of boron and carbon in natural mereheadite. This stems from the impossibility of ensuring the purity of samples for wet-chemical analysis, and from the predominance of lead in the structure of the mineral which has meant that the location of boron and carbon within the mereheadite structure is unresolved, 11B MAS NMR does show, however, that boron is present as BO3 groups. The structure consists of alternating PbO sheets and layers of chlorine atoms. Each lead atom is coordinated to four chlorines and four O/OH in a square antiprism configuration. As such, it is structurally-related to nadorite, thorikosite and schwartzembergite. Comparisons with structurally analogous phases such as bismuth oxychlorides and bismutite (Bi2O2CO3) suggest that the BO3 and CO3 groups are likely to replace chlorine in the layer between PbO sheets. The composition of natural mereheadite is defined by three end-members: the mereheadite end-member Pb2O(OH)Cl, and two fictive end-members Pb2(OH)2CO3 and Pb4O(OH)3BO3.

Abstract: Fluid inclusions in hematite and hausmannite from high-grade Wessels-type ore from the Kalahari manganese field were studied using infrared microscopy. The data obtained suggest that both minerals were deposited cogenetically, along with gangue minerals such as calcite and datolite, from high-salinity fluids. The minerals constituting the high-grade manganese ore were deposited at temperatures below 200 degrees C, well below the temperature range of 200 degrees to 400 degrees C that was estimated on the basis of the thermodynamic stability of common mineral assemblages. The observed melting behavior suggests that the enclosed fluids are rich in Ca and/or Mg salts. It is concluded that an important part of the salt load of the hydrothermal fluid can be attributed to metasomatic interaction with the Ca-Mn-Mg carbonate-rich sedimentary protore to the Wessels-type ore.