ABSTRACT. Honessite and hydrohonessite from Unst, Shetland, and Linden, Wisconsin, are sulphate-containing members of the pyroaurite group and are analogous to reevesite. The structural formula can be written (Nis.ssMg0.10Fe2a~s)(OH)16(SO4)l.ls .XH20 for the Unst material, with sulphate groups replacing the interlayer carbonate groups in reevesite. Microprobe analyses of the Unst materiaJ show amounts of SO 3 in excess of that required to balance all trivalent cations and this may be due to the presence of an amorphous nickel sulphate material. We find no evidence for the presence of trivalent nickel in honessite.The Unst material occurs in both 8.8 (honessite) and 1t.1/~ (hydrohonessite) forms, depending on composition, humidity, and temperature, similar to carrboydite, motukoreaite, and SO4-exchanged takovite. The Wisconsin material occurs only with an 8.8 A basal spacing. The increase from the 7.7/~ basal spacing of reevesite is due to the difference in molecular geometry of carbonate and sulphate groups. The presence of the sulphate molecule is unambiguously shown by the infra-red spectra, and the observed bands are consistent with the sulphate groups lying in the interlayer with their trigonal axes parallel to c. The Unst material is found intimately mixed in variable proportions with reevesite, distributed in patches on chromite, and the mixed nature is revealed by X-ray powder diffraction, by variations in total SO3 content, and by the presence of absorptions due to both SO~-and CO ]-in the infra-red spectra.

Synopsis Mattheddleite, a new lead member of the apatite group with sulphur and silicon totally replacing phosphorus, occurs as tiny crystals (<0.1 mm) forming drusy cavities in specimens from Leadhills. Optically, the mineral is colourless in transmitted light and is uniaxial with ω2.017 and ε1.999. X-ray powder diffraction data are similar to the synthetic compound lead hydroxyapatite and may be indexed on a hexagonal cell with a 9.963 and c 7.464 Å (the cell volume is 642 Å 3 ). The calculated density is 6.96 g/cm 3 . The strongest lines in the powder pattern are [ d , ( I ) ( hkl )]: 2.988 (100) (112, 211), 4.32 (40) (200), 4.13 (40) (111), 2.877 (40) (300), 3.26 (30) (210). Single crystal Weissenberg photographs are close to those of pyromorphite, space group P6 3 /m. Chemically, mattheddleite does not contain S and Si in the expected 1:1 ratio, and the ideal formula may be expressed as Pb 20 (SiO 4 ) 7 (SO 4 ) 4 Cl 4 . The infrared spectrum is very similar to that of hydroxyellestadite. Associated minerals are lanarkite, cerussite, hydrocerussite, caledonite, leadhillite, susannite, and macphersonite. The mineral is named after Matthew Forster Heddle (1828–1897), a famous Scottish mineralogist.

The crystal structure of macphersonite (Pb4SO4(CO3)2(OH)2, Pcab, a = 9.242(2), b = 23.050(5), c = 10.383(2) Å) from Leadhills, Scotland has been determined to an R = 0.053. The structure has many features in common with its polymorph leadhillite including three distinct types of layers. Layer A includes sulphate tetrahedra, Layer B is composed of Pb and OH, while Layer C is composed of Pb and CO3 with topology identical to that in cerussite. In both macphersonite and leadhillite these layers are stacked along [010] as …BABCCBABCC… The double CC layer is almost identical in the two structures and forms a structural backbone and occurs in other structures including hydrocerussite and plumbonacrite. The sulphate layer shows the greatest difference between the two structures and can be described by a pattern of up or down pointing tetrahedra. For macphersonite the sequence along [001] is …UDUDUD… while in leadhillite the sequence along [010] is …UDDUUDDU… This latter sequence effectively doubles b relative to the equivalent direction in macphersonite. Susannite, a third polymorph, may have yet another sequence of sulphates to give trigonal symmetry; by heating leadhillite, displacive movements of sulphate groups may occur with a conversion to susannite.

ABSTRACT. Macphersonite is white, resinous to adamantine, hardness (Mohs) 2 89 3, density 6.50-6.55 grn/cm 3 and possesses a perfect cleavage on {010}.

SYNOPSIS A glossary of Scottish mineral species, listing some 443 mineral names, and containing 399 valid species has been Compiled to fulfil a long recognized need. Wherever possible x-ray diffraction and chemical data have been used in the selection of the species. Entries for the glossary have been derived from an extensive literature survey, examination of many specimens from various museums and from data provided by individuals. The value of museum collections in works of this nature is highlighted, for without them coverage would be inadequate. Modern nomenclature in reference works is used as a yardstick for species status, but in some instances personal decisions had to be taken regarding the status of certain minerals. The mineral lists of Heddle have been revised and are now accredited with 162 valid species; this forms the nucleus of the glossary. Entries follow a standard sequence of name, ideal formula, crystal system, assemblage and locality details, except for Heddle entries. The first, or earliest found, reference is given for minerals discovered after 1901. Some minerals well entrenched in the literature are unsubstantiated. The glossary is accompanied by a text and three appendices, viz, minerals grouped anionically; mineral groups; and minerals which were discovered first in Scotland. Some 15 per cent of the species listed in this glossary have not been recorded previously for the United Kingdom.