Alkali feldspars in plutonic igneous rocks vary both in their exsolution textures and in the structural state of their components. The primary factor leading to this diversity is the availability of hydrothermal fluids during their cooling history. In many plutons feldspar variation is related to degree of fractionation as indicated by rock chemistry. The variation reflects build-up of water with magmatic evolution and implies that fluids did not circulate freely in the intrusives in the temperature range of unmixing and ordering. Experimental work bearing on the role of fluids in subsolidus changes in feldspars is reviewed, and points of overlap between crystallographic studies of feldspars and stable isotope studies are discussed.

The Helmholtz dispersion equations have been rewritten in a form that enables the optical constants of both transparent and opaque media to be calculated from their spectra. Both Helmholtz equations are used to describe the optical properties of opaque media, and to obtain values of reflectance, refractive index, and absorption coefficient. The Sellmeier dispersion equation is a special case of the dispersive Helmholtz equation applicable to weakly absorbing media (including the great majority of minerals studied in thin section): it is used to derive the wavelength-, composition-, direction-, and volume-dependence of the principal indices in mixed crystals of monoclinic or higher symmetry. The treatment can be extended to triclinic crystals.

The birefringence of transparent phases is the expression in the visible region of the pleochroism of absorption bands in the ultra-violet. The bireflectance of opaque phases depends also upon the pleochroism of bands in the visible and near infra-red, resulting in extreme sensitivity of the optics of opaque materials to changes in wavelength, composition, and structure. The optical anisotropy of both transparent and opaque phases may be calculated if the dependence of the spectra on structure can be established by measurement or by calculation from structure data. The quantitative application of Bragg's method is restricted to phases of very simple chemistry and structure (e.g. calcite and rutile), but it may be applied qualitatively to rationalize the optic orientations of phases containing only closed-shell ions of neon or argon configuration, including many pyroxenes, amphiboles, micas, and chain aluminosilicates. When open-shell (transition metal) ions enter the structure, the general rule is that the anisotropy becomes more closely related to the distortion of the coordination polyhedron about the metal ion, and for ions of formal charge ≥ 3, this source of anisotropy is usually dominant.

Emeleusite (Li2Na4Fe2IIISi12O30) is an orthorhombic (pseudohexagonal) mineral occurring as euhedra within a facies of a peralkaline trachyte dyke. The composition is: SiO2 70·75; TiO2 0·55; ZrO2 0·10; Al2O3 1·34; Fe2O3 12·13; MgO 0·10; MnO 0·03; Na2O 11·98; Li2O 2·78:99·76. The density is 2·775 g/cm3 (calculated) and 2·76(7) g/cm3 (determined); hardness 5–6. It is colourless, transparent, and has a glassy lustre. Emeleusite is biaxial negative with 2Va, varying from near-zero to c. 30° r » v. The refractive indices are α 1·596; β 1·597; y 1·597; with α ∥ b; β ∥ a; y ∥ c. Emeleusite can occur as interpenetration triplets with {110} as twin planes. The space group is Acam or Aba2 with a = 10·073±0·002 Å; b = 17·350±0·005 Å and c = 14·010±0·005 Å. Z = 4. Prominent powder diffraction lines are 7·001(5) 002, 4·352(10) 220, 040, 3·501(7) 004, 3·209(8) 311, and 3·090(7) 124. While the structure is as yet unknown it is suggested that it is an orthorhombic associate of the milarite group.

Bazirite, BaZrSi3O9, is one of several late-stage interstitial minerals present in the aegirine-riebeckite granite of Rockall Island. Crystals are hexagonal, probably ¯6m2, colourless, chiefly as anhedral grains. Only forms noted {10¯10} and probably {10¯14}, cleavage {0001} and probably {10¯14}. ‘Low’ and ‘high’ titanium varieties with respective mean compositions: SiO2 39·71%, ZrO2 26·38%, BaO 33·69%, TiO2 0·17%, SnO2 0·06% and SiO2 39·59%, ZrO2 26·12%, BaO 34·12%, TiO2 0·51%, SnO2 0·11%. Low and high refractive index types exist; uniaxial positive with ω = 1·6751±0·0003. ɛ = 1·6850±0·0003 and ω = 1·6813±0·0003, ɛ = 1·691 (deduced); probably due to differences in titanium content. Fluorescence (2537 Å), moderately strong, pale whitish blue. Space group almost certainly P¯6c2. Cell dimensions: low R.I. variety, a = 6·7690±0·0005 Å, c = 10·020±0·001 Å; high R.I. variety, a = 6·7662± 0·0005 Å, c = 10·158±0·0010 Å. Strongest lines are 3·80 Å (100), 2·800 (100), 5·85 (35), 3·38 (35), Z = 2. Dcalc = 3·82. Related minerals, benitoite and pabstite.

The deformation of biotite single crystals submitted to experimental shock-pressures of about 200 and 400 kb with direction of the shock wave parallel to [001] was investigated by X-ray precession and Weissenberg techniques. The X-ray analysis reveals a ‘fracturing’ of the biotite crystals upon pressure release into relatively large lattice blocks (> 1000 Å), which increases with the degree of shock compression. Deformation is inferred to take place by gliding and/or microfracturing processes parallel to the (010) and (110) lattice planes. Chains of tetrahedra running parallel to [100] and [110] probably exert a controlling influence on shock deformation mechanisms.

Metasomatic zones formed at the borders of various mafic inclusions in the Wairere Serpentinite, central North Island, New Zealand, show eucritic and hornblende gabbros grading through distinct zones of altered gabbro (hydrogarnet-free) to coarse and finegrained rodingites (hydrogarnet-bearing). The hydrated, calcium-rich, aluminium-silicate mineral assemblages of each reaction zone change between the zones, with one mineral phase being altered to another at each replacement front. Initial alteration of the gabbros albitized, prehnitized, and epidotized plagioclase and chloritized hornblende and clinopyroxene. Continuing metasomatism garnetized prehnite and generated secondary amphibole and pyroxene while the original host harzburgite was highly serpentinized and, adjacent to the inclusions, the serpentinite was chloritized so as to form a further distinct zone between peridotite and gabbro. These mineral replacement reactions are reflected in the bulk chemistry of the reaction zones, which show that increasing alteration of the gabbro is accompanied by a marked increase in Ca and H2O, a possible small gain in Mg, and depletion in Si, Al, and alkalis. These trends are balanced in the host peridotite by Si enrichment and Ca loss upon serpentinization. Clinopyroxenes and hornblendes of the zones show an increase in Ca and Mg and a decrease in Si with increasing metasomatic alteration. Garnets also decrease in Si but chlorites show an increase in both Si and Mg. Additional minerals analysed from both primary and altered rocks include olivine, orthopyroxene, biotite, plagioclase, pectolite, rosenhahnite, sphene, and chrome spinel. The zones are regarded as the result of reaction between the inclusions and Ca- and Al-rich solutions derived, in turn, by action of alkaline, CO2-poor solutions, which percolated through fractures in the original host peridotite and, in the process of serpentinization, released Mg and enhanced Ca and Al levels. At the contact of the mafic inclusions, chemical gradients were generated down which various chemical species diffused to different levels forming the zonation. The reactions probably took place in the vicinity of 260 to 350 °C, 2 to 3 kb, and at constant volume. Cataclasis and brecciation of the outer zones indicate more than one metasomatic episode.

Plots of partition coefficients (mineral/ matrix) against ionic radius of the alkali and alkaline earth elements in forty-four alkali feldspars from rhyolites and trachytes show two kinds of diagram, one for homogeneous alkali feldspars with a single peak, and the other for cryptoperthites with a wide spread of points, which cannot represent a single peak, but may probably be explicable in terms of two peaks. The difference in behaviour does not seem to be due to the chemical composition, but is apparently related to the crystallo-graphic features of the alkali feldspar structures. In particular the presence of two peaks in the diagrams of the cryptoperthites seems to be due to the existence of lattice positions suited to receive bigger ions (K, Ba, Rb) and smaller ions (Na, Ca, Sr) in the potassic and sodic domains.

The chemistry and petrography of a meta-ultrabasic lens containing peridotite with poikilitic pyroxenes and hornblende, both of probable igneous origin, are given. The body also contains feldspathic-bearing hornblende meta-gabbros and is a tectonic fragment in hornblende-plagioclase rock belonging to the same Errismore intrusion. Chemical analyses fail to discern any appreciable systematic differentiation trend across the body despite modal variation to hornblende plagioclase layers. The high Niggli mg near the structural bottom of the intrusion suggests that this part of the Errismore intrusion is right way up, contrary to a previous suggestion.

An X-ray and chemical examination of deweylites reveals that they are intimate mixtures of very poorly ordered trioctahedral 2:1 and 1:1 layer silicates. The 2:1 mineral exhibits no swelling in the presence of ethylene glycol or water and is best described as an extremely fine-grained and highly disordered form of talc, for which the term kerolite is often used. Stevensite is not a component of deweylites. The 1:1 component most closely resembles a disordered chrysotile.

Evaluation of the chemical data, including values for H2O +, shows that both end-members are hydrated to various degrees. This hydration is most probably attributable to the presence of water molecules and hydroxyls associated with the large surface areas and unbalanced surface bonds. Formulae for the end-members are approximately R3Si4O10(OH)2·0·3 0·7 H2O and R3Si2O3(OH)4·0·3–0·7 H2O where R is principally Mg, but samples with more or less H2O are entirely possible due to variations in crystallinity. No specific formula can be given to deweylites as they are mixtures with variable proportions of the components. The name is useful, however, as a field or ‘box’ term, similar to the use of garnierite and limonite.

The first occurrence of pumpellyite in the Lower Ordovician volcanic rocks of North Pembrokeshire is reported and its form and chemistry are described. The significance of pumpellyite in relation to the grade of metamorphism attained during the Caledonian orogenic episode is discussed.

Manganoan ilvaite forms as an early retrograde metamorphic phase from calcic rhodonite in the polymetamorphosed Pb-Zn sulphide/manganese silicate rocks at Broken Hill and from grossular-andradite in a skarn at Ban Ban. Ilvaite exists as a probable metastable phase into which Mn is apparently strongly partitioned. The manganoan ilvaites analysed contain up to 13·5 % MnO substituting for FeO and this substitution shifts the 020 peak.

Two new analyses of magnesian cordierites (MgO ∼ 12 %) reveal about 1% by weight of H2O +, and one sample contains 15 × 10−5 cm3STP/gm argon, which is twenty times in excess of argon expected for its age. Host granulites contain unexpectedly light oxygen, and three cordierites have δ 18O values of 2·0, 2·8, and 3·1 ‰, respectively.

Five members of the linnaeite series of sulphides linnaeite Co3S4, siegenite (Co, Ni)3S4, carrollite Co2CuS4, violarite Ni2FeS4, and polydymite Ni3S4 occur in nickel-sulphide-mineralized rocks in areas adjacent to the Kambalda and Scotia deposits of the Kalgoorlie district, Western Australia. Linnaeite, siegenite, carrollite, and polydymite occur in trace amounts as disseminated grains and veinlets. Violarite is also found as disseminations but its major occurrence is in the supergene enriched zones of the ore deposits. Aspects of their mineralogy, including mode of occurrence, general mineragraphy, reflectance at 546 nm, 589 nm and 644 nm, microhardness, and electron probe microanalyses are described.

Titaniferous clinohumite occurs as a major constituent in the centres of late magmatic veins in the Tertiary ultramafic, strongly alkaline Gardiner Plateau Complex in East Greenland (69° N. and 32° W.). The complex is briefly described and an electron microprobe analysis of titaniferous clinohumite is presented. This gives the following: 38·44% SiO2, 2·99% TiO2, 4·87% FeO, 51·89% MgO, 1·3% F, H2Ocalc 1·50%, and total 100·44%. Optical data, density, and unit cell dimensions are also presented. Of magmatic occurrences, titaniferous clinohumite has otherwise only been reported in certain kimberlites and carbonatites.

A quantitative X-ray diffraction method has been successfully applied to the determination of analcime in pumice rock samples. A calibration line was constructed from spiked pumice standards (range: 0 to 42%) and the mean relative error of the standards on the calibration line was < 0·6%. The major-element compositions of the samples and standards were available, and this allowed the total mass absorption coefficients to be calculated. The latter were then used to correct the sample and standard intensities for absorption effects resulting from compositional variations. When compositional data already exist, the calculation of the total absorption coefficient provides a rapid and accurate alternative to direct measurement, or to the use of an internal standard.

Six specimens of talc studied by the electron microscope lattice-imaging and diffraction techniques were found to have the triclinic one-layer (1 Tr) structure. Since no evidence of a monoclinic two-layered structure was found in any specimen it is concluded that the 1 Tr structure is the common one for talc, and it may, moreover, be the only one. Extremely fine polysynthetic twinning and single intrinsic stacking faults were found in talc.

The separation of iron-titanium oxide droplets and the formation of olivine, in the Freetown layered gabbro, depleted the magma in iron and enriched the magma in copper. In magma injections in which the sulphur fugacity was sufficiently high, this resulted in the separation of immiscible copper sulphide droplets closely associated with the iron-titanium oxide droplets, and both were subsequently enclosed by olivine. A lower sulphur fugacity in other injections of magma suppressed the separation of the copper, which later appeared as native copper associated with euhedral titanomagnetite, and was enclosed by feldspar. Crystallization of the iron-titanium oxide droplets and subsolidus diffusion has resulted in granular exsolution of ilmenite from the droplets and lamellar exsolution of ilmenite in the euhedral titanomagnetite.

A reinvestigation of uranospathite from the type locality, Redruth, Cornwall, shows that it is an aluminium uranyl phosphate belonging to the torbernite series but more highly hydrated than other members thereof. The composition (HAl)0·5(UO2)2(PO4)2.20H2O was derived by indirect methods and by making use of analysis by the electron microprobe, which showed an Al2O3 content 2·6% in the partially dehydrated state. Unit-cell dimensions: a 7·00, c 30·02 Å, Z = 2. Probable space group P42/n. Strongest lines of the powder pattern: 15·22 (10) 002, 7·60 (10) 004, 4·93 (10) 110, 111, 3·50 (8) 116, 200, 201.

The mineral is unstable and unless preserved at low temperature or in a humid atmosphere converts into a new phase with a loss of 10- 12H2O. This phase (HAl)0.5 (UO2)2(PO4)2.8–10H2O is identical with the mineral sabugalite. Experimental work led to the synthesis of a hydrate with the composition (HAl)o·5(UO2)2(PO4)2. 16H2O, but not to fully hydrated uranospathite. This synthetic compound also dehydrates rapidly under normal conditions, converting into the same phase as does natural uranospathite. Arsenian uranospathite occurs at Menzenschwand in the Southern Black Forest.

Arsenuranospathite proper is a new mineral found at Menzenschwand and also at Wittichen in the Central Black Forest. It forms lathlike or wedge-shaped crystals of orthorhombic symmetry. Forms: {100}, {010}, {110}, {001}. Cleavage {001} perfect, {100} and {010} good. Hardness probably about 2, ρcak2·54 g cm −3. White to pale yellow. Streak white. Fluoresces greenish in ultra-violet light with variable intensity.

Welshite, triclinic, Ca2Mg4Fe3+Sb5+O2 [Si4Be2O18], a = 10·28 Å, b = 10·69 Å, c = 8·83 Å, α = 106·1°, β = 96·3°, γ = 124·8°, P¯1 with pseudo-monoclinic cell am = 9·68(1) Å, bm = 14·77(2) Å, cm = 5·14(1) Å, βm = 101·5(2)°, c-centred, occurs as lustrous subadaman-tine, thick prismatic reddish-black crystals up to 3 mm in greatest dimension associated with roméite, adelite, swedenborgite, berzeliite, manganoan phlogopite, richterite in crystalline dolomite from Làngban, Sweden. Forms include b {010}, a{100}, k{130}, m{110}, j{021}, p{111}, q{121}, v{131}, r{¯111}, s{¯121}, t{¯131}, and v{¯141} (monoclinic setting). Streak pale-brown, hardness 6, specific gravity 3·77, fracture conchoidal, no cleavage, α = 1·81(1), γ = 1·83(1), 2E(obs.) ≈45°, no noticcable pleochroism.

Welshite is a new member of the aenigmatite-rhönite group and obtains from the substitution Sb5+→ Ti4+ and Be2+ → Al3+ in rhönite.

Manganhumite, orthorhombic, a 10·54(1), b 21·45(2), c 4·822(9) Å, space group Pnma, Z = 4, (Mn0·68Mg0·30Fe0·01Ca0·01)7(OH)2(SiO4)3, is the manganese analogue of humite whose type locality is the Brattfors mine, Nordmark, Province of Värmland, Sweden. It is a skarn assemblage mineral and occurs with katoptrite, manganostibite, magnussonite, galaxite, etc., which replace manganosite grains in magnetite-manganosite ore.

The mineral is biaxial (+), α = 1·707(3), β = 1·712(3), 7 = 1·732(3), 2V7(obs.) = 37(4)° r > v perceptible, cleavage {010} perfect, hardness = 4, specific gravity = 3·83(5). The five strongest powder lines are 1·777(10), 2·500(7), 3·371(6), 2·628(6), and 1·525(6).