Eugene E. Foord

Mineralogic Evidence for an Impact Event at the Cretaceous-Tertiary Boundary

A thin claystone layer found in nonmarine rocks at the palynological Cretaceous-Tertiary boundary in eastern Montana contains an anomalously high value of iridium. The nonclay fraction is mostly quartz with minor feldspar, and some of these grains display planar features. These planar features are related to specific crystallographic directions in the quartz lattice. The shocked quartz grains also exhibit asterism and have lowered refractive indices. All these mineralogical features are characteristic of shock metamorphism and are compelling evidence that the shocked grains are the product of a high velocity impact between a large extraterrestrial body and the earth. The shocked minerals represent silicic target material injected into the stratosphere by the impact of the projectile.

Magnesioferrite from the Cretaceous-Tertiary boundary, Caravaca, Spain

Abstract Magnesioferrite grading toward magnetite has been identified as a very small but meaningful constituent of the basal iron-rich portion of the Cretaceous-Tertiary (K-T) boundary clay at the Barranco del Gredero section, Caravaca, Spain. This spinel-type phase and others of the spinel group, found in K-T boundary clays at many widely separated sites, have been proposed as representing unaltered remnants of ejecta deposited from an earth-girdling dust cloud formed from the impact of an asteroid or other large bolide at the end of the Cretaceous period. The magnesioferrite occurs as euhedral, frequently skeletal, micron-sized octahedral crystals. The magnesioferrite contains29 ± 11 ppb Ir, which accounts for only part of the Ir anomaly at this K-T boundary layer(52 ± 1 ppb Ir). Major element analyses of the magnesioferrite show variable compositions. Some minor solid solution exists toward hercynite-spinel and chromite-magnesiochromite. A trevorite-nichromite (NiFe 2 O 4 -NiCr 2 O 4 ) component is also present. The analyses are very similar to those reported for sites at Furlo and Petriccio, Umbria, Italy. On the basis of the morphology and general composition of the magnesioferrite grains, rapid crystallization at high temperature is indicated, most likely directly from a vapor phase and in an environment of moderate oxygen fugacity. Elemental similarity with metallic alloy injected into rocks beneath two known impact craters suggests that part of the magnesioferrite may be derived from the vaporized chondritic bolide itself, or from the mantle; there is no supporting evidence for its derivation from crustal target rocks.

Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak batholith (central Colorado)

Abstract The crystal structure and M-site populations of a series of micas-1M from miarolitic pegmatites that formed within host granitic rocks of the Precambrian, anorogenic Pikes Peak batholith, central Colorado, were determined by single-crystal X-ray diffraction data. Crystals fall in the polylithionitesiderophyllite- annite field, being 0 ≤ Li ≤ 2.82, 0.90 ≤ Fetotal ≤ 5.00, 0.26 ≤ [6]Al ≤ 2.23 apfu. Ordering of trivalent cations (mainly Al3+) is revealed in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C12/m(1) (siderophyllite and annite crystals) to C12(1) diperiodic group (lithian siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length, the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M3 ≠ M2, M2 = M3 ≠ M1 and M1 ≠ M2 ≠ M3. As the siderophyllite content increases, so do the a, b, and c unit-cell parameters, as well as the refractive indices, primarily nβ. The tetrahedral rotation angle, α, is generally small (1.51 ≤ α ≤ 5.04°) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Δz, is sensitive both to octahedral composition and degree of order (0.0 ≤ Δz ≤ 0.009 Å for siderophyllite and annite, 0.058 ≤ Δz ≤ 0.144 Å for lithian siderophyllite and ferroan polylithionite crystals).

Proposed nomenclature for samarskite-group minerals: New data on ishikawaite and calciosamarskite

Abstract The current definition of samarskite-group minerals suggests that ishikawaite is a uranium rich variety of samarskite whereas calciosamarskite is a calcium rich variety of samarskite. Because these minerals are chemically complex, usually completely metamict, and pervasively altered, their crystal chemistry and structure are poorly understood. Warner and Ewing (1993) proposed that samarskite is an A3+B5+O4 mineral with an atomic arrangement related to α-PbO2. X-ray diffraction analyses of the recrystallized type specimen of ishikawaite and the Ca-rich samarskite reveal that they have the same structure as samarskite-(Y) recrystallized at high temperatures. Electron microprobe analyses show that the only significant difference between samarskite-(Y), ishikawaite, and calciosamarskite lies in the occupancy of the A-site. The A-site of samarskite-(Y) is dominated by Y+REE whereas the A-site of ishikawaite is dominantly U+Th and calciosamarskite is dominantly Ca. Additionally, a comparison of these data to those of Warner and Ewing (1993) show that in several cases Fe2+ or Fe3+ are dominant in the A-site. We propose that the name samarskite-(REE+Y) should be used when one of these elements is dominant and that the mineral be named with the most abundant of these elements as a suffix. The name ishikawaite should be used only when U+Th are dominant and the name calciosamarskite should only be used when Ca is the dominant cation at the A-site. Finally, because of the inability to quantify the valence state of iron in these minerals, the exact nature of the valence state of iron in these minerals could not be determined in this study.

Meurigite, a new fibrous iron phosphate resembling kidwellite

Abstract Meurigite is a new hydrated potassium iron phosphate related to kidwellite and with structural similarities to other late-stage fibrous ferric phosphate species. It has been found at four localities so far - the Santa Rita mine, New Mexico, U.S.A.; the Hagendorf-Sud pegmatite in Bavaria, Germany; granite pegmatite veins at Wycheproof, Victoria, Australia; and at the Gold Quarry Mine, Nevada, U.S.A. The Santa Rita mine is the designated type locality. Meurigite occurs as tabular, elongated crystals forming spherical and hemispherical clusters and drusy coatings. The colour ranges from creamy white to pale yellow and yellowish brown. At the type locality, the hemispheres may reach 2 mm across, hut the maximum diameter reached in the other occurrences is usually less than 0.5 ram. A wide variety of secondary phosphate minerals accompanies meurigite at each locality, with dufrenite, cyrilovite, beraunite, rockbridgeite and leucophosphite amongst the most common. Vanadates and uranates occur with meurigite at the Gold Quarry mine. Electron microprobe analysis and separate determination of H2O and CO2 on meurigite from the type locality gave a composition for which several empirical formulae could be calculated. The preferred formula, obtained on the basis of 35 oxygen atoms, is (K0.85Na0.03)∑0.88(Fe7.013+Al0.16Cu0.02)∑7.19 (PO4)5.11(CO3)0.20(OH)6.7·7.25H2O, which simplifies to KFe73+(PO4)5(OH)7·8H2O. Qualitative analyses only were obtained for meurigite from the other localities, due to the softness and openness of the aggregates. Because of the fibrous nature of meurigite, it was not possible to determine the crystal structure, hence the exact stoichiometry remains uncertain. The lustre of meurigite varies from vitreous to waxy for the Santa Rita mine mineral, to silky for the more open sprays and internal surfaces elsewhere. The streak is very pale yellow to cream and the estimated Mohs hardness is about 3. Cleavage is perfect on {001 } and fragments from the type material have a mean specific gravity of 2.96. The strongest lines in the X-ray powder pattern for the type material are (dobs,lobs,hkl) 3.216(100)404; 4.84(90)111; 3.116(80)205; 4.32(70)112; 9.41(60)201; 3.470(60)800. The X-ray data were indexed on the basis of a monoclinic unit cell determined from electron diffraction patterns. The cell parameters, refined by least squares methods, are a = 29.52(4), b = 5.249(6), c = 18.26(1) Å, β= 109.27(7) °V = 2672(3) Å3, and Z = 4. The calculated density is 2.89 gcm x−3. The space group is either C2, Cm or C2/m. X-ray powder data for meurigite are closely similar to those for kidwellite and phosphofibrite, but meurigite appears to be characterised by a strong 14 Å reflection. The relationship between these three minerals remains uncertain in the absence of structural data. On the available evidence, meurigite and kidwellite are not the respective K and Na-endmembers of a solid solution series. The meurigite cell parameters suggest it belongs to a structural family of fibrous ferric phosphates, such as rockbridgeite, dufrenite and beraunite, which have a discrete 5 Å fibre axis. Meurigite occurs in widely varying environments, its formation probably favoured by late-stage solutions rich in K rather than Na.

The chemistry, mineralogy, and petrology of the George Ashley Block pegmatite body

and career of Eugene E. Foord. Gene was a mineral enthusiast like few this profession has ever seen. He loved everything about minerals—their occurrence, their chemistry and structure, and their aesthetics. He will be remembered especially for sharing his enthusiasm with so many of us. For university researcher and amateur collector alike, Gene was a source of samples, assistance, and abundant anecdotal humor. In this issue, a few of the many individuals who benefited from Gene’s acquaintance or publications pay their respects to this very gifted and personable Fellow of the Society. Gene’s knowledge of minerals was expansive (e.g., Gaines et al. 1997), but he focused his attention on the mineralogy of granitic pegmatites, particularly of the Nb-Ta oxides, micas, and feldspars. Gene’s mineralogical investigation for his Ph.D. dissertation on the Himalaya dike, Mesa Grande district, San Diego County, still represents one of the most comprehensive mineral-chemical studies of a single pegmatite body (Foord 1976). His work on the Himalaya dike included mineral structure and ordering phenomena, morphological variation, and the nature of chemical zonation and fractionation within individual phases, for example the evolution of garnet compositions across the pegmatite-aplite (also see Kleck and Foord, this issue). Gene’s knowledge of pegmatites was truly first hand: he assisted many of the gem specimen miners in the San Diego County districts with the back-breaking labor of hardrock mining. All of the manuscripts for this issue follow the pegmatite theme in various ways. Some of Gene’s close colleagues have completed a manuscript (Foord et al.) that reports the new mineral, simmonsite. It is named for Gene’s close friend, William B. “Skip” Simmons. Simmonsite, which is related chemically and structurally to cryolite, comes from the Zapot pegmatite, an amazonite-bearing NYF (Nb-Y-F) type ( Černý 1991) located in Nevada. This class of pegmatites greatly interested Gene, and he is well known for his studies of similar amazonite-bearing pegmatites in the Pikes Peak batholith, Colorado (Foord et al. 1995; Foord and Martin 1979; Kile and Foord 1998). Six other papers in this issue pertain to the NYF type of granites and pegmatites. In a manuscript by Gene’s USGScolleagues, Kyle and Eberl use the crystal-size distributions of amazonite from miarolitic cavities in pegmatites of the Pikes Peak Batholith to deduce the principal mechanisms of crystal nucleation and growth. Smerekanicz and Dudas have investigated the compositions of fluid inclusions in the amazonitebearing Morefield pegmatite (Amelia), Virginia, to map out the history of fluid evolution within and mixing between pegmatite and host rocks. A comprehensive mineralogical paper by Pezzotta et al. scrutinizes the crystal chemistry of gadolinite from NYF-type granites and pegmatites of the famous Baveno and Cuasso al Monte regions of northern Italy. Three papers present new data—one a new mineral species—for the complex minerals of Nb, Ta, and Ti. Galliski et al. describe the new species ferrotitanowodginite, one of the growing family of Nb-Ta-Ti-Sn oxides that formed the nexus of Gene’s mineralogical investigations. Černý et al. further define the complex relations of niobian rutile with analyses of this and related phases from the McGuire (NYF) pegmatite in Colorado. Cooper et al. have defined the crystal chemistry and structure of sogdianite, a complex Li-Zr-silicate originally described from the alkaline granites of Dara-i-Pioz, Tajikistan. The crystal chemistry of pegmatitic micas was another of Gene’s primary interests (e.g., Foord et al. 1991, 1995; Kile and Foord 1998). In this issue, Hawthorne et al. have refined the crystal structure of a RbCs-phlogopite from the exocontact of the Red Cross Lake pegmatite, a chemically evolved member of the LCT (Li-Cs-Ta) class of deposits ( Černý 1991). Enrichment in beryllium is common to both the NYF and the LCT pegmatite types. That enrichment is typically expressed by the presence of beryl. Evensen et al. have examined experimentally the solubility of beryl in metaluminous to peraluminous hydrous melts as functions of temperature and the activities of Al and Si components in melt. Their results show a sharply retrograde solubility for beryl with decreasing temperature. This behavior helps to explain why beryl is a common phase in pegmatites, even at very low whole-rock Be values, if pegmatites arise from rapid cooling in relation to the rates of crystal nucleation and growth. The crucial question of relating crystal habits to environment of growth is the subject of a striking new model presented by Baker and Freda. Their simulation of crystal growth using the Ising model produces habits and crystal sequences that are consistent with natural pegmatitic fabrics, albeit at radically different scales. Two papers provide important new contributions to the geology of pegmatites in San Diego Country, California—Gene Foord’s favorite stomping ground. In Kleck and Foord, a longtime friend and colleague of Gene’s has completed their study of the mineral chemistry of garnet and other indicator minerals through a composite layered aplite-pegmatite dike known as the George Ashley Block (Pala district). Webber et al. (including Foord) present a complementary investigation of the cooling history and crystallization dynamics in this and sevPreface to the Gene Foord issue

Ruby and sapphire from Jegdalek, Afghanistan

GEMS & GEMOLOGY Summer 2000 he gem mines of Afghanistan are some of the oldest in the world. The lapis lazuli mines at Sar-eSang, in the Badakhshan region, have been worked for at least 6,500 years (see, e.g., Wyart et al., 1981). Today, Afghanistan continues to be an important source of various gem minerals—including emerald, ruby, sapphire, aquamarine, tourmaline, and spodumene (see, e.g., Bowersox and Chamberlin, 1995). Yet relatively little is known about many of the gem localities. This article reports on the only known source of ruby in Afghanistan: the Jegdalek region. A historical review, the geology, mining methods, and current production of gem corundum (figure 1) from Jegdalek are given below, together with the results of our research on the gemological properties of this material.

Reassessment of the volkonskoite-chromian smectite nomenclature problem.

The name volkonskoite was first used in 1830 to describe a bright blue-green, chromiumbearing clay material from the Okhansk region, west of the Ural Mountains, U.S.S.R. Since that time, the name has been applied to numerous members of the smectite group of clay minerals, although the reported chromium content has ranged from 1% to about 30% Cr2O3. The name has also been applied to some chromian chlorites. Because volkonskoite has been used for materials that differ not only in their chromium content but also in their basic structure, the species status of the mineral has been unclear.To resolve this uncertainty, two specimens of volkonskoite from (1) Mount Efimiatsk, the type locality in the Soviet Union (USNM 16308) and (2) the Okhansk region in the Perm Basin, U.S.S.R. (USNM R4820), were examined by several mineralogical techniques. Neotype sample 16308 has the following structural formula: (Ca0.11Mg0.11Fe2+0.03K0.02)(Cr1.18Mg0.78Fe3+0.29Ca0.02)(Si3.50Al0.51)O10(OH)2 · 3.64H2O. Sample R4820 has the following structural formula: (Ca0.25Mg0.05Fe2+0.01K0.03Mn0.01)(Cr1.07Mg0.75Fe3+0.35(Si3.59Al0.43)O10(OH)2 · 4.22H2O. Mössbauer spectroscopy indicates that 91% and 98% of the iron is present as Fe3+ in samples 16308 and R4820, respectively. X-ray powder diffraction patterns of both samples have broad lines corresponding to minerals of the smectite group.On the basis of these data, volkonskoite appears to be a dioctahedral member of the smectite group that contains chromium as the dominant cation in the octahedral layer. Smectites containing less than this amount of octahedral chromium should not be called volkonskoite, but should be named by chemical element adjectives, e.g., chromian montmorillonite, chromian nontronite.

Geochemical characteristics and KAr ages of rare-metal bearing pegmatites from the Birimian of southeastern Ghana

Abstract The pegmatite-aplite rocks at Mankwadzi (Ejisimanku Hills) in southeastern Ghana are part of the pegmatite district that extends from Cape Coast to Winneba along the Atlantic coastline. The pegmatites are associated with the Cape Coast granite complex and were intruded during the waning phase of the Eburnian Orogeny (∼2.0 Ga). Three muscovite separates from pegmatite give KAr retention ages of 1909 ± 13 Ma , 1965 ± 13 Ma and 2019 ± 14 Ma. A biotite separate from granite yields a KAr age of 1907 ± 13 Ma. These ages are similar to KAr dates previously reported for the Cape Coast granites, indicating that the granites and pegmatites are coeval and probably genetically linked. The pegmatites are enriched in Li, Be, Nb and Sn and considerably impoverished in Rb, Th, Y and REEs. Microscopic examination of quartz from the pegmatites shows a large number of low salinity fluid inclusions that can be divided into two types: (1) one-phase liquid or gas-filled inclusions; and (2) two-phase liquid-vapour inclusions, with the vapour occupying 2–5% of the volume. The homogenisation temperature of the fluid inclusions clusters between 129 and 144°C. These homogenisation temperatures lead to an inferred entrapment temperature of ∼300°C at a pressure of ∼2.5 kbar, which is estimated for the metamorphism of host hornblende schists. The pegmatite fluid inclusions are interpreted as being secondary to the quartz hosts.

Phosphates in some Missouri refractory clays

This paper describes in detail phosphate minerals occurring in refractory clays of Missouri and their effect on the refractory degree of the clays. The minerals identified include carbonate-fluorapatite (francolite), crandallite, goyazite, wavellite, variscite and strengite. It is emphasized that these phosphates occur only in local isolated concentrations, and not generally in Missouri refractory clays.The Missouri fireclay region comprises 2 districts, northern and southern, separated by the Missouri River. In this region, clay constitutes a major part of the Lower Pennsylvanian Cheltenham Formation. The original Cheltenham mud was an argillic residue derived from leaching and dissolution of pre-Pennsylvanian carbonates. The mud accumulated on a karstic erosion surface truncating the pre-Cheltenham rocks. Fireclays of the northern district consist mainly of poorly ordered kaolinite, with variable but minor amounts of illite, chlorite and fine-grained detrital quartz. Clays of the southern district were subjected to extreme leaching that produced well-ordered kaolinite flint clays. Local desilication formed pockets of diaspore, or more commonly, kaolinite, with oolite-like nubs or burls of diaspore (“burley” clay).The phosphate-bearing materials have been studied by X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectral analysis (SEM-EDS) and chemical analysis. Calcian goyazite was identified in a sample of diaspore, and francolite in a sample of flint clay. A veinlet of wavellite occurs in flint clay at one locality, and a veinlet of variscite-strengite at another locality.The Missouri flint-clay-hosted francolite could not have formed in the same manner as marine francolite. The evidence suggests that the Cheltenham francolite precipitated from ion complexes in pore water, nearly simultaneously with crystallization of kaolinite flint clay from an alumina-silica gel. Calcian goyazite is an early diagenetic addition to its diaspore host. The wavellite and variscite-strengite veinlets are secondary, precipitated from ion complexes in ground water percolating along cracks in the flint clay. The flint clay host of the variscite-strengite veinlet contains strontian crandallite. All of the phosphates contain significant amounts of strontium. The source of P, Ca and Sr was the marine carbonates. Dissolution of these carbonates produced the argillic residue that became the primordial Cheltenham paludal mud, which ultimately altered to fireclay.Preliminary firing tests show that the presence of phosphates lowers fusion temperature. However, it is not clear whether poor refractoriness is due to the presence of phosphates, per se, or to Ca, Sr and other alkaline elements present in the phosphates.