Geology topics

W.B. Simmons

Publications and source records attributed to W.B. Simmons.

3 recordsLinked to original sources

USGS2018

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

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 A 3+ B 5+ O 4 mineral with an atomic arrangement related to α-PbO 2 . 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 Fe 2+ or Fe 3+ 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.

Mineralogical Magazine↗

USGS1997

The role of diffusion-controlled oscillatory nucleation in the formation of line rock in pegmatite-aplite dikes

The George Ashley Block (GAB), located in the Pala Pegmatite District, San Diego County, California, is a composite pegmatite–aplite dike of 8 m thickness displaying striking mineralogical layering in the aplite portion of the dike, referred to as line rock. Rhythmic layering is characterized by garnet-rich bands alternating with albite–quartz–muscovite-rich bands. Cumulus textures are notably absent from the layered portion of the dike. Elongated quartz megacrysts are oriented perpendicular to the garnet-rich layers and poikilitically include garnet, albite, and muscovite. Calculated crystal-free magma viscosity with 3% H 2 O is 10 6.2 Pa s and the calculated settling velocity for garnet is 0.51 cm/year. Conductive cooling calculations based on emplacement of a 650°C dike into 150°C fractured gabbroic country rock at 1.5 kbar, and accounting for latent heat of crystallization, demonstrate that the line rock portion of the dike cools to 550°C in about 1 year. Crystal size distribution studies also suggest very rapid nucleation and crystallization. Diffusion-controlled gel crystallization experiments yield textures virtually identical to those observed in the layered aplite, including rhythmic banding, colloform layering, and band discontinuities. Thus, observed textures and calculated magmatic parameters suggest that mineralogical layering in the GAB results from an in situ diffusion-controlled process of oscillatory nucleation and crystallization. We propose that any event that promotes strong undercooling has the potential to initiate rapid heterogeneous nucleation and oscillatory crystal growth, leading to the development of a layer of excluded components in front of the crystallization front, and the formation of line rock.

Journal of Petrology↗

USGS1986

Minasgeraisite, a new member of the gadolinite group from Minas Gerais, Brazil.

Minasgeraisite, Y2CaBe2Si2O10, a 9.833(2), b 7.562(1), c 4.702(1) A, beta 90.46(6)o, P21/a, Z = 2, is a new member of the gadolinite group. Found in quantities of only several hundred mg in the Jaguaracu pegmatite, Minas Gerais, Brazil, it occurs as 0.2 to 1.0 mm-diameter rosettes, coating and intergrown with milarite, albite, quartz and muscovite. The mineral is lavender to lilac purple and has a sheaf-like habit, with a grain size of <3-5 mu m. It is not magnetic and not fluorescent under either short-wave or long-wave ultraviolet radiations; H. approx 6-7, D(meas.) >4.25, D(calc.) 4.90 g/cm3, lustre is earthy to subvitreous and the streak is pale purple. Minasgeraisite is slowly soluble in common acids. The mineral has one excellent cleavage, (100) by analogy with gadolinite, and another good cleavage (001). Minasgeraisite is biaxial positive with alpha (colourless) 1.740(4), beta (pale greyish yellow) 1.754(4), gamma (lavender purple) 1.786(4), 2Vgamma 68o. Strongest powder XRD lines include 3.11(100), 2.830(100), 2.540(90), 1.768(35) A.-J.A.Z.

American Mineralogist↗