Naslov
Two generations of microcline from Mount Malosa pegmatite, Zomba District, Malawi

Autori
Bermanec, Vladimir ; Horvat, Marija ; Žigovečki Gobac, Željka ; Zebec, Vladimir

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Contributions to the 6th International Symposium on Granitic Pegmatites / Simmons, William B. ; Webber, Karen L. ; Falster, Alexander U. ; Roda-Robles, Encarnación ; Márquez-Zavalía, María Florencia ; Galliski, Miguel Ángel - New Orleans (LA) : Rubellite Press, 2013, 20-21

Skup
6th International Symposium on Granitic Pegmatites

Mjesto i datum
Bartlett (NH), Sjedinjene Američke Države, 26.05.2013. - 31.05.2013

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
microcline; structural ordering; Mount Malosa pegmatite

Sažetak
The Zomba-Malosa pluton is composed of syenite and quartz-syenite, with an inner ring of quartz-microsyenite, and an outer ring of peralkaline granite (Bloomfield, 1965) associated with miarolitic, granitic and alkaline pegmatites (Guastoni et al., 2003). Structural ordering in twinned and untwinned microcline crystals from Mount Malosa pegmatites was investigated. The microcline examined is a 10 cm white crystal that exhibits a Baveno twin with the surface of some crystal faces covered with late-stage, transparent colorless feldspar crystals. A black aegirine crystal, a common mineral in the alkali pegmatites of Mount Malosa, is also present. Thin sections that contained both observable feldspar phases were cut from the sample and analyzed with polarizing microscope. Both the early-white and late-colorless phases were analyzed with Philips X’pert Pro powder diffractometer (CuKα radiation filtered with a graphite monocrystal monochromator, running at 40 kV and 40 mA). XRPD data were collected from 4 to 65 °2θ, and identified and indexed using the X'Pert program Highscore Plus v. 2.1 software (PANalytical, 2004) and ICDD Powder Diffraction File 2 database (2004). Unit cell parameters were calculated using the program “UnitCell” (Holland & Redfern, 1997). Triclinicity (Δ) was calculated using the formula of Goldschmidt & Laves (1954) and the ordering index and the ΣAl at the T1 sites, according to Neves & Godinho (1995). Polarizing microscope observations confirm two generations of microcline growth, the early phase exhibits a characteristic microscopic tartan twin pattern where the later crystals occur single crystals do not. XRPD analyses show that in addition to microcline, minor quartz is also present. Both the early-white and late-colorless microcline show split {;131}; potassium feldspar reflections reflecting their triclinic symmetry. Calculated values of triclinicity (Δ = 0.981 (late/transparent/colorless) and Δ = 0.968 (basic crystal body/white) and lattice parameters (a = 8.583(1) Å, b = 12.974(2) Å and c = 7.224(1) Å, with α = 90.62(2)°, β = 115.93(1)°, γ = 87.67(1)° and a = 8.581(1) Å, b = 12.969(2) Å and c = 7.225(1) Å, with α = 90.67(2)°, β = 115.94(1)°, γ = 87.60(1)° respectively, are characteristic for low microcline. The ordering index and the ΣAl at the T1 sites, calculated according to Neves & Godinho (1995), for both investigated zones is 100% confirming maximal ordering, i.e. low microcline (Kroll & Ribbe 1987). The degree of ordering in the transition zone was not determined. The investigated microcline sample from the Mount Malosa pegmatite contains two generations of low microcline: an early twinned crystal that is coated on some faces with late-stage crystals. Microcline crystals of both generations revealed maximum triclinicity and a high ordering state for potassium feldspars. The transition zone was not investigated due to an inability to separate it. The inner part of the crystal (early phase) likely crystallized as monoclinic feldspar and subsequently inverted to triclinic symmetry. The second generation of microcline (late phase) crystallized directly as maximally triclinic microcline. Further research of transition zone and its interface to both investigated microcline generations is necessary.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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