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Pseudomorphs of low microcline after adularia fourlings from the Alto da Cabeça (Boqueirão) and Morro Redondo pegmatites, Brazil (CROSBI ID 187358)

Prilog u časopisu | izvorni znanstveni rad | međunarodna recenzija

Bermanec, Vladimir ; Horvat, Marija ; Žigovečki Gobac, Željka ; Zebec, Vladimir ; Scholz, Ricardo ; Škoda, Radek ; Wegner, Reinhard ; de Brito Barreto, Sandra ; Dódony, István Pseudomorphs of low microcline after adularia fourlings from the Alto da Cabeça (Boqueirão) and Morro Redondo pegmatites, Brazil // Canadian mineralogist, 50 (2012), 4; 975-987. doi: 10.3749/canmin.50.4.975

Podaci o odgovornosti

Bermanec, Vladimir ; Horvat, Marija ; Žigovečki Gobac, Željka ; Zebec, Vladimir ; Scholz, Ricardo ; Škoda, Radek ; Wegner, Reinhard ; de Brito Barreto, Sandra ; Dódony, István

engleski

Pseudomorphs of low microcline after adularia fourlings from the Alto da Cabeça (Boqueirão) and Morro Redondo pegmatites, Brazil

Potassium feldspar crystals from the Alto da Cabeça (Boqueirão), Rio Grande do Norte (sample BQ) and the Morro Redondo. Minas Gerais (sample MR) pegmatites in Brazil have a typical adularia fourling habit consisting of a combination of Baveno and Manebach twins. Crystal morphology suggests a monoclinic symmetry, but XRPD measurements and TEM experiments show that the samples are low microcline. Calculated values of triclinicity [Δ = 0.915 (BQ) and Δ = 0.9225 (MR)], cell parameters [a 8.587(4), b 12.969(4), c 7.223(2) Å, α 90.66(3) β 115.97(4), γ 87.69(3)° for BQ and a 8.584(3), b 12.970(3), c 7.225(2) Å, α 90.66(3) β 115.97(3), γ 87.74(3)° for MR), t1o occupancy of Al [0.99 (BQ) and 0.98 (MR)], order index [ΔSM= 0.97 (BQ) and ΔSM = 0.99 (MR)] and ΣAl at the T1 sites [98.6% (BQ) and 99.5% (MR)] reveal a triclinic symmetry and a high state of order for the samples of potassium feldspar. Transmission electron microscopy and selected-area electron-diffraction investigations of the crystals show that they are compositionally homogeneous, without exsolution lamellae of other phases. The electron-diffraction patterns show no splitting of any reflection and no streaking, and thus indicate ideal order in both samples. According to crystal morphology and the results on submicroscopic scale, we conclude that in this case adularia samples have a completely ordered structure. The measurements and calculations demonstrate that they are highly ordered, i.e., maximum microcline. Adularia pseudomorphs are highly ordered triclinic feldspar developed through transformation of a monoclinic parent.

low microcline; adularia; pseudomorph; granitic pegmatite; Brazil

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Podaci o izdanju

50 (4)

2012.

975-987

objavljeno

0008-4476

10.3749/canmin.50.4.975

Povezanost rada

Geologija

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