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.

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