Na-feldspar: temperature, pressure and the state of order

In feldspars, mean tetrahedral T–O bond lengths (T = Al,Si) are the standard measure of the tetrahedral Al content. However, for a sophisticated assessment of the Al,Si distribution, factors have to be accounted for (1) that cause individual T–O bond lengths to deviate from their tetrahedral means and (2) that cause mean tetrahedral lengths to deviate from values specified by the Al content. We investigated low albite, Na[AlSi3O8], from six X-ray crystal structure refinements available in the literature. The Al,Si distribution of low albite is fully ordered so that Al,Si–O bond length variations result only from bond perturbing factors. For the intra-tetrahedral variation ΔT–O≡T–O-〈T–O〉, only two factors turned out to be effective: (1) the sum of bond critical point electron densities in the Na–O and T–O bonds neighbouring the T–O bond under consideration and (2) the fractional s-bond character of the bridging oxygen atom. This model resulted in a root mean square (rms) value for ΔT–O of only 0.002 Å, comparable to the estimated standard deviations (esd's) routinely quoted in X-ray and neutron structure refinements. In the second step, the inter-tetrahedral differences Δ〈T–O〉≡〈T–O〉-〈〈T–O〉〉 were considered. Here, apart from the tetrahedral Al content, the only size-perturbing factor is the difference between the tetrahedral and the grand mean fractional s-characters. The resulting rms value was as small as 0.0003&thinsp;Å. From this analysis, Al site occupancies, t, can be derived from observed mean tetrahedral distances, 〈T–O〉obs, as t=0.25(1+nAn)+〈T–O〉adj-〈〈T–O〉〉/0.12466(17), with the observed distance 〈T–O〉obs adjusted for the influence of the fractional s-character, 〈T–O〉adj=〈T–O〉obs+0.1907(51)[〈fs(O)〉-〈〈fs(O)〉〉]. This equation served to determine the site occupancies of 16 intermediate to high albites and one analbite from their mean tetrahedral distances. It was found that the individual site occupancies t10, t1m and t20= t2m all vary linearly with the difference Δt1= t10− t1m. Δt1, in turn, varies linearly with the length difference, Δtr[110], between the unit cell repeat distances [1∕2a, 1∕2b, 0] and [1∕2a, -1/2b, 0]. Then, from the Δtr[110] indicator, values of t were obtained as t10=(1-b0)+b0(b1+b2Δtr[110])t1m=(1-b0)-(1-b0)(b1+b2Δtr[110])t20=t2m=(b0-0.5)-(b0-0.5)(b1+b2Δtr[110]), with b0=0.7288(16), b1=0.1103(59) and b2=3.234(32)&thinsp;Å−1. Finally, from an expression that converts the Δ2θ(131) measure of order into Δtr[110] and thus into site occupancies, it was possible to obtain from the unique suite of bracketed high-pressure experiments performed on albites by Goldsmith and Jenkins (1985) the evolution with equilibrium temperature of the thermodynamic order parameter Qod and of the individual Al site occupancies t at a pressure of 1&thinsp;bar. For that purpose, since the Goldsmith and Jenkins experiments were performed at ≈18&thinsp;kbar, a procedure was devised that accounts for the effect of pressure on the state of order. At 1&thinsp;bar, low albite is stable up to 590&thinsp;∘C, where it begins to disorder, turning into high albite above 720&thinsp;∘C. The highly though not fully disordered monoclinic state (monalbite) is reached at 980&thinsp;∘C, 1&thinsp;bar, and 1055&thinsp;∘C, 18&thinsp;kbar, respectively. Eventually, when applying the determinative equations given above to low microcline, full order is predicted as in low albite.</p

Rubidian microcline from Red Cross Lake, northeastern Manitoba

Dark grey, slightly perthitic and grid-twinned to non-perthitic and Baveno-twinned, near-maximum rubidian microcline occurs in sheared dikes of pollucite-bearing pegmatites at Red Cross Lake, northeastern Manitoba. The composition of the most rubidium-rich sample, free of mineral inclusions on optical microscopic scale, is Or70.3Ab11.4An0.4Rb-f16.9Cs-f0.9Tl-f0.05; the potassic phase of the perthite is close to Or76.5Ab3.9An0.2Rb-f18.4Cs-f1.0Tl-f0.05. Unit cell dimensions are a 8.6373(7), b 12.9691(5), c 7.2264(6)Å, α 90.588(5)°, ß 116.002(5)°, γ 87.842(5)°, V 727.01(13)Å3; triclincity 0.93; α 1.518‒1.520, ß 1.522‒1.524, γ 1.525‒1.526, (‒)2V 76‒80°, X' (001) 19‒21°, X' (010) 7‒8°; Euler angles φ = +74°, ψ = 95.5°, θ = 72.5°; D(meas.) 2.612, D(calc.) 2.614; characteristic IR absorption bands at 15.455 and 18.585 μm. This rubidian microcline is the most rubidium-rich natural feldspar described to date. Its properties support the conclusion that high (Rb,Cs) contents may retard ordering in K-feldspar but are not a primary control of triclinization. Available data on K-feldspars show no consistent correlation between the highest Rb-contents and specific structural states in individual pegmatites

Feldspat-Mischkristalle : eine Übersicht

Compounds having the feldspar structure can be represented by the general formula (A¹⁺ₓA²⁺₁₊ₓ) [T³⁺₂₋ₓT⁴⁺₂₊ₓO₈], 0 < x < 1. As far as known the possible substitutions are A¹⁺ : Na, K, Rb, Tl, NH₄, A²⁺ : Ca, Sr, Ba, Pb, Mn, T³⁺ : Al, B, Ga, Fe, T⁴⁺ : Si, Ge. These compounds can be classified as belonging to one of the following three groups : 1. Alkali feldspar type and its solid solutions ; 2. Alkaline earth feldspar type and its solid solutions ; 3. Plagioclase type : solid solutions between members of the first and second group. A review of the solid solutions formed by these three groups is given, and the crystal chemical implications are discussed.Résumé. — Les composés ayant la structure d'un feldspath peuvent être représentés au moyen de la formule générale : (A¹⁺ₓA²⁺₁₊ₓ) [T³⁺₂₋ₓT⁴⁺₂₊ₓO₈], 0 < x < 1. Autant qu'on le sache, les substitutions possibles font intervenir : — pour A¹⁺ : Na, K, Rb, Tl, NH₄, — pour A²⁺ : Ca, Sr, Ba, Pb, Mn, — pour T³⁺ : Al, B, Ga, Fe, — pour T⁴⁺ : Si, Ge. On peut classer ces composés selon leur appartenance à l'un des trois groupes suivants : 1. Celui du type des feldspaths alcalins et ses solutions solides ; 2. Celui du type des feldspaths alcalino-terreux et ses solutions solides ; 3. Celui du type des plagioclases, lesquels sont des solutions solides entre membres du premier et du second groupe. On passe en revue les solutions solides formées par ces trois groupes et on en discute les implications cristallochimiques.Bambauer Hans Ulrich, Kroll Herbert, Nager H. E., Pentinghaus H. Feldspat-Mischkristalle : eine Übersicht. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 97, 2-5, 1974. Les solutions solides en minéralogie. Colloque international du C.N.R.S. n° 234. 27-30 mai 1974, Orléans, France

Zur Prozesschemie der Verglasung hochradioaktiver Abfaelle in einem keramischen Schmelzer Experimente mit einem Laborschmelzer

Zur Untersuchung der chemischen Vorgaenge bei der Verglasung von hochradioaktiven Abfallkonzentraten (HAWC) in einem direktbeheizten keramischen Schmelzer wurden zwei Laborschmelzanlagen inaktiv betrieben. Insgesamt wurden 10 Verglasungskampagnen mit z.T. mehrmonatiger Dauer durchgefuehrt, die Betriebsdaten ausgewertet und die produzierten Glaeser analysiert. Bei der Mehrzahl der Kampagnen war das Verhalten der Platinmetalle unter Prozessbedingungen der Hauptuntersuchungsgegenstand. Daneben wurde die Verglasung von MAWC/HAWC Gemischen und von Ru- bzw. Te-freiem HAWC erprobt. Der Feststoffaustrag beim Verglasungsprozess, die Bildung anderer Fremdphasen in der Silikatschmelze sowie die Auswirkung eines Zusatzes von Si bzw. C wurden bestimmt. Die Korrosion am Elektrodenmaterial und an der kermaischen Auskleidung des Schmelzers wurde dokumentiert. Es zeigte sich, dass die in Silikatschmelzen kaum loeslichen Platinmetalle Ru, Rh und Pd im Schmelzer innerhalb weniger Stunden separate Phasen bilden, die sich in ihrer Zusammensetzung und Partikelgroesse kaum noch aendern. In Schmelzern mit 45 Bodenneigung vergroeberte sich diese fliessfaehige Platinmetallsuspension in Totzonen des Schmelzers im Verlauf von mehreren tausend Betriebsstunden unter Ausbildung eines stationaeren Sediments, in dem die elektrisch leitfaehigen Platinmetallpartikel (Pd-Rh-Te Legierung und Ru(Rh)O_2) ein dreidimensionales Netzwerk bildeten. Im stationaeren Sediment herrschten Temperaturen bis weit ueber 1200 C. Der Badwiderstand ging im Verlauf einer Kampagne auf 30-35% des Ausgangswertes zurueck. Bei Schmelzern mit 60 Bodenneigung wurde kein stationaeres Sediment beobachtet. Der Widerstandsrueckgang betrug nur etwa 45%. Ein Zusatz von 0,7 bzw. 1,2% Si verringerte den Widerstandsrueckgang auf nur 10% bis 15%. (orig.)High level liquid waste (HLLW) simulates were vitrified on the lab scale in a liquid fed ceramic melter to study the chemical processes during the vitrification. The process data of 10 vitrification campaigns, some of them with a duration of several months, and the HLLW glasses were analyzed. In most of the campaigns the interest was focused on the behaviour of the platinum metals which formed separate phases of a Pd-Rh-Te alloy and Ru(Rh)O_2 in the glass melt. Other campaigns were tests for the vitrification of MLLW/HLLW mixtures and HLLW solution which did not contain ruthenium and tellurium, respectively. The entrainment of the vitrification off-gas with solids, the formation of other separate phases and the effect of additives like Si- and C-powder were investigated as well as the corrosion of the electrodes and the melter ceramic. Within a few hours of operation the platinum metals Ru, Rh and Pd formed insoluble particles in the glass melt which did not change further size and composition. Coarsening of this mobile suspension of platinum metal phases occured in melters with a sloped floor of 45 in the transition zone wall/floor after some 1000 hours of operation. The resulting sediments were immobile and consisted of a three dimensional networks of electrically conductive particles. The temperature in this sediments exceeded 1200 C; the over all resistance of the glass melt decreased to about 30%-35% of the starting value. No immobile sediments were observed in melters with a sloped floor of 60 . The decrease of melter resistance was only about 45%. With an addition of 0,7 and 1,2 wt.%, respectively of silicon powder the resistance decreased only for 10 to 15%. (orig.)SIGLEAvailable from TIB Hannover: ZA 5141(5289) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Synthesis and crystal structure of low ferrialuminosilicate sanidine

Iron-containing potassium feldspar crystals are prepared using the hydrothermal synthesis in an alkaline medium at temperatures ranging from 500 to 526°C. The crystal structure of the synthetic potassium feldspar is refined [Ital Structures diffractometer, MoKalpha radiation, 1327 unique reflections with F > 4sigma(F), anisotropic approximation, R(F) = 0.044]. It is established that, under the given preparation conditions, the synthesis leads to the formation of the monoclinic modification with the following unit-cell parameters: a 8.655(7) Å, b = 13.101(9) Å, c = 7.250(g) Å, beta = 116.02(2)°, space group C2/m, and Z = 4. The cation distribution over crystallographically inequivalent tetrahedral positions T(1) and T(2) is determined and justified using X-ray diffraction data. According to this distribution, the iron-containing potassium feldspar is assigned to the low ferrialuminosilicate sanidine. The proposed structural formula K A=0.99 (Si1.2Fe0.5Al0.3) T(1)=2 (Si1.81Al0.19) T(2)=2 O8 agrees well with the data of the electron microprobe analysis. It is revealed that iron occupies the T(1) position and manifests itself as a majority rather than minority impurity element with respect to aluminum