key words [Na-Mg-silicates phase transformation melt]

Interest in Na-Mg-silicates as plausible mantle minerals was aroused by the discovery of high-pressure reaction between forsterite (Mg₂SiO₄) and jadeite (NaAlSi₂O₆) at pressures above 4.5 GPa (Litvin and Gasparik, 1995). Forsterite interacts with jadeite through the reaction:

4Mg₂SiO₄+2NaAlSi₂O₆=3MgSiO₃+Mg₃Al₂Si₃O₁₂ +
+Na₂Mg₂Si₂O₇,

where the Na-Mg-silicate of Na₂Mg₂Si₂O₇ composition is formed in addition to enstatite and pyrope. Na₂Mg₂Si₂O₇ is a new compound of experimental mineralogy at high pressures, and its existence on the solidus of alkaline olivine-normative mantle as an individual phase is highly probable (Gasparik and Litvin, 1996).

Na₂Mg₂Si₂O₇ and other Na-Mg-silicates (Na₂Mg₂Si₆O₁₅ and Na₂MgSiO₄) of interest as plausible mantle minerals were experimentally studied by differential thermal analysis (DTA), high-pressure quenching, microprobe, and X-ray diffraction methods.

Crystalline powders of Na-Mg-silicates, which were obtained by recrystallization of stoichiometric

gels at 700^oC for 1 day, were used as starting materials for the thermal analysis. All the compounds studied melt congruently at 1 atm. Melting temperatures determined were with accuracy of +5^oC. Enthalpies of melting H^o(melt) of these compounds were found by comparison of the areas of respective endothermal DTA peaks and reflexes of a standard (Na₂SO₄ was used as the standard). Entropies of melting S^o(melt) were also determined. The enthalpies of melting were estimated with in +15% accuracy. The results of DTA measurements are given in Table 1.

Table 2. Melting temperatures (^oC) of Na₂Mg₂Si₂O₇, Na₂MgSiO₄, and jadeite (Na₂AlSi₂O₆) in dependence of pressure.

The dependences of melting points of Na₂Mg₂Si₂O₇ and Na₂MgSiO₄ on pressure in the pressure range up to 20 GPa were experimentally studied with a high-pressure split-sphere multianvil press. Some results are presented in Table 2 together with data on melting of jadeite (alkaline high-pressure mantle component) (Litvin and Gasparik, 1993).

These data indicate that the melting points of alkaline Na-Mg-silicates are by 250-560 degrees lower than the melting point of jadeite in the pressure range 4-16 GPa. Participation of Na-Mg-silicates in the solidus parageneses of mantle peridotites is responsible for some peculiar properties of the mantle magma formation, including generation of alkaline magmas.

Some high-pressure experiments were performed with the apparatus “anvil-with-hole” to identify high-pressure polymorphism. X-ray diffraction analysis of quenched Na₂Mg₂Si₂O₇, Na₂Mg₂Si₆O₁₅, and Na₂MgSiO₄ samples after the five-hours runs at 3.7 GPa and 1200^oC and 30-min runs at 7.0 GPa and 1300^oC indicated polymorthic transition of Na₂Mg₂Si₂O₇ as a result of pressure effect. X-ray microanalysis showed that the samples were single-phase and were represented by the stoichiometric compound Na₂Mg₂Si₂O₇. The other compounds disproportionate to Na₂SiO₃ and MgSiO₃ or MgO. Therefore, these substances underwent polymorphic and chemical transitions, and the high-pressure phase Na₂Mg₂Si₂O₇ proved to preserve at metastable conditions.

key words [polymorphic modifications Mg₃N₂-BN]

The interaction of magnesium and boron nitrides was repeatedly studied. It was established that the only compound forming in the system Mg₃N₂-BN at atmospheric pressure is magnesium boronitride Mg₃BN₃ (L). When heated at pressures of more than 20 kbar, the hexagonal modification of Mg₃BN₃ (L) was shown to undergo polymorphic transformation into the high-pressure tetragonal phase Mg₃BN₃ (H).

Several papers describe at least one more compound forming in this system at high pressures. The composition, X-ray characteristics, and stability field of this phase (phases) are hotly debated. To this end, the objective of the present work was to study phase relations in the system Mg₃N₂-BN at high pressures, primarily in the boron nitride-rich area.

The standard high-pressure cell of toroid type was used. Boron nitride and magnesium nitride or boronitride mixtures were heated up to 1600^oC at 15-60 kbar for several hours (with an intermediate grinding). The resulting powders were analyzed by X-ray diffraction. The sample densities were determined by hydrostatic weighing in CCl₄.

Formation of a new phase previously unknown was fixed within the pressure range 15-40 kbar (new phase 1). Sufficient purity of this compound was only attained at 15 kbar and the charge composition close to Mg₃B₂N₄. The substance thus obtained was almost free from Mg₃BN₃ admixture, but contained some quantities of unreacted boron nitride and magnesium nitride, judging from X-ray diffraction data. The attempts to obtain the samples of better purity by increasing the run time and varying the initial

charge composition or synthesis conditions were unsuccessful.

When the X-ray pattern of Mg₃B₂N₄ (new phase 1) was indexed, the hexagonal syngony with the unit cell parameters a = 5.397(2)Å, c=10.585(5)Å, and v = 267.0 (3 Å were assumed. The experimental density determination gives _exp = 2.67(5) g/cm³ which is in agreement with the roentgenographic density of the sample (_theor=81 g/cm³). The lower experimental density value is probably due to the initial boron nitride (-BN) impurity in the analyzed sample. The density of this compound is significantly lower. The De-Wolf parameter M(20) is 15.3 which suggests that the X-ray indexing is correct. Obtained magnesium boronitride (Mg₃B₂N₄ - new phase 1) appears as a brick-red substance gradually hydrolyzed on air. When heated in nitrogen atmosphere up to 600^oC, it remains unchanged, but at 900^oC decomposes to yield Mg₃BN₃ (L) and boron nitride:

Another phase (new phase 2) of assumed composition Mg₃B₂N₄ was established in the reaction products within the pressure interval 40 - 60 kbar. The attempts to extract this phase in the pure state were not successful. Its composition, properties, and structural features will be discussed in later publications. The X-ray diffraction data on the two new magnesium boronitrides of the assumed composition Mg₃B₂N₄ (new phase 1 and new phase 2) compared to the previous data on known boronitrides Mg₃BN₃ (atmospheric and high pressures) are presented in Table 1. Both new phases and the known Mg₃BN₃ (H) are likely to play a major part in catalytic transition of the hexagonal graphite-like -BN into the cubic diamond-like -BN that occurs in the Mg₃N₂-BN system at elevated tempartures and pressures.

Table 1. Comparison of X-ray diffraction data on two new high-pressure magnesium boronitrides with previous data

key words [nitrides synthesis high pressure]

Urgency. Magnesium and aluminium nitrides are good catalysts for the - transformation in boron

nitride, however, the phase diagrams of the systems metal nitride-boron nitride did not attract much attention with researchers until the last decade. Some data are available for the systems AlN-BN and Mg₃N₂-BN [1,2], and no data exist on the interaction in the system Bn-AlN-Mg₃N₂.

Goal. The goal of this work is an X-ray study in the system BN-AlN-Mg₃N₂ at normal and elevated pressure.

Technique. A mixture of the starting nitrides in preset molar ratios was heated under normal and elevated (30-75 kbar) pressures in hermetically sealed refractory steel capsules at 1200^oC for 50-100 h and at 1600^oC for 15-60 min, respectively. All the preparatory work with the samples was performed in a “dry” box.

AlN and Mg₃N₂ synthesized under laboratory conditions were used as the starting ones. The samples were subjected to X-ray (DRON-2) and microcrystalline optic examination.

The density was determined by a method of hydrostatic weighing.

Results. 37 mixtures of various compositions have been studied. It has been found that in the binary subsystem AlN-Mg₃N₂ under normal pressure there form five compounds having the wurtzite-type crystalline structure and belonging to the homologic series Mg₃Al_nN_n+2, where n assumes the values from 1 to 4 at normal pressure, and from 2 to 5 at elevated one. The layering of these compounds is (n+3)Z, where Z is the number of the formula units per cell. The individual compound Mg₃AlBN₄ forms in the system BN-AlN-Mg₃N₂ at normal pressure. At pressures in excess of 50 kbar Mg₃AlBN₄ becomes unstable, at temperatures above 1200^oC it decomposes by the reaction:

_P,t

Mg₃AlBN₄ Mg₃Al₃N₅ + Mg₃B₂N₄ + Mg₃BN₃.

The volumetric effect of this reaction is 9.4 %. Isothermal joins of the system have been plotted at normal (1 atm) and elevated (50 kbar) pressures.

1. Zhukov A.N., Burdina K.P., Semenenko K.N. (1994) // Jour. Organ. Khimii., V.64, Iss.3, pp.357-360.
2. Polushkin N.I., Burdina K.P.et al. (1989) // Dokl. AN SSSR, V.306, N.6, pp.1413-1420.