Melilite structure

A composition such as Ln2Si2A104N3 is dose to the limit of Al-substitution in the melilite structure, and in the low-Z rare earth systems. This composition is also close to the limit of nitrogen substitution in the liquid (and hence glass-forming) region in these systems. 

Fig. 10. Projection onto the (001) plane of the RjSijOjN melilite structure in which both T, and Tj tetrahedra are occupied by silicon atoms (after Lejus et al. 1994).

LaSrGasOy adopts the melilite structure, which consists of alternating cationic (La/Sr)2 and corner-sharing tetrahedral anionic GasOy layers and features fivefold tunnels that accommodate the eight-coordinate La/Sr as chains of cationsJ This composition is an insulator and was found as a secondary phase in the synthesis of the Sr, Mg-doped lanthanum gallate (LSGM) electrolyte. Rozumek etal. showed that the La/Sr... 

On the other hand, fine-grained CAIs consist of aggregates of nodules with a concentric structure consisting of spinel cores surrounded by melilite, anorthite, and diopside, or melilite cores surrounded by anorthite and diopside (Fig. 7.5). The sequence of minerals in the nodules is consistent with a condensation sequence. However, the textures of the overall objects are complicated and often indicate extended and multistage histories. For... 

The phenomenon of increased hardness occurs principally in minerals of sheet and chain structures, which link together through the cations (silicates and aluminosilicates, as well as hydrated sheet minerals, such as glauconite, melilite and gypsum—M ranging from 0 to about 1.25), and also in minerals of skeletal structures (borates, phosphates, sulphates, nitrates, carbonates, such as calcite, dolomite and others—Ah from 0 to about 1.15). For this reason, the hardness analysis of minerals with weak bonds demands consideration of the fact that just as the basic crystallo-chemical factors, so is hardness influenced by the form of domains (component parts of structures) in all anisodesmic minerals of chain, sheet or skeletal structure. Depending on the form of domain (and also according... 

As resulting from XRD measurements, the lines of a-sialon (PDF-JCPDS 42-0251), together with a-Si3N4 (PDF-JCPDS 41-0360), mellilite (Y2Si3N403, PDF-JCPDS 30-1460) and small additions of P-Si3N4 (PDF-JCPDS 33-1160), can be observed (sample PLS-1). After 1% carbon addition (Fig. 19.6, samples PLS-3 and PLS-7) the same main structural lines could be observed as in the reference sample (PLS-1). After we increased the carbon to 10% (PLS-5 and PLS-9) the a-sialon and melilite lines disappeared. In this case the main constituents of the microstructures are a-Si3N4, P-Si3N4 and SiC (PDF-... 

Inclusions that are predominantly melilite with minor spinel, perovskite, and hibonite are referred to as Type A. Most Type-A CAIs have a porous structure and are called fluffy Type-A CAIs. Some Type-A CAIs have a compact form and generally rounded shapes. These are referred to as compact Type-A CAIs. Type-B1 CAIs are characterized by coarse-grained, melilite-rich mantles surrounding cores composed of melilite, spinel, fassaite, and anorthite. Type-B2 inclusions have the same mineralogy, but lack the melilite-rich mantle. Type-B3 inclusions contain significant amounts of forsterite in addition to melilite. Type-C inclusions are similar to Type B2s, but anorthite is more abundant than melilite. All Type-B and Type-C inclusions have compact morphologies. 

Regourd (R34) reviewed structural and other aspects of slags. XRD patterns show an asymmetric, diffuse band from the glass peaking at about 31 20 (CuK d = 0.29 nm) and extending from about 20 to about 37 and a weaker band at about 48 20 (0.19 nm). Crystalline phases, if present in sufficient quantity, give superimposed, sharper peaks melilite and merwinite are the most usual. Neither periclase nor lime is found and, since it is present either in the glass or in these inert, crystalline phases, MgO is not a potential cause of expansion, as it may be in a clinker (S89). 

In terms of sheer numbers, probably the most abundant variety of CAl in most chondrites consists largely of spinel and calcic pyroxene. Some of these contain lesser amounts of anorthite, melilite, hibonite, and perovskite, but pyroxene-spinel-rich assemblages are almost universal. These objects range in size from a few tens of micrometers up to 1-2 cm (in CV3 chondrites). Large or small, from whatever kind of chondrite, the essential structure consists of small spinel grains or dense nodules of spinel grains or even chains of spinels, enveloped in a continuous thin rim of aluminum diopside that binds the entire structure together. 

All the layers are in fact slices of the cristobalite structure, which is the end-member of the series, and the Si307 layer is the anion in Na2Si307. This A3X7 layer, in which all circuits are rings of six tetrahedra, may be compared with the simple A3X7 layer in melilite and related compounds which is composed entirely of rings of five tetrahedra. We referred to this layer under pyrosilicates it is illustrated in Fig. 5.7(c), p. 163. In both A3X7 layers two-thirds of the tetrahedra share 3 vertices and the remainder 4 vertices. 

Local structural relaxation around Co along the hardystonite-Co-akermanite melilite solid solution. Phys, Chem. Miner., 39, 713-723. 

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