Intergrowth occurrence

Alkali feldspars (K.NajAlSijOg) form a continuous solid solution series at high temperatures, but tend to exsolve upon cooling into a potassium-rich phase and a sodium-rich phase, resulting in perthitic intergrowths (2). Sodic plagioclases and potassium-rich feldspars tend to be studied together, in part because of similarities in structure, composition, occurrence, and weatherability (e.g., 1[). 

These few examples illustrate the range of habits exhibited by minerals whose basic structural units are single chains. In addition, the pyroxenes, which have compositions quite similar to those of the amphiholes, are often found intimately intergrown with the latter double-chain minerals in what appears to be a single large crystal. An intergrowth of amphibole and pyroxene as a fiber, or in a fibrous aggregate, has not yet been described, but it is a likely occurrence. 

Sibieude has reported the occurrence of several ordered or partly ordered W structures at high temperatures in these systems. The evidence is from X-ray powder-diffraction and electron microscopy. In the light of our earlier discussion of B1, C1 and A-La2C>3 it is not surprising that these ternaries are also intergrowths of 111 layers of the first two - or the last two. 

Twinning in crystals results front the intergrowth of two or more individuals in such a way as to y ield parallelism in the ease of certain parts of the different individuals und. at the same time, other parts of the different individuals are in reverse positions in respect to each other. For example, an octahedral crystal of magnetite is twinned when one-half of the crystal is rotated ISO parallel to an octahedral facial plane. This type of twinning is known as spinel rn inirini . owing to its common occurrence in the spinel group of minerals. 

The occurrence of intergrowths of zeolite Y and ZSM-20, the cubic and hexagonal forms, is analogous to similar intergrowths in SiC and ZnS crystals. Intergrowths in zeolite Y and ZSM-20 do not block channels, as is the case in the erionite-offretite family, where rotation of cancrinite layers blocks the 12MR channels, but are more like intergrowths in the ZSM-5/ZSM-11 family, which modify the channel system. 

Giorgetti G., Tropper P., Essene E. J., and Peacor D. R. (2000) Characterization of non-equihbrium and equihbrium occurrences of paragonite/muscovite intergrowths in an eclogite from the Sesia-Lanzo Zone (Western Alps, Italy). Contrib. Mineral. Petrol. 138, 326—336. 

Occurrence of ewaldite in syntactic intergrowth with mackelveyite. 1017 ... 

Donnay G. and Donnay J. D. H. Ewaldite, a new barium calcium carbonate I. Occurrence of ewaldite in syntactic intergrowths with mackelveyite. Tschermaks miner, petrogr. Mitt., 15, 1971, 185-200. 

The U2Ni2Sl7 structure has not yet been found with R-T-M compounds, but its occurrence in R-T-M systems seems likely. The structure is shown in fig. 52b. It can be interpreted as an intergrowth of CeNi8i2- and CaF2-type slabs. These kinds of intergrowth structures are discussed with 8050 CeNi28i2. 

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