Gehlenite

Fig. 1. Phase equilibria ia the C—A—S (CaO—AI2O2—Si02) system (3,4) temperatures are ia °C. Shaded areas denote two Hquids compositional index marks on the triangle are indicated at 10% intervals B denotes cristobaUte [14464-46-17, and D denotes tridymite [15468-32-3] both of Si02 composition E is anorthite [1302-54-17, Al2CaSi20g G is muUite [55964-99-3]-, H, gehlenite [1302-56-3], Ca2Al2Si02 and J is the area of Pordand cement compositions.

Ca(Ali)2Si20g + Ca2(Al )2Si07 + SCaSiOj anorthite gehlenite wollastonite... 

Most orthosilicates reacted completely with poly(acrylic acid) solution an exception was andradite, CagFOg [SiOJg. Even so, the cements of gehlenite and hardystonite were very weak and affected by water. Only gadolinite and willemite formed cements of some strength which were unaffected by water, probably because one contained beryllium and iron and the other zinc. 

Figure 5.6 Triangular composition diagram for SiO -Alj Oj-CaO glasses, showing that glasses with cement-forming ability fall within the gehlenite and anorthite composition region, and that only glasses with less than 61 to 62% by mass of silica have the potential to form a cement (Hill Wilson, 1988a).

Figure 5.7 Triangular composition diagram for SiOj-Al Oj-CaO glasses. Glasses in the gehlenite region yield stronger cements (95 to 104 MPa in compression) than those in the anorthite region (29 to 56 MPa) (Hill Wilson, 1988a).

When X was 10, fluorite and anorthite crystallites were formed. With glasses of lower fluoride content (x < 10) gehlenite crystallites were also found. As X decreased, increasingly more gehlenite was formed at the expense of anorthite and fluorite. In coimection with this observation it should be noted that the chemical composition of the glass corresponds to gehlenite when x = 0 and to a mixture of anorthite and fluorite when x = 1. 

X-ray diffractograms (Fig. 4) and petrographic analyses (scanning electron microscopy also used for estimating the surface roughness and micro-heterogeneity of samples) indicate the presence of diverse silicates and oxides on the surface and in the bulk of the vitrocrystalline samples. In addition to the nearly ubiquitous quartz, other minerals were found in several samples gehlenite, albite, diopside, portlandite, pyroxenes... 

Halite (NaClk sylvite (KC.I). gehlenite (CaiALSiO/). anhydrite CaS04 Halite (NaCl)... 

Melilite - solid solution between Ca2Al2Si07 (gehlenite) and Ca2MgSi207 (akermanite). 

Bulk chemical compositions of CAIs, AOAs, and chondrules, compared to trends for calculated total condensed solids formed by equilibrium condensation of a hot solar gas at two different pressures. Condensed minerals are corundum (Cor), hibonite (Hib), end members of melilite series gehlenite (Geh) and akermanite (Ak), anorthite (An), diopside (Di), and forsterite (Fo). Modified from MacPherson etal. (2005). 

Figure 7. Upper Diffractogram of the sample MJ0240, representative of the unaffected Sevillian pottery. Lower Diffractogram of the sample MJ0237, representative of the affected Sevillian pottery, anl analcime cal calcite gh gehlenite hm hematite ill illite-muscovite kfs alkaline feldspar pg plagioclase px pyroxene qtz quartz...

As noted in Section 3.3.1, the decarbonation of calcite is greatly favoured by intimate mixing with quartz or clay minerals. Under these conditions, much CO2 is lost before any free lime can be detected. The formation of C2S as an early product is well established, but the situation with the calcium aluminate phases is more complex, no one phase being dominant as an initial product (W11). The aluminate or aluminosilicate phases most often reported as early or intermediate products in laboratory experiments with pure chemicals or raw meals have been CA, CjjA, and, less frequently, gehlenite (W12,D6-D8,W13,L8,L9,I6,C9,C10,R10). CjA can form by 850°C (L9,C10) but seems usually to be a later product. Ferrite phase forms readily, and is initially of low Al/Fe ratio (D7,R10). 

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