Cristobalite occurrences

The occurrence of the moganite-type phase in PON confirms that moganite is really a new structure type in AX2 compounds. The molar volumes at ambient of the three phases, cristobalite, moganite, and quartz (22.58, 21.05, and 20.64 cm3, respectively) follow the same trend as in silica. The smaller size of the P02N2 tetrahedron relative to that of Si04 (cf. Table VI) widens the pressure fields of stability, so that PON appears to be very useful to shed light on the detailed behavior of silica, which is still ill defined. 

The occurrence of secondary Si02- H20 has been observed in several tailings impoundments, predominantly as amorphous hydrated pseudo-morphs after biotite. Cristobalite [Si02] has also been detected in the pseudomorphs. Partial replacement of various other minerals by amorphous silica has also been observed. 

Figure 3. Number of occurrences of recrystalUzed cristobalite or quartz from selected Deep-Sea Drilling Sites as a function of the age of the sample. Numbers within boxes refer to drilling sites. While the listing is not exhaustive, we do feel that it is representative.

Figs. 5 atid 6 show experiments with an Fe-containing batches. It can be seen that the crystallization occurres faster than in the presence of Ni. Similar to the Ni containing batch a recrystallisation to cristobalite is observed (Fig.5) EDAX experiments show that in contrast to the Ni-product the Fe is preferably inside the crystals regardless whether the Fe-source has been two- or threevalent. This agrees with results which have been obtained from MoBbauer experiments (ref. 18). 

A species name implies the simultaneous presence of three distinct features (1) a fixed crystal structure (2) a fixed chemical composition or, more reasonably, a fixed range of composition and (3) an occurrence in nature. Thus, artificial minerals synthesized in a laboratory should not have a name unless they are the equivalents of a naturally occurring species. Therefore it is quite wrong to make statements implying that Si02 is quartz because (SiOs) occurs in many different structures which have different names (e.g., tridymite, cristobalite, coesite etc.). The phrase can be turned around to say that quartz has the composition Si02, but it also has the structure P3,2i (see Sec. Ill), which distinguishes it from the other mentioned species called polymorphs of quartz because several different structures share the same chemical composition. 

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