Tridymite occurrences

Normal ice is analogous to tridymite water has the quartz structure with a greater density. The occurrence of the density maximum at 4°, a unique property of water, must be attributed to the gradual transition of the tridymite structure into the quartz-like structure, while at higher temperature the normal thermal expansion again gets the upper hand. 

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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