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What is Which of the following statements is not true about use of silica in the investment materials? and explain in detail?
Silica exists in at least four allotropic forms: quartz, tridymite, cristobalite, and fused quartz. 
When quartz, tridymite, or cristobalite is heated, a change in crystalline form occurs at a transition temperature characteristic of the particular form of silica. For example, when quartz is heated, it inverts (transforms) reversibly from a “low” room-temperature crystal form, known as α quartz, to a “high” form, called β quartz, at a temperature of 573 °C. This α to β phase transformation is called an inversion.
The β-allotropic forms are stable only above the transition temperature noted, and an inversion to the lower α form occurs on cooling in each case.
Ref: Phillip’s 12th edition page 201,202