<prompt> Which of the following statements is not true about use of silica in the investment materials?It acts as refractoryIt regulates thermal expansionQuartz and cristobalite are the allotropic form used in dental investmentsBeta allotropic form is stable below transition temperature  <response>: 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: Phillips 12th edition page 201,202 