Distinguishing well ordered opal-CT and opal-C from high temperature cristobalite by x-ray diffraction

Abstract

A critical step in quantifying the amount of crystalline silica in mineral deposits is the accurate identification of all mineral constituents. It is particularly important to correctly identify the low-temperature opaline silica polymorphs opal-C and opal-CT which, depending on degree of ordering, can be mistaken for α-cristobalite in standard x-ray diffraction patterns. Misidentification occurs because there is limited x-ray diffraction data available from the literature and because these minerals have diffraction maxima that coincide with those of high temperature cristobalite, X-ray diffraction patterns of opal have been collected in 20 bentonite, fuller's earth, and diatomaceous earth deposits to illustrate the range in ordering that naturally occurs in these polymorphs. Two opaline silica polymorphs are commonly observed. Opal-C is characterized by sharp, intense (101) reflections centered near 4.0 Å with peak widths ranging from 0.222 to 0.453 Å and opal-CT is characterized by broader, less intense (101) reflections centered near 4.07 to 4.10 Å with peak widths ranging from 0.506 to 0.883 Å. Opal-A was observed in one sample. Opal-A is easily distinguished from the other opaline silica polymorphs and from α-cristobalite using x-ray diffraction. Opal-C and opal-CT, however, are not as readily distinguished from α-cristobalite. The position and width of the (101) peak can be used to distinguish these polymorphs from one another or a simple heating test can be used. Because opaline silica is hydrated, the position of the (101) reflection shifts and sharpens as a result of heating When α-cristobalite is heated to change in peak position is observed. It has also been found that opaline silica, when reacted with phosphoric acid, potassium pyrosulfate, and sodium sulfide solutions, is considerably more chemically reactive than the crystalline silica minerals. Based on results of the heating test and the position and width of the 4 Å peak of the unheated sample, none of the 20 bentonite, fuller's earths, or diatomaceous earth deposits analyzed contain high temperature α-cristobalite.