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Opaline Spheres: Loosely-packed Aggregates from Silica Nodule in Diatomaceous Miocene Fuller's Earth
Charles O. Pollard, Jr., Charles E. Weaver
Spheres of opaline silica, 1,000 A to 8,300 A in diameter, occur individually and in small aggregates around cavities of diatom fragments in a silica nodule from the middle Miocene Hawthorne Formation in south Georgia. The spheres are apparently equivalent to the silica spheres of precious opal, except that the spheres in precious opal are imperfectly close-packed and the spheres in this nodule are not close-packed, probably because of lack of sedimentation. The spheres are a high-silica variety of opal and presumably form in voids which are relatively inaccessible to other ions such as Al, Fe, and Mg. These opaline spheres are not present in isolated diatoms in the clay bed.
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