Editing Opal (section)

=== Noncrystalline opal ===
Two broad categories of noncrystalline opals, sometimes just referred to as "opal-A" ("A" stands for "amorphous"),<ref>{{cite journal |last1=Wilson |first1=M.J. |title=The structure of opal-CT revisited |journal=Journal of Non-Crystalline Solids |date=2014 |volume=405 |page=68 |doi=10.1016/j.jnoncrysol.2014.08.052 |bibcode=2014JNCS..405...68W |url=https://doi.org/10.1016/j.jnoncrysol.2014.08.052 |access-date=16 June 2022|url-access=subscription }}</ref> have been proposed. The first of these is opal-AG consisting of aggregated spheres of silica, with water filling the space in between. Precious opal and potch opal are generally varieties of this, the difference being in the regularity of the sizes of the spheres and their packing. The second "opal-A" is opal-AN or water-containing amorphous silica-glass. [[Hyalite]] is another name for this.

Noncrystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of [[diagenesis]], due to the increasing overburden pressure in [[sedimentary]] rocks, as some of the stacking disorder is removed.<ref>{{cite journal |url=http://minsocam.org/msa/AmMin/TOC/Articles_Free/1996/Cady_p1380-1395_96.pdf |title=HRTEM of microcrystalline opal in chert and porcelanite from the Monterey Formation, California |last1=Cady |first1=S.&nbsp;L. |last2=Wenk |first2=H.-R. |last3=Downing |first3=K.&nbsp;H. |date=1996 |journal=American Mineralogist |volume=81 |issue=11–12 |pages=1380–1395 |url-status=live |archive-url=https://web.archive.org/web/20111001053527/http://minsocam.org/msa/AmMin/TOC/Articles_Free/1996/Cady_p1380-1395_96.pdf |archive-date=1 October 2011 |df=dmy-all |bibcode=1996AmMin..81.1380C |doi=10.2138/am-1996-11-1211 |s2cid=53527412 }}</ref>

[[File:Schematic silica gel surface.png|thumb|center|upright=3|Schematic representation of the hydrated opal surface.]]