Editing Opal (section)

== Local atomic structure ==
[[File:Alphacrist.png|thumb|The crystal structure of crystalline α-[[cristobalite]]. Locally, the structures of some opals, opal-C, are similar to this.]]
[[File:Lussatite-France.jpg|thumb|Lussatite (opal-CT)]]

The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica. As a [[mineraloid]], no [[unit cell]] describes the structure of opal. Nevertheless, opals can be roughly divided into those that show no signs of crystalline order ([[amorphous]] opal) and those that show signs of the beginning of crystalline order, commonly termed [[cryptocrystalline]] or microcrystalline opal.<ref>{{cite journal|last=Graetsch|first=H.|title=Structural characteristics of opaline and microcrystalline silica minerals. Silica, physical behavior, geochemistry and materials applications|journal=Reviews in Mineralogy|volume=29|date=1994|editor1-first=P.&nbsp;J.|editor1-last=Heaney|editor2-first=Connecticut|editor2-last=Prewitt|editor3-first=G.&nbsp;V.|editor3-last=Gibbs}}</ref> Dehydration experiments and [[infrared spectroscopy]] have shown that most of the H<sub>2</sub>O in the formula of SiO<sub>2</sub>·''n''H<sub>2</sub>O of opals is present in the familiar form of clusters of molecular water. Isolated water molecules, and [[silanol]]s, structures such as SiOH, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The structure of low-pressure [[Polymorphism (materials science)|polymorphs]] of anhydrous [[silica]] consists of frameworks of fully corner bonded tetrahedra of SiO<sub>4</sub>. The higher temperature polymorphs of silica [[cristobalite]] and [[tridymite]] are frequently the first to crystallize from amorphous anhydrous silica, and the local structures of microcrystalline opals also appear to be closer to that of cristobalite and tridymite than to quartz. The structures of tridymite and cristobalite are closely related and can be described as [[hexagonal close packing|hexagonal]] and [[cubic close packing|cubic close-packed]] layers. It is therefore possible to have intermediate structures in which the layers are not regularly stacked.

=== Microcrystalline opal ===
Microcrystalline opal or ''Opal-CT'' has been interpreted as consisting of clusters of stacked [[cristobalite]] and [[tridymite]] over very short length scales. The spheres of opal in microcrystalline opal are themselves made up of tiny nanocrystalline blades of cristobalite and tridymite. Microcrystalline opal has occasionally been further subdivided in the literature. Water content may be as high as 10&nbsp;wt%.<ref>[https://www.mindat.org/min-32185.html Opal-CT on Midat]</ref> Opal-CT, also called ''lussatine'' or ''lussatite'', is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder. Typical water content is about 1.5&nbsp;wt%.

=== 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}}</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.]]

===Opal surface chemical groups===
The surface of opal in contact with water is covered by [[siloxane]] bonds (≡Si–O–Si≡) and [[silanol]] groups (≡Si–OH). This makes the opal surface very [[hydrophile|hydrophilic]] and capable of forming numerous [[hydrogen bond]]s.