Editing Torbernite (section)

== Metatorbernite ==
[[Image:Metatorbernite sapin.jpg|thumb|Metatorbernite from [[Margabal Mine]], [[Entraygues-sur-Truyère]], France (Size:&nbsp;4&nbsp;cm&nbsp;×&nbsp;3&nbsp;cm&nbsp;×&nbsp;1.8&nbsp;cm)]]
Torbernite dehydrates readily to metatorbernite with the sum formula Cu[(UO<sub>2</sub>)(PO<sub>4</sub>)]<sub>2</sub>(H<sub>2</sub>O)<sub>8</sub>. It forms as torbernite withers, and can also be obtained by artificially heating torbernite above 75&nbsp;°C.<ref name="Hallimond1916" /> The crystals are rather opaque and only weakly translucent with a glassy lustre.<ref name="MindatMeta" />

Metatorbernite crystallises tetragonally-dipyramidally in space group ''P''4/''n'' with the lattice parameters ''a''&nbsp;=&nbsp;6.9756(5)&nbsp;Å and ''c''&nbsp;=&nbsp;17.349(2)&nbsp;Å and 2 formula units per unit cell.<ref name="LocockBurns" />

[[Image:Metatorbernite - Locock, Burns - packing.png|thumb|left|Packing of metatorbernite. Colour code: <span style="color: green">uranium</span>, <span style="color: lime">copper</span>, <span style="color: yellow">phosphorus</span>, <span style="color: red">oxygen</span>, <span style="color: grey">water</span>, <span style="color: silver">hydrogen</span>]]

The crystal structure of metatorbernite is different from torbernite as every second uranyl phosphate layer is moved about one half of the length of the crystallographic ''a''-axis in the [[Miller index|directions]] [100] and [010].<ref name="LocockBurns" /> The analysis by Locock and Burns confirms the finding by Stergiou et al., that the Cu<sup>2+</sup> ions only have an 88% crystallographic occupancy. The authors assume that by protonation of some of the water molecules there is a charge compensation for electronic neutrality, as it is discussed with the mineral [[chernikovite]].<ref name="LocockBurns" /> The same is postulated by the same authors for autunite.<ref name="LocockBurnsAutunit" /> Due to the limitations of [[X-ray crystallography|X-ray diffraction]] this postulate is practically not verifiable with this method.

The analysis by Locock and Burns shows eight molecules of water per formula unit in metatorbernite. This is in accord with the works by Arthur Francis Hallimons<ref name="Hallimond1916" /><ref name="Hallimond1920" /> and [[Kurt Walenta]],<ref name="Walenta" /> who show that the different steps of hydration between torbernite and metatorbernite have clear boundaries, and the water content of each compound remains constant and does not vary, in contrast for instance, as seen in minerals of the [[zeolite]] group. Therefore, sum formulae indicating varying degrees of water for torbernite and metatorbernite must not be used.<ref name="LocockBurns" />