Wadsleyite like several other of the minerals that have meteoritic type localities has a lab synthesized origin story. The synthetic material [đť›˝- (Mg,Fe)â‚‚SiOâ‚„)] was created in 1966 by Alfred Edward Ringwood and Alan Major, both of whom were honored with minerals named after them that happen to also have meteoritic type localities. In 1970 Ringwood and Major proposed that should the material they created in the lab ever be found in nature, earning it a mineral status, it should be named after Arthur David Wadsley. Wadsley was a fellow Australian crystallographer known for his contribution to the field of crystallographic shearing. đť›˝- ((Mg,Fe)â‚‚SiOâ‚„) was discovered by G.D. Price, A. Putnis, S.O. Agrell, and D.G.W. Smith of the University of Alberta within a shock feature of the Peace River meteorite and was submitted and accepted as a new mineral in 1983. Not ironically, Peace River also happens to contain both Ringwoodite and Majorite.
Peace River is an L6 ordinary chondrite, at the time of writing the metbul lists 13,376 meteorites of that type, making it the most common ordinary chondrite and most common class overall. Martin Horejsi did an amazing article on the witness accounts and the hunting of the fall available here if you are interested in a great read on those aspects of the meteorite. It always impresses me to think that with the abundance of available material, and being the first recorded witness fall type (Nogata, Japan May 19, 861 A.D.) that L6s still turn up surprises and are a bit of a trove for new minerals.
Wadsleyite is a high-pressure polymorph of the olivine group. Stable pressure values vary a bit in literature, but a rough breakdown of the relationship is at around 10-14 GPa olivine (more towards the forsterite end member) will transform into the beta phase denoted by the đť›˝ symbol which is Wadsleyite. Wadsleyite at around 18-23 GPa in turn will transform into the gamma phase (Îł) ringwoodite, making wadsleyite the intermediate pressure member polymorph.
As forsterite olivine transforms to wadsleyite, all sorts of interesting things are occurring from a mineralogical classification standpoint. Forsterite and the rest of the olivine group solid solution series are nesosilicates, meaning their individual silicon-oxygen tetrahedrons are bonded thought their interstitial cations of magnesium and iron. The high pressure and temperature rearrangement converts the structure into one that falls into the sorosilicate structure. In sorosilicates the silicon-oxygen tetrahedrons are no longer independent but rather share an oxygen between two paired tetrahedrons (Si2O7). As the pressure increases and wadsleyite transforms to ringwoodite the shared oxygen bond breaks and goes back to a nesosilicate structure.
Wadsleyite also happens to be a spinelloid mineral, which can cause some confusion as there is overlap in mineral classification schemes. Spinelloid denotes structure and also composition. To further add to the confusion its usages doesn’t have great bounding on the limits to how spinelloid something should be for it to be applied. The term itself means a mineral is spinel-like, spinel being a distinct mineral with composition [(MgAl)2O4] for which the “spinel group” of minerals was named after. Since it is both a distinct mineral and the namesake of a mineral grouping, it is common to see the term “spinel” thrown around copiously, rather than the more accurate “spinel group” or “spinelloid”. Minerals that are “spinel group” or “spinelloid” are any mineral of the form (AB)2X4 where A and B are cations and X is an anion with a 2:4 ratio. The most common anion is oxygen (O4). From a structural perspective this grouping of minerals forms tightly packed cubic arrangements as their characteristic feature. Wadsleyite highlights that the terms sorosilicate and spinelloid aren’t mutually exclusive since it is described as both. Wadsleyite is considered both a sorosilicate to account for its paired (Si2O7) tetrahedrons and a “spinelloid” as it displays a distorted and imperfect cubic habit called “spinelloid III” with the distortion exaggerated by the fact that wadsleyite is able to absorb hydroxyl ions due to it having large amounts of underbonded oxygen atoms in its structure.
Wadsleyite is important as it is theorized to be a major constituent of the earth’s mantle transition zone. Based on accessibility limitations, meteoritic wadsleyite provides valuable information and a proxy to the earth’s wadsleyite formative environment. This zone separates the lower and upper parts of the mantle, and thus the material characteristics of wadsleyite are believed to play a critical role in dynamic interaction between the earth’s lower and upper mantle as far as mantle flow, convection and seismic properties. Additionally the fact that wadsleyite can be hydrated makes it a major potential water-trap within the mantle as part of the deep water cycle within the earth.
