Hardness can be defined microscopically as the combined resistance of chemical bonds in a material to indentation. The current review presents three most popular microscopic models based on distinct scaling schemes of this resistance, namely the bond resistance, bond strength, and electronegativity models, with key points during employing these microscopic models addressed. These models can be used to estimate the hardness of known crystals. More importantly, hardness prediction based on the designed crystal structures becomes feasible with these models. Consequently, a straightforward and powerful criterion for novel superhard materials is provided. The current focuses of research on potential superhard materials are also discussed.

Microscopic theory of hardness and design of novel superhard crystals

On the characterization of disordered and heterogeneous carbonaceous materials by Raman spectroscopy.

X-ray diffraction, Raman and infrared spectroscopic evidence for the inclusion of water-rich ringwoodite in diamond from Juina, Brazil, indicates that, at least locally, the Earth’s transition zone is hydrous to about 1 weight per cent. It is not clear just how much water resides within the solid Earth, and where it is to be found, with many indirect measurements yielding conflicting results. Here Graham Pearson and co-authors present evidence from a diamond inclusion from Juina, Brazil, for the first known terrestrial occurrence of ringwoodite — a high-pressure polymorph of olivine first identified in meteorites and thought to be a major constituent of the Earth's mantle transition zone. The water-rich nature of this inclusion provides direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The ultimate origin of water in the Earth’s hydrosphere is in the deep Earth—the mantle. Theory1 and experiments2,3,4 have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth’s transition zone, at depths between 410 and 660 kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine—wadsleyite and ringwoodite—to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics5,6,7, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results8,9,10, and the issue of whether the transition zone contains abundant water remains highly controversial11. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juina, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region.

/pdf/hydrous-mantle-transition-zone-indicated-by-ringwoodite-4xqrsfww3b.pdf

Hydrous mantle transition zone indicated by ringwoodite included within diamond

We review the radiometric ages of the 16 currently known Martian meteorites, classified as 11 shergottites (8 basaltic and 3 lherzolitic), 3 nakhlites (clinopyroxenites), Chassigny (a dunite), and the orthopyroxenite ALH84001. The basaltic shergottites represent surface lava flows, the others magmas that solidified at depth. Shock effects correlate with these compositional types, and, in each case, they can be attributed to a single shock event, most likely the meteorite’s ejection from Mars. Peak pressures in the range 15 – 45 GPa appear to be a “launch window”: shergottites experienced ~ 30 – 45 GPa, nakhlites ~ 20 ± 5 GPa, Chassigny ~35 GPa, and ALH84001 ~35 – 40 GPa. Two meteorites, lherzolitic shergottite Y-793605 and orthopyroxenite ALH84001, are monomict breccias, indicating a two-phase shock history in toto: monomict brecciation at depth in a first impact and later shock metamorphism in a second impact, probably the ejection event.

/pdf/ages-and-geologic-histories-of-martian-meteorites-3pkv192wbm.pdf

Ages and Geologic Histories of Martian Meteorites

Rutile stability and rutile/melt HFSE partitioning during partial melting of hydrous basalt: Implications for TTG genesis

Shock veins in the Sixiangkou (L6) chondrite contain two high-pressure assemblages: (i) majorite-pyrope solid solution plus magnesiowustite that crystallized at high pressures and temperatures from a shock-induced silicate melt of bulk Sixiangkou composition and (ii) ringwoodite plus low-calcium majorite that were produced by solid-state transformation of olivine and low-calcium pyroxene. The morphology and chemistry of the majorite-pyrope garnet and the size of the magnesiowustite crystals indicate a longer duration at high pressure and temperature than predicted by impact scenarios. This pressure-temperature regime is constrained by the olivine-ringwoodite and orthopyroxene-majorite phase transformations, fusion of the meteorite constituents, and crystallization of majorite-pyrope solid solution plus magnesiowustite from that melt under high pressure.

https://science.sciencemag.org/content/sci/271/5255/1570.full.pdf

The Majorite-Pyrope + Magnesiowüstite Assemblage: Constraints on the History of Shock Veins in Chondrites

The hollandite high-pressure polymorph of plagioclase has been identified in shock-induced melt veins of the Sixiangkou 16 chondrite. It is intimately intergrown with feldspathic glass within grains previously thought to be "maskelynite." The crystallographic nature of the mineral was established by Laser micro-Raman spectroscopy and x-ray diffraction. The mineral is tetragonal with the unit cell parameters a = 9.263 +/- 0.003 angstroms and c = 2.706 +/- 0.003 angstroms. its occurrence with the Liquidus pair majorite-pyrope solid solution plus magnesio-wustite sets constraints on the peak pressures that prevailed in the shock-induced melt veins. The absence of a calcium ferrite-structured phase sets an upper bound for the crystallization of the hollandite polymorph near 23 gigapascals.

Natural NaAlSi3O8-Hollandite in the shocked Sixiangkou meteorite

The nature of maskelynite in shocked meteorites: Not diaplectic glass but a glass quenched from shock-induced dense melt at high pressures

A post-stishovite phase of silica was identified in the Shergotty meteorite by x-ray diffraction and field emission scanning electron microscopy. The diffraction pattern revealed a monoclinic lattice, similar to the baddeleyite-structured polymorph with the cell parameters a = 4.375(1) angstroms, b = 4.584(1) angstroms, c = 4. 708(1) angstroms, beta= 99.97(3), rho = 4.30(2) grams per cubic centimeter, where the numbers in parentheses are the maximum deviations. Transmission electron microscopy investigations indicate the presence of the alpha-lead dioxide-like polymorph, stishovite, and secondary cristobalite in the same silica grain. The mixture of high-density polymorphs suggests that several post-stishovite phases were formed during the shock event on the Shergotty parent body.

https://science.sciencemag.org/content/sci/288/5471/1632.full.pdf

A Monoclinic Post-Stishovite Polymorph of Silica in the Shergotty Meteorite

We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.

https://science.sciencemag.org/content/sci/293/5534/1467.full.pdf

Ahmed El Goresy

Papers

The Majorite-Pyrope + Magnesiowüstite Assemblage: Constraints on the History of Shock Veins in Chondrites

Natural NaAlSi3O8-Hollandite in the shocked Sixiangkou meteorite

The nature of maskelynite in shocked meteorites: Not diaplectic glass but a glass quenched from shock-induced dense melt at high pressures

A Monoclinic Post-Stishovite Polymorph of Silica in the Shergotty Meteorite

An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater.