Ancient minerals: artefacts of the primordial solar system

Reflecting on refractory minerals

The discovery of the oldest mineral in the solar system, krotite, found in an unusual refractory inclusion of the meteorite NWA 1934 from northwest Africa, provides an unprecedented look back into deep time to the first planetary materials formed in our solar system.

The inclusion within this particular meteorite is known affectionately to the scientists studying it as "Cracked Egg" for its peculiar but distinctive appearance. It was observations by Harold Connolly, Jr. and student Stuart Sweeney Smith at the City University of New York (CUNY) and the American Museum of Natural History (AMNH) who first recognized that the cracked egg grain might be yet more intriguing than a superficial investigation would suggest. They revealed that it is a calcium-aluminium-rich refractory inclusion. The refractory epithet alludes to the behaviour of such mineral-containing grains at very high temperatures where they are rather stable. Such stability implies that they must have formed as very primitive, high-temperature condensates from the primordial solar nebula temperatures.

Caltech's Chi Ma received the inclusion from the team in order to carry out a detailed nano-mineralogical investigation using Scanning electron microscopy-electron backscatter diffraction (SEM-EBSD), micro-Raman spectroscopy and electron microprobe (for mean chemical composition), it was then forwarded to Anthony Kampf, Curator of Mineral Sciences at the Natural History Museum of Los Angeles County (NHM), for X-ray diffraction studies. Kampf corroborated the tests carried out at Caltech and together the data showed that the main component of the cracked egg grain is a low-pressure calcium aluminium oxide (CaAl₂O₄). Although synthetic polymorphs had been seen at both low and high pressure, this is the first time that the mineral has been observed in nature. Bizarrely, structure determination by XRD and EBSD revealed this form to be the same as that seen in a synthetic component of high- temperature concrete.

Honourable mineral

The mineral was named krotite in honour of cosmochemist Alexander Krot of the University of Hawaii at Manoa in Honolulu, Hawaii. Krot's pioneering work has involved major discoveries about the very early solar system.

The researchers explain that krotite, a name now adopted by the International Mineralogical Association, occurs as the dominant phase in the inclusion and occupies the central and mantle portions of the inclusion together with minor perovskite, gehlenite, hercynite, and chlorine-bearing mayenite, and a trace of hexamolybdenum. The regions of the object containing krotite are surrounded by a rim of grossite, mixed hibonite, and spinel, then gehlenite with an outermost layer composed of aluminium-rich diopside. It is a colourless, transparent and brittle substance with a vitreous lustre. The single-crystal XRD revealed krotite to be monoclinic with a "stuffed tridymite structure"

Materials scientists are well aware that the synthetic component of concrete that shares krotite's characteristics forms only at temperatures above 1500 Celsius but at low pressure. This coupled with the fact that the cracked egg grain is known to have formed 4.5 billion years ago suggests that the krotite refractory phase must have formed in the solar nebula as one of the first minerals. The researchers are now investigating the unique cracked egg refractory inclusion further with the hope of learning more about the physical and chemical conditions under which it presumably formed and subsequently changed. Intriguingly, among the additional eight minerals found in the inclusion, one was previously unknown to scientists.

"Another new mineral has been discovered in the same refractory inclusion, brearleyite," Ma revealed to SpectroscopyNOW. Details are to be published in the August issue of American Mineralogist, he adds.

The discovery team comprised: Chi Ma, John Beckett and George Rossman of the Division of Geological and Planetary Sciences, at California Institute of Technology, Caltech, in Pasadena, California, working with Anthony Kampf of the Mineral Sciences Department at the Natural History Museum of Los Angeles County, Harold Connolly Jr. of the Department of Physical Sciences, at Kingsborough Community College of CUNY, Brooklyn, the American Museum of Natural History, New York and the Lunar and Planetary Laboratory, University of Arizona, Tucson, together with UArizona's Devin Schrader and Stuart Sweeney Smith of UArizona and the Department of Geology, at Carleton College, in Northfield, Minnesota.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.