Home

Process for Making Single-Domain Magnetite Crystals

Crystals can be chemically pure and free of defects.

A process for making chemically pure, single-domain magnetite crystals substantially free of structural defects has been invented as a byproduct of research into the origin of globules in a meteorite found in Antarctica and believed to have originated on Mars. The globules in the meteorite comprise layers of mixed (Mg, Fe, and Ca) carbonates, magnetite, and iron sulfides. Since the discovery of the meteorite was announced in August 1996, scientists have debated whether the globules are of biological origin or were formed from inorganic materials by processes that could have taken place on Mars. While the research that led to the present invention has not provided a definitive conclusion concerning the origin of the globules, it has shown that globules of a different but related chemically layered structure can be grown from inorganic ingredients in a multistep precipitation process.

As described in more detail below, the present invention comprises the multi-step precipitation process plus a subsequent heat treatment. The multistep precipitation process was demonstrated in a laboratory experiment on the growth of submicron ankerite crystals, overgrown by submicron siderite and pyrite crystals, over-grown by submicron magnesite crystals, overgrown by submicron siderite and pyrite. In each step, chloride salts of appropriate cations (Ca, Fe, and Mg) were dissolved in deoxygenated, CO2-saturated water. NaHCO3 was added as a pH buffer while CO2 was passed continuously through the solution. A 15-mL aliquot of the resulting solution was transferred into each of several 20 mL, poly (tetrafluoroethylene)-lined hydrothermal pressure vessels. The vessels were closed in a CO2 atmosphere, then transferred into an oven at a temperature of 150 °C. After a predetermined time, the hydrothermal vessels were removed from the oven and quenched in a freezer. Supernatant solutions were decanted, and carbonate precipitates were washed free of soluble salts by repeated decantations with deionized water.

ImageThe procedure as described thus far was repeated for each subsequent step, except that a chemically different solution was added to the washed carbonate precipitates left in the hydrothermal vessels from the previous steps. Sulfur was included in the second and fourth steps to form Fe sulfides in addition to siderite. Hence, each globule comprised an ankeritic core (formed in step 1) followed by concentric zones of siderite + pyrite (formed in step 2), magnesite (formed in step 3), and siderite +pyrite (formed in step 4). The carbonate +pyrite globules thus synthesized were heated to, then cooled from, 470 °C in a differential scanning calorimeter at the rate of 20 °C /min in a stream of CO2 at a pressure of 13.3 kPa. This heat treatment converted the siderite +pyrite to magnetite + pyrrhotite.The magnetite crystals are believed to have formed from the thermal decomposition of the siderite crystals in the reaction 3FeCO3Fe3→O4 + 2CO2 + CO. The magnetite crystals were found to have a variety of shapes, to have linear dimensions of predominantly 10 to 100 nm (in the superparamagnetic-to- single-domain size range), to be chemically pure, and to be free of structural defects (see figure). Magnetite powders are essential ingredients of magnetic recording tapes. Powders made from single-domain magnetite crystals may enable the production of magnetic tapes capable of storing data at densities greater than are now possible, provided that the process can be refined so that the magnetite globules are mostly elongated along their (111) crystallographic axes. At the time of reporting the information for this article, scanning-electron-microscopy and electron-diffraction experiments to determine length-to- width ratios and crystallographic orientations were under way.

This work was done by D. C. Golden of Hernandez Engineering; Douglas W. Ming, Richard V. Morris, Gary E. Lofgren, and Gordan A McKay of Johnson Space Center; and Craig S. Schwandt, Howard V. Lauer, Jr., and Richard A. Socki of Lockheed Martin.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23326.