An international team of scientists led by Dr David Bish of Indiana University has analyzed recent data from the CheMin, a miniaturized X-ray diffraction instrument on board NASA’s Curiosity rover, to characterize the mineralogy of Martian soil and rocks.
This two-dimensional X-ray diffraction pattern for the John Klein mudstone sample reveals the presence of more than a dozen minerals. Image credit: Bish, D. et al.
The identification of minerals in Martian soil and rocks is crucial for the Curiosity mission’s goal to assess past environmental conditions.
Each mineral records the conditions under which it formed. The chemical composition of a rock provides only ambiguous mineralogical information, as in the textbook example of the minerals diamond and graphite, which have the same chemical composition, but strikingly different structures and properties.
The Chemistry and Mineralogy instrument (CheMin) – one of ten instruments on the Curiosity rover – uses X-ray diffraction, the standard practice for geologists on Earth using much larger lab instruments.
This method provides more accurate identifications of minerals than any method previously used on Mars.
It reads minerals’ internal structure by recording how their crystals distinctively interact with X-rays.
“CheMin performs what we call X-ray diffraction measurements on powdered rocks and soil samples. An X-ray diffraction is the best method for telling us what minerals are present in a rock or a soil because it is sensitive to the arrangements of atoms in minerals,” said Dr Bish, who is a co-investigator on the CheMin instrument and the lead author of the paper in the journal IUCrJ (published by the International Union of Crystallography).
“As the X-rays strike the soil sample, CheMin shows us how mineral crystals distinctively interact with X-rays, and this image shows our first X-ray diffraction results. The diffraction signals appear on the detector as rings that represent the fingerprint of the individual minerals. The rings tell us not only what minerals are present in the soil but also how abundant they are.”
To date, four different samples have been analyzed by the instrument: Rocknest, an aeolian bedform that was sampled using the rover’s scoop, two mudstone samples from locations called John Klein and Cumberland, and one of a sandstone.
The Rocknest data reveal abundant plagioclase (30 weight %), Fe-forsterite (16%), augite (11%), pigeonite (10%), magnetite (1.5%), and traces of anhydrite, quartz, sanidine, hematite, and ilmenite. In addition, the sample contains 27 percent amorphous, or non-crystalline, material.
“The CheMin-determined mineralogy of the Rocknest material is similar to the normative mineralogy of other basaltic rocks on Mars and of basaltic Martian meteorites. Perhaps the most interesting aspect of these results is the presence of major amounts of amorphous material, consistent with the lack of significant long-term interactions with liquid water,” the researchers wrote in the IUCrJ paper.
X-ray diffraction analysis of the John Klein and Cumberland samples reveals abundant phyllosilicates (18-22%), plagioclase (22%), Fe-forsterite (1-3%), augite (4%), pigeonite (6-8%), orthopyroxene (3-4%), magnetite (4%), anhydrite (1-3%), bassanite (1-2%), sanidine (1-2%), amorphous material (28-31%), and traces of quartz, hematite, ilmenite, akaganeite, pyrite, and pyrrhotite.
“The presence of poorly ordered phyllosilicates in both John Klein and Cumberland supports alteration involving liquid water, although the persistence of a significant amorphous component (∼30%) in both of these samples suggests either that interactions with liquid water were not long lived or that the hydrous minerals in these samples formed elsewhere and were later transported,” Dr Bish and his colleagues wrote.
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Bish, D. et al. 2014. The first X-ray diffraction measurements on Mars. IUCrJ 1, 514-522; doi: 10.1107/S2052252514021150
