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Gray hematite has the same chemical formula (Fe2O3) as its rusty-red cousin, but a different crystalline structure. Red rust is fine and powdery; typical grains are hundreds of nanometers to a few microns across. Gray hematite crystals are larger, like grains of sand.
"Red and gray iron oxides on Mars are really just different forms of the same mineral," explained Hamilton. "If you ground up the gray hematite into a fine powder it would turn red because the smaller grains scatter red light."
Hematite minerals, on the other hand, might have been formed by hydrothermal water deep underground.
The Hematite Group is a more or less informal group of closely related trigonal oxides. Their relationship is linked through their similar structures. The general formula for this group is A 2O3. The A cations can be either iron, titanium, aluminum, chromium, vanadium, magnesium, antimony, sodium, zinc and/or manganese.
The structure is composed of alternating layers of cations and oxygens. The cations occupy spaces in layers between the oxygen layers and each are bonded to three oxygens in the above layer and three oxygens in the bottom layer. Not all of the sites for these cations are occupied as only two out of three are filled. If all the sites were filled then the formula would be AO in stead of A 2O3.
In the Ilmenite Subgroup, alternating layers of cations are occupied by only titanium or antimony and the other cation layers are occupied by a dissimilar action
Hematite is made up of iron and oxygen-a type of iron oxide. It takes its name from the Greek word for "blood," and is a rusty color in powdered form. Fine-grained hematite helps gives Mars its characteristic red hue.
"We want to know if the grains of hematite appear to be rounded and cemented together by the action of liquid water or if they're crystals that grew from a volcanic melt," says Crisp. "Is the hematite in layers, which would suggest that it was laid down by water, or in veins in the rock, which would be more characteristic of water having flowed through the rocks."