NMNH - Bennu, Mineral examples

Earth-originating examples of minerals found in Bennu samples. These specimens are from the U.S. National Mineral Collection at the Smithsonian’s National Museum of Natural History and show the minerals observed in Bennu samples as formed and found on Earth. Four of these minerals had not been previously observed in extraterrestrial samples: gaylussite, villiaumite, thenardite, trona.

Foreground, left to right: calcite, gaylussite, sylvite with halite, villiaumite.

Calcite from Saint Joe Lead District, Missouri, U.S. (NMNH 134335). Calcite (calcium carbonate) is the first mineral to form from evaporation on Bennu.

• Gaylussite (white coating) from Searles Lake, California, U.S. (NMNH 102881) on trona.  Gaylussite (water-bearing sodium calcium carbonate) is typically the first sodium-bearing mineral to form from evaporating brines on Earth and likely formed the same way on Bennu.
• Sylvite with associated halite from Stassfurt, Germany (NMNH 56103). Sylvite (potassium chloride) and halite (sodium chloride; common table salt) form late in salty, evaporating fluids.  As in this sample, these two minerals are found in association in Bennu.
• Villiaumite from Murmanskaja, Russia (NMNH 176297). Villiaumite (sodium fluoride) is the last phase formed during evaporation of the salty, sodium-rich water on Bennu.

Background, left to right: magnetite, thenardite, trona.

• Magnetite from St. Lawrence County, New York, U.S. (NMNH R 20522, from the Roebling Collection). Magnetite (iron oxide) is formed by the precipitation of iron dissolved in fluid early in the evaporation sequence. The distinctive cubic form is observed in samples of Bennu, although at a scale of micrometers.
• Thenardite from Deep Springs Valley, California, U.S. (NMNH 94538). Thenardite (sodium sulfate) first appears when the brine becomes sodium-rich, lending the moniker “soda lakes” to bodies of water on Earth. On Bennu, thenardite is the first sodium-rich mineral to form and the first occurrence of oxidized sulfur.

Trona from Searles Lake, California, U.S. (NMNH 166722). Trona (water-bearing sodium carbonate) forms in the middle of the evaporation sequence, often forming long, thin needles.   Trona forms similar needles in the Bennu samples, although thousands of times smaller.