The probe and petrographic data indicate that Monahans 1998 white and black lithologies are H5, with the latter lithology being slightly more equilibrated and considerably more shocked than the former.

The matrix appears to consist of a pulverized mixture of the white and black lithologies, with one exception: the matrix contains grains of dark blue to purple, vitreous, transparent to opaque mineral, not found in either of the unbrecciated lithologies. The grain size of the mineral ranges up to 1 mm. At first it was believed that this phase was sodalite. However, EDX and WDS analyses showed it to be actually Halite (NaCl), with minor inclusions of the related halide Sylvite (KCl). To our knowledge this is the first report of these minerals within an ordinary chondrite, and they appear to represent the coarsest examples of these minerals known from any meteorite. Halite and Sylvite have been reported from a ureilite [1]. The compositions of these minerals were determined by probe analyses using a Cameca Camebax instrument, using a rastered beam, and employing natural mineral standards for Br and K, and pretzel salt (Snyder's) for Na and Cl.

The crystals of sylvite are present as inclusions within the larger halite crystals, with a heterogenous distribution. This is similar to the occurrence of halite and sylvite in terrestrial rocks.

It is well known that exposure of halite to ionizing radiation produces the same blue to purple color as is observed in Monahans 1998, although this can also be caused by presence of colloidal inclusions [2]. We suggest that this coloration was produced in the meteorite either by exposure to solar and galactic cosmic rays, or by exposure to beta decaying ⁴⁰K (in the sylvite). The exact mechanism for the coloration; i.e. electrons caught in anion holes, etc., has yet to be determined for this meteorite. The critical point here is that the blue color proves that the halite has a preterrestrial origin, since simple evaporite halite is not blue or violet. The presence of halite/sylvite solely within the brecciated matrix shows that it formed on the parent asteroid, during brecciation and impact gardening.

Several interesting possiblities are suggested by the halite occurrence in Monahans 1998. (1) The most likely paragenesis for the halite is asteroidal brines. If this origin is correct, then fluid inclusions may be present. It may even be possible to date the halite/sylvite by Rb/Sr systematics.(2) If brines were responsible for the halite/sylvite then other traces of aqueous alteration may be present. We will examine this possibility through TEM characterization of the matrix. Certainly, any consequent gain in understanding of the chemical properties of aqueous fluids on asteroids would be very valuable. (3) The halite was noticed in Monahans because of its attractive blue/purple color, and this required special sampling and thin sectioning procedures to be employed which preserved the halides. Exposure of the meteorite to a humid environment would certainly have caused dissolution of the halite/sylvite, and bleaching of the halite. Exposure to heat or light would also cause the blue color to be bleached out. In other words, if not noted within a few days of itÇs fall, any halite present in a chondrite may be routinely overlooked or destroyed. It is therefore possible that halite is commonly present in chondrites, but has been overlooked. It is also possible that a fraction of the sulfate/halide efflorescence noted in Antarctic meteorite is derived from halite, rather than from indigenous contaminants in ice.