As the authors note, the mineralogy of enstatite chondrites is distinctive, with some normally lithophile elements (Ca, Mg, Mn, K, Si) having chalcophile or siderophile behavior in these meteorites. For example, much of the Ca and Mn are present as sulfides (CaS, oldhamite; and MnS, alabandite) instead of as solid solution in silicates. Even Mg and Si, normally lithophile elements, are present in these chondrites partially in the form of sulfide (MgS, niningerite) and metal alloy (Si-bearing metal). (Fig. 7 of the paper shows that the Si content in metal alloy varies from ~10 to 35 mg/g or ~1-3.5 wt%, which is much higher than the <0.01 wt% typical in most meteorites.)
The distinctive mineralogy of enstatite chondrites is generally agreed to reflect the formation of these meteorites in a reducing, oxygen-poor environment. In this type of environment, the partial pressure of oxygen gas (expressed as oxygen fugacity or fO2) was very low. Under such low-fO2 conditions, most of the iron is present either as Fe-rich metal or as iron sulfides (such as FeS, troilite). Indeed, so little iron and calcium are present in pyroxene that the composition of the chief mineral in these chondrites approaches that of pure enstatite (close to En100).
Another noteworthy aspect of enstatite chondrites is that they have an achondrite (igneous) analogue-- the enstatite achondrites or aubrites. These meteorites resemble type 6 enstatite achondrites in many ways. Mainly, these two types of meteorites differ in Fe-metal content (little for enstatite achondrites) and grain size (coarser for enstatite chondrites). It seems likely that the aubrites formed by melting of an enstatite-chondrite-like precursor.
Instrumental neutron activation analysis (INAA) - This is a method of bulk geochemical analysis that relies on gamma ray emission from a sample that has been irradiated by neutrons in a nuclear reactor. The gamma ray spectra allow the abundance of different elements to be determined even for very small-mass samples (e.g., 200 mg). Determinations are usually made for selected trace elements (those present in low concentrations, such as <0.1 wt% or <1000 ppm) and selected major elements (>0.1 wt%). Sulfur and silicon are never analyzed by INAA. A complete major-analysis cannot be obtained with INAA.
Wet-chemical analysis -- This is the difficult and largely lost art of obtaining a complete bulk major-element analysis using a series of gravimetric techniques (e.g., weighing of precipitates following a dissolution step). With this technique one can determine not only the amount of total iron in the sample but also its form, whether as FeO (in silicate), FeS (in sulfide), or Fe-metal. A drawback is that large quantities (tens of grams) of sample are consumed.
Moderately-volatile element -- This is an element that can be considered to be fairly volatile (i.e. to prefer a gaseous state) but which is not "overly" or highly volatile. The volatility scale for elements can be quantified by something known as the 50% condensation temperature (Tc,50), which represents the temperature at which half of the element is in the gaseous state and half is in a condensed (solid or liquid) state. These Tc,50 values can be calculated from thermodynamic data if one specifies for the system the total pressure (usually 0.001-0.000001 bars, or atmospheres, corresponding to an estimate for the midplane of the solar nebula) and bulk composition (usually solar proportions of elements). A moderately-volatile-chalcophile element is one that has a fairly low Tc,50 value (e.g., <1000 K or <730 C) and is mainly present in the form of sulfides.
REE - rare-earth-element. This is the series of lanthanide elements from La to Lu in the 6th period (row) of the periodic table. The REE are normally lithophile elements, but in the enstatite chondrites they are largely bonded with S in the mineral oldhamite. This makes them chalcophile in these meteorites.
Solvus - this is a subsolidus relationship that shows for a particular temperature how the compositions of co-existing minerals vary in equilibrium.
Open-system vs. closed-system - This refers to whether components are free to move out of the system of interest (open system) or are not free to move (closed system). The more volatile the element, the more likely it is to form a gas, and the more likely it is to be in an open system. Sulfur is a fairly volatile element (Tc,50 ~ 650 K); oxygen is even more volatile.
Lithology - a particular mineral assemblage.
fS2 - This is the fugacity (partial pressure) of S2 gas, analogous to fO2 (see above).
Mesostasis/Mesostases - This is the fine-grained intersitial material between larger crystals.
Modal composition - The amount of various minerals
present.