# Hermiominerals

"By comparing NWA enstatite chondrite impact melts to Mercury, we infer that they represent imperfect petrological analogs to this planet given their high metal abundances, but they could represent important geochemical analogs for the behavior and geochemical affinities of elements on Mercury. Furthermore, the enstatite chondrite impact melts represent an important petrological analog for understanding high-temperature processes and impact processes on Mercury, due to their similar mineralogies, Fe-metal-rich and FeO-poor silicate abundances, and low oxygen fugacity."
"The highly reduced nature and mineralogy of enstatite-rich meteorites is similar to the 𝑓O $_2$ of Mercury estimated using data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft (McCubbin et al. 2012, 2017; Zolotov et al. 2013)."

## Aubrites

"Aubrites are a rare group of meteorites (only 30 classified at the time ...) that hold important implications for reducing magmatic systems in our solar system, and have also been suggested as a potential petrologic analog for rocks on Mercury (Burbine et al. 2002)."
"The equigranular texture of the enstatite and the presence of the metal surrounding enstatite indicate that [such] rocks were not formed through igneous processes like the aubrites, but rather by impact processes."
Aubrites are "also known as enstatite achondrites".
Aubrites "do not originate from melted enstatite chondrite parent bodies as shown by their mineralogical differences, including their lower Fe,Ni/troilite ratio, the higher abundances of Ti in troilite, and the presence of diopside and olivine in aubrites, which contrasts with enstatite chondrites (aside from a few exceptions) (Keil 1968; Watters and Prinz 1979; Grossman et al. 1985; Brett and Keil 1986; Kitamura et al. 1987; Okada et al. 1988; Crozaz et al. 2003; Fogel 2005; Lin et al. 2005; McCoy et al. 2018)."
"Formation on a differentiated parent body, such as that of the aubrites, would have effectively segregated metals from silicates, which is observed among aubrites, and is not what is observed in NWA 7214, NWA 7809, and NWA 11071.(2) The dominant silicate phase in the four NWA meteorites is enstatite. Enstatite is the only pyroxene present in all four samples, which is significantly different from what has been reported in aubrite samples. In addition to enstatite, aubrites have diopside (up to 20% in Norton County), as distinct grains and as exsolution lamellae in enstatite (Okada et al. 1988; Keil 1989; McCoy et al. 2018). The lack of diopside in the four investigated samples is in agreement with an affinity to enstatite chondrites, which lack diopside as well, although diopside has been reported in some enstatite chondrites (Kitamura et al. 1987; Floss et al. 2003). In addition, aubrites have forsteritic olivine with modal abundances in the range 0.3–10 vol % (Keil 2010). In particular, Norton County has up to 10 vol % olivine (Watters and Prinz 1979). In contrast, both enstatite chondrites and enstatite chondrite impact melts are typically devoid of olivine. Olivine is also entirely absent from NWA 4799, NWA 7214, NWA 7809, and NWA 11071, further supporting the notion that these samples are more akin to enstatite chondrites rather than aubrites."

## Enstatite-rich meteorites

"A unique characteristic of all enstatite-rich meteorites is their highly reduced conditions of formation resulting in their depletion in FeO."
"Enstatite-rich meteorites, including EH chondrites, EL chondrites, and aubrites, which have previously been shown to originate from at least three distinct parent bodies, show identical O isotopic compositions (Clayton et al. 1984; Newton et al. 2000; Miura et al. 2007; Barrat et al. 2016)."

## Enstatite chondrite impact melts

The "bulk compositions of enstatite chondrite impact melts and the compositions of any individual terrane on Mercury are likely a mismatch because lavas on Mercury formed from partial melting of a differentiated mantle, and enstatite chondrite impact melts have chondritic, undifferentiated bulk compositions."
The "bulk compositions of enstatite chondrite impact melts are a mismatch for the alkali-rich boninitic and komatiitic volcanic rocks on the surface of Mercury (e.g., Vander Kaaden et al. 2017)."
Enstatite "chondrite impact melts have exotic sulfide phases, graphite, albitic plagioclase, Fe-rich Si-bearing metal, and ferromagnesian silicates with low abundances of FeO, all of which are inferred to be present on the surface of Mercury (Nittler et al. 2011; Weider et al. 2012; Charlier et al. 2013; Zolotov et al. 2013; Namur et al. 2016a; Vander Kaaden and McCubbin 2016; McCubbin et al. 2017; Namur and Charlier 2017; Vander Kaaden et al. 2017; McCoy et al. 2018)."
"Although aubrites and enstatite chondrite impact melts both formed from melting processes, aubrites formed as a result of incipient melting due to radioactive heating, while the source of heating for impact melt rocks is linked to collisions."

## Mercury

"Mercury was shown to have formed under highly reducing conditions as well (McCubbin et al. 2012; Zolotov et al. 2013). The composition of Mercury was not well understood until the arrival of the MESSENGER spacecraft, which launched in 2004 (e.g., Nittler et al. 2011). It orbited Mercury from March 18, 2011, until it made its planned descent into the surface of the planet on April 30, 2015, with a primary goal of investigating the structure of the planet's interior and the unusual conditions under which its magma formed (Solomon et al. 2001). Data from the MESSENGER X-Ray Spectrometer (XRS) and the Gamma Ray and Neutron Spectrometer (GRNS) show that Mercury's surface exhibits a high-S abundance (up to 4 wt%), low-Ti (<0.8 wt%), and low-Fe abundances (<2 wt%) (Nittler et al. 2011; Evans et al. 2012; Starr et al. 2012; Weider et al. 2015). A unique characteristic of Mercury from a geochemical standpoint is the extremely low oxygen fugacity at its surface (IW−7.3 to IW−2.6) and within its interior (~IW−3) compared to Earth, the Moon, and Mars (McCubbin et al. 2012, 2017; Zolotov et al. 2013). Additionally, Mercury exhibits volatile element abundances that are similar to the Earth, as exemplified by K/Th, K/U, and K/Cl ratios (Peplowski et al. 2011, 2012; Evans et al. 2015)."

## Ilafegh 009

Ilafegh 009 is an enstatite chondrite, (EL7).
All "four meteorites [NWA 4799, NWA 7214, NWA 7809, and NWA 11071] are more akin to enstatite chondrite impact melts, such as Ilafegh 009, rather than aubrites based on prior studies of both meteorite groups (Keil 1968; Watters and Prinz 1979; Brett and Keil 1986; Bischoff et al. 1992; McCoy et al. 1995; Lin and Kimura 1998; Lin and El Goresy 2002)."
"The O isotopic compositions for the four NWA samples are within error of previously published O isotopic compositions (Clayton et al. 1984; Clayton and Mayeda 1996; Miura et al. 2007). However, the mean isotopic compositions are slightly heavier in δ$^{18}$ O and δ$^{17}$ O than previously reported enstatite chondrites, including impact melts, such as Ilafegh 009 (Clayton et al. 1984; McCoy et al. 1995; Newton et al. 2000) and aubrites (Clayton and Mayeda 1996; Miura et al. 2007; Barrat et al. 2016). Newton et al. (2000) reported a few enstatite chondrites with heavier O isotopic values such as Indarch (EH4), and Grein 002 (EL4-5). The NWA sample ∆$^{17}$ O compositions are slightly lighter than the other enstatite-rich meteorites, except the EH values from Newton et al. (2000)."
The "presence of plagioclase suggests that the NWA meteorites are not enstatite-rich partial melt residues, such as NWA 2526 and Itqiy (Patzer et al. 2001; Keil and Bischoff 2008). Troilite and metal inclusions in enstatite grains also indicate that these rocks were formed from complete melting similar to the Queen Alexandra Range (QUE) 94204 and paired impact melt rocks and Ilafegh 009, as indicated by troilite and metal inclusions in enstatite grains (McCoy et al. 1995; Van Niekerk et al. 2014)."
"The presence of troilite or enstatite surrounded by metal suggests that gravitational segregation of metal and sulfide did not occur, indicating rapid melting and subsequent cooling in the four investigated meteorites [...]. In addition, the texture of enstatite, which is euhedral and equigranular, in a matrix of plagioclase, metal, and sulfide in the four NWA meteorites [...], likely indicates superheating of the impact melts followed by rapid crystallization, which is evident for impact melting. Pyroxene size, especially in NWA 11071, is finer than what is usually observed in aubrites (up to 5 mm; Keil 2010). Similar textures were observed in the enstatite chondrite impact melt Ilafegh 009 and Happy Canyon (McCoy et al. 1992, 1995). Most enstatite chondrite impact melts consist of euhedral enstatite laths. Euhedral laths of enstatite compared to the subhedral enstatite observed [...] likely indicate a lower nucleation density and slower cooling rate than that of NWA samples, as well as Ilafegh 009 and Happy Canyon."
"The ungrouped meteorite Zaklodzie, which has an enstatite chondrite precursor, was shock melted at 4.50 Ga and the enstatite chondrite impact melt Ilafegh 009, which shows similar textures as the four NWA meteorites, has $^{39}$ Ar-$^{40}$ Ar ages of 4.34–4.44 Ga representing resetting during shock metamorphism (McCoy et al. 1995; Bogard et al. 2010)."

## NWA 4799

The "parent bodies have undergone impact events from at least 4.5 Ga (NWA 11071) until at least 4.2 Ga (NWA 4799) according to $^{39}$ Ar-$^{40}$ Ar ages, indicating that this region of the solar system was heavily bombarded early in its history."
"Eighty-six pieces, totaling 365 g, of NWA 4799 were found in Algeria in 2007."
"NWA 4799 [contains] euhedral-subhedral pyroxene with small troilite and terrestrial alteration of Fe,Ni metals."
NWA 4799 sample A contains 40 vol % enstatite, 14 vol % feldspar, 44 vol % Fe-rich weathering products, and 7.4 vol % pre-weathering metal.
NWA 4799 sample B contains 41 vol % enstatite, 16 vol % feldspar, 43 vol % Fe-rich weathering products, and 7.1 vol % pre-weathering metal.
NWA 4799 "shows high degrees of terrestrial weathering (43–44 vol % terrestrial alteration products), with alteration products consisting of hydrated iron oxides that surround nearly all grains [...]. Alteration products also consist of altered metallic Fe,Ni veins, up to 1.1 mm in width, that crosscut both investigated thin sections. Alteration zones measure up to ~4 mm in width [...]. [NWA] 4799 does not include metal, but we calculated ~7.1–7.4 vol % of pre-weathering metal (Van Niekerk et al. 2014). This sample showing an igneous texture, consists of 40–41 vol % of fine- to medium-grained (up to 1.1 mm) euhedral to subhedral and striated enstatite grains and 14–16 vol % interstitial plagioclase up to 250 μm in length [...]. [NWA] 4799 shows significantly lower abundances and a lesser diversity of sulfides compared to NWA 7214, NWA 7809, and NWA 11071. Anhedral minor daubréelite (FeCr $_2$S $_4$) measuring up to 200 μm in length and a single troilite (FeS) grain are the only sulfides observed in the studied sections. Graphite is also present in alteration veins. Bunch et al. (2008) previously reported observing kamacite [Fe,Ni metal], niningerite [MgS], perryite [(Ni,Fe) $_5$(Si,P) $_3$], brezinaite [Cr $_3$S $_4$], and schreibersite [(Fe,Ni) $_3$P] within NWA 4799; however, the two thin sections we have analyzed lack such minerals, likely due to biased sampling or terrestrial weathering."

## NWA 7325

"NWA 7325 is actually a group of 35 meteorite samples discovered in 2012 in Morocco. They are ancient, [...] dating the rocks to an age of about 4.56 billion years."
"NWA 7325 has a lower magnetic intensity — the magnetism passed from a cosmic body's magnetic field into a rock — than any other rock yet found. Data sent back from NASA's Messenger spacecraft currently in orbit around Mercury shows that the planet's low magnetism closely resembles that found in NWA 7325."
"NWA 7325 has olivine in it that is insanely magnesium-rich. Iron and magnesium are two elements that are almost always found together in rocks; the ions they make have the same size and charge so they happily occupy the same positions in crystal lattices. It's weird to have a rock that is so dominantly magnesium-rich. Mercury's surface rocks are known (thanks to MESSENGER) to be unusually low in iron."
"NWA 7325's oxygen isotope ratios do not match any known meteorites from any other planet-size body. In fact, they're not particularly similar to much of anything that we've measured oxygen isotope ratios for."
"The ratios of Al/Si (0.224) and Mg/Si (0.332) plus the very low Fe content of NWA 7325 are consistent with the compositions of surface rocks on Mercury, but the Ca/Si ratio (0.582) is far too high. However, since NWA 7325 is evidently a plagioclase cumulate (and presumably excavated from depth), it may not match surface rocks on its parent body. The abundance of diopside rather than enstatite might be consistent with some earlier spectral observations of Mercury [7]."
"[I]t's about 23 times harder to get a rock from Mercury to Earth than it is from Mars to Earth. Given that we've got more than 70 known Mars meteorites in our collections, that means we ought to have found 3 (give or take a couple) Mercury meteorites by now."
Northwest Africa 7325 is a unique igneous meteorite which crystallized as a basalt on a large asteroid or planetesimal approximately 4.56 billion years ago, classified as an ungrouped achondrite, and is notable for its green fusion crust and high-magnesium/low-iron composition. It was purchased from anonymous finders in a marketplace in Erfoud, Morocco in April 2012. The original find was composed of 35 fragments with a combined weight of approximately 345 grams (12.2 oz), however many additional fragments with a total weight of over 1,100 grams (39 oz) were subsequently recovered.
Investigation of the meteorite determined that the meteorite's composition might be consistent with that of Mercury as determined by the MESSENGER spacecraft. NWA 7325 could also have come from a smaller differentiated planetesimal. A different class of basaltic meteorites, angrites might be from Mercury.
Later studies cast doubt upon the meteorite's association with Mercury by comparing FTIR spectra obtained from the meteorite to astronomical results of the planet's surface; the observed differences suggest that the meteorite is not from Mercury. Additionally, NWA 7325's relatively early crystallization age suggests that it formed on a small body that cooled rapidly after accreting. Mercury is far larger and any samples from the planet's surface would likely be ~10^8 years younger.

## Forsterites

Mg$_2$SiO$_4$ is the formula for forsterite, with 28.6 at % magnesium.
Forsterite is a member of the olivine group of minerals.