# Selenominerals

Def. "a meteorite that is known to have originated on the Moon" is called a lunar meteorite, or perhaps a selenometeorite.
About 371 lunar meteorites have been discovered so far (as of July, 2019), perhaps representing more than 30 separate meteorite falls (i.e., many of the stones are "paired" fragments of the same meteoroid). The total mass is more than 190 kilograms (420 lb).
All lunar meteorites have been found in deserts; most have been found in Antarctica, northern Africa, and the Sultanate of Oman, but none have yet been found in North America, South America, or Europe.
Lunar origin is established by comparing the mineralogy, the chemical composition, and the isotopic composition between meteorites and samples from the Moon collected by the Apollo missions.
Cosmic ray exposure history established with noble gas measurements have shown that all lunar meteorites were ejected from the Moon in the past 20 million years. Most left the Moon in the past 100,000 years.
All six of the Apollo missions on which samples were collected landed in the central nearside of the Moon, an area that has subsequently been shown to be geochemically anomalous by the Lunar Prospector mission. In contrast, the numerous lunar meteorites are likely to be random samples of the Moon and consequently provide a more representative sampling of the lunar surface than the Apollo samples. Half the lunar meteorites, for example, likely sample material from the farside of the Moon.
So far seifertite has only been found in Martian and lunar meteorites.
"A felsite clast in lunar breccia Apollo sample 14321 [arrowed in the image on the left], which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta [...] indicates a pre-impact depth of 30–70 km, i.e. near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce$^⁎$ anomalies corresponding to an oxygen fugacity +2 to +4 log units higher than that of the lunar mantle, with crystallization temperatures of 771 ± 88 to 810 ± 37 °C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9 ± 1.2 kbar, corresponding to a depth of crystallization of 167 ± 27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO$_2$, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas."
"The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin."
The "felsite and its zircon crystallized on Earth at a modest depth of 19 ± 3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia."

## Allan Hills 81005

The meteorite called Allan Hills 81005 resembled some rocks brought back from the Moon by the Apollo program.

## NWA5000

NWA 5000 was found in 1999 with a mass of 11,500 gm composed of feldspathic breccia.
NWA5000, the largest known lunar meteorite, was found in southern Morocco in the Sahara desert in 2007.
Physical characteristics: "A single, large cuboidal stone (11.528 kg) with approximate dimensions 27 cm × 24 cm × 20 cm. One side (which appears to have been embedded downward in light brown mud) has preserved regmaglypts and is partially covered by translucent, pale greenish fusion crust with fine contraction cracks. Abundant large beige to white, coarse-grained clasts up to 8 cm across (some of which have been eroded out on exterior surfaces of the stone, likely by eolian sand blasting) and sparse black, vitreous clasts up to 2 cm across (containing irregular small white inclusions) are set in a dark gray to black, partially glassy breccia matrix. One partially eroded clast exposed on an exterior surface contains both the coarse grained beige lithology and the more resistant black, vitreous lithology in sharp contact."
Petrography: "Almost monomict fragmental breccia dominated by Mg-suite olivine gabbro clasts consisting predominantly of coarse-grained (0.5-2 mm) calcic plagioclase, pigeonite (some with fine exsolution lamellae), and olivine with accessory merrillite, Mg-bearing ilmenite, Ti-bearing chromite, baddeleyite, rare zirconolite, silica polymorph, K-feldspar, kamacite, and troilite. Some gabbro clasts have shock injection veins composed mostly of glass containing myriad fine troilite blebs and engulfed mineral fragments. Black, vitreous impact melt clasts consist of sporadic, small angular fragments (apparently surviving relics) of gabbro and related mineral phases in a very fine grained, non-vesicular, ophitic-textured matrix of pigeonite laths (up to 20 microns long × 2 microns wide) and interstitial plagioclase with tiny spherical grains of kamacite, irregular grains of schreibersite and rare troilite."
Geochemistry: "Gabbro clasts: plagioclase (An $_{(96.1-98.0)}$Or $_{(<0.1)}$), pigeonite (Fs $_{(32.0-64.5)}$Wo $_{(6.7-13.1)}$; FeO/MnO = 51.1-62.0), olivine in different clasts range from Fa $_{(23.9-24.2)}$, Fa $_{40.4}$ to Fa $_{58.8}$ (with FeO/MnO = 81-100), chromite [(Cr/(Cr + Al) = 0.737, Mg/(Mg + Fe) = 0.231, TiO $_2$ = 5.9 wt%], ilmenite (4.1 wt% MgO)."
Bulk composition: National Institute of Archaeology and Art "INAA of 6 subsamples gave mean values of 5.3 wt% FeO and 0.4 ppm Th."
Classification: "Achondrite (lunar, feldspathic breccia)."

## Yamato 791197

Yamato 791197 is another lunar meteorite.

## Pigeonites

In the image on the right is a polarized light microscope image of part of a grain of orthopyroxene containing exsolution lamellae of augite from the Bushveld intrusion. The texture documents a multistage history: (1) crystallization of twinned pigeonite, followed by exsolution of augite; (2) breakdown of pigeonite to orthopyroxene plus augite; (3) exsolution of augite parallel to the former twin plane of pigeonite.
Formula: (Ca,Mg,Fe)(Mg,Fe)Si $_2$O $_6$. The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.
Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group.
Pigeonite is found as phenocrysts in volcanic rocks on Earth and as crystals in meteorites from Mars and the Moon. In slowly cooled intrusive igneous rocks, pigeonite is rarely preserved. Slow cooling gives the calcium the necessary time to separate itself from the structure to form exsolution lamellae of calcic clinopyroxene, leaving no pigeonite present. Textural evidence of its breakdown to orthopyroxene plus augite may be present, as shown in the accompanying microscopic image.
Pigeonite is named for its type locality on Lake Superior's shores at Pigeon Point, Minnesota, United States. It was first described in 1900.

## Seifertites

Seifertite is a silicate mineral with the formula SiO $_2$ and is one of the densest polymorphs of silica.
X-ray diffraction reveals that the mineral has a scrutinyite (α-PbO $_2$) type structure with an orthorhombic symmetry and Pbcn or Pb2n space group. Its lattice constants a = 4.097, b = 5.0462, c = 4.4946, Z = 4 correspond to the density of 4.294 g/cm$^3$, which is among the highest for any forms of silica (for example, the density of quartz is 2.65 g/cm$^3$). Only stishovite has a comparable density of about 4.287 g/cm$^3$.

## Hypotheses

* Passage through the Earth's magnetic field and natural electric field of magnetic meteors may cause deflection and a slowing down during flight such that collision with the Earth does not create a crater.