* Astronomy

Title: Mineralogy and Shock Textures in the Padvarninkai Eucrite
Authors: Yamaguchi, A.; Mori, H.; Takeda, H.

The Padvarninkai eucrite was found in Androniski, Lithuania. This eucrite is unique in that it was once identified as shergottite because of the presence of maskelynites [1]. Mason et al. [2] classified it as a pyroxene-plagioclase achondrite (eucrite). However, Padvarninkai has not been previously studied in detail. To compare this shock texture with those of experimentally shocked synthetic eucrites, we investigated the mineralogy of a new specimen of Padvarninkai (19.6 g) by optical microscope, EPMA, SEM equipped with EDS. The host material of Padvarninkai has an ophitic texture and is heavily shocked. Hand specimen observation indicates that this rock consists of two lithologies; an unbrecciated grey host and suevitic black veins (<0.5 cm thick). Most plagioclases were converted into maskelynites. Some of the plagioclases display extensive mottled extinctions when viewed in cross polarizers. Pigeonite is severely fractured and mosaicked. In spite of the shock textures of the minerals, the igneous texture of the host is well preserved. The veins consist of tiny fragmented pyroxenes, deformed plagioclases (glassy phase?), impact melt, opaque minerals, and many vesicles (<0.2 mm). Fine droplet-shaped troilites (<10 micrometers) were often observed in the veins. Plagioclases and mesostases near the vein are often deformed. The boundary between some minerals and melt show crystallization of fine (<5 micrometers) crystals. The veins are not totally glass phases. Some areas are partly devitrified, and most areas are substantially crystalline (<1 micrometer). In the SEM, the glassy zones show flow texture, which indicates heterogeneous compositions. Most compositions of the veins fall along the pyroxene-plagioclase line in the silica-olivine-anorthite pseudoternary system [3], which indicate incomplete mixing of the melt or presence of fine remnant minerals as observed in the SEM. The mineral assemblage of this eucrite, which is composed of pyroxene, plagioclase, and minor minerals such as ilmenite, chromite, troilite, silica mineral, and Ca-phosphate, is compatible with other ordinary eucrites such as Juvinas. Pyroxene has (001) exsolution lamellae (<10 micrometers thick), and its compositions fall along a single tie line in the pyroxene quadrilateral ranging from Ca1Mg37Fe62 to Ca43Fe31Fe26 (bulk composition: Ca9Mg36Fe55). Clouding of pyroxenes is minor and less than other monomict eucrites. Plagioclases are zoned from An81 to An95. Recrystallized mesostasis consists of silica, Na-rich plagioclase (An81-90), fine pyroxene, troilite, ilmenite, and Ca-phosphate. The compositions of ilmenite (Ti7.7-8.0Fe7.2-7.7Mn0.1- 0.2Mg0.2Ca0-0.1O24) and chromite (Fe6.0-9.0Mg0.1Mn0.1Cr4.8-8.4Al0.7-2.9Ti0.5- 3.0O24) are also within the ranges of ordinary eucrites. The mineralogy indicates that Padvarninkai is a ordinary monomict eucrite with homogenized pyroxenes of type 5 [4], despite preservation of the plagioclase zoning. The ophitic texture suggests that the initial cooling rate is slower among the ordinary eucrites. The shock features are unusual compared to other ordinary eucrites. Although impact melted veins are sometimes observed in other eucrites [5], the presence of maskelynites in the host is very rare. The presence of suevitic vein suggests that the rock has been melted by shock heating. The presence of the maskelynite in the unbrecciated host suggests that Padvarninkai suffered from intense shock effect. Padvarninkai is the most highly shocked eucrite. An experimentally shocked eucrite shows similar texture at <47 GPa [6]. According to Bischoff and Stoffler [5], estimated equilibrium shock pressure is from -42-45 to -60 GPa. After the primary crystallization of Padvarninkai, this eucrite suffered from thermal annealing. The presence of glassy phase (maskelynite and glassy vein) such as observed in shergottites and lunar meteorites indicate that Padvarninkai was shocked by the final cratering event on the surface of the parent body, which may have ejected the mass of this eucrite into space.  

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