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Angrites

The achondrites of this group are named for their type specimen, Angra dos Reis, a meteorite that fell in Rio de Janeiro, Brazil, in early 1869. Angra dos Reis is a unique igneous rock that is largely composed of the rare pyroxene fassaite, an aluminium-titanium diopside. During the last two decades, a handful of similar fassaite-rich meteorites have been recovered, leading to the establishment of the angrite group, presently comprising six members.

The angrites are primarily composed of varying amounts of fassaitic pyroxene, anorthitic plagioclase, minor olivine, kirschsteinite, along with other accessory minerals and phases. They are basaltic rocks with cumulate textures, often containing porous areas and abundant round vesicles with diameters up to 2.5 cm. These vesicles have been interpreted as remnants of gas-bubbles that formed prior to the crystallization of the rock. However, current research suggests that the vesicles originally were solid spheres that have been exsolved in subsequent stages of rock-formation. Both theories are consistent with a magmatic origin of the angrites, making them the most ancient igneous rocks known. They show crystallization ages of ~ 4.55 billion years, which suggests their formation occurred in the early days of the unfolding solar system. The angrites are thought to have formed on one of the earliest differentiated asteroids from the igneous processing of CAI-rich chondritic matter, similar to carbonaceous chondrites of the CI or CM group.

By comparing the reflectance spectra of the angrites to that of several main belt asteroids, two analogs were identified - 289 Nenetta, and 3819 Robinson. Further research will determine whether one of these asteroids actually represents the angrite parent body. Only three angrites are currently available to the private collector; these are Sahara 99555, a single stone that was found in the Sahara desert in 1999,  D'Orbigny, an Argentinian find from 1979 that wasn't recognized as a meteorite until 1998 and NWA 1670 with a total weight of only 30gr.... 

 

D'Orbigny Angrite

D’Orbigny, Buenos Aires Province, Argentina 

Found in 1998

TKW : 16.5 kg 

 


contact us

 

 

D'orbigny 00

incredible slice with many bubbles, druses filled of crystals and fusion crust.... no comments !!

70.43gr

Price on request

 

D'orbigny 01

crusted end cut with many bubbles

71.80gr

SOLD           

D'orbigny 02

Thin slice with bubbles and druses

9.30gr 

 SOLD

D'orbigny 03

Broken fragment

0.38gr

SOLD

D'orbigny 04

Very nice slice !

0.344 gr

SOLD

D'orbigny 05

Small endcut with a patch of crust !

0.318gr

SOLD

D’ORBIGNY: A NEW WINDOW INTO ANGRITE GENESIS*. G. Kurat1, M. E. Varela2, F. Brandstätter1, E. Wäsch3 and M. A. Nazarov4, 1Naturhistorisches Museum, Postfach 417, A-1014 Vienna, Austria (gero.kurat@univie.ac.at); ² Dept. de Geologia, Universidad Nacional del Sur, 8000 Bahia Blanca, Argentina (evarela@criba.edu.ar); 3Museum für Naturkunde, Humboldt-Universität Berlin, D-10115 Berlin, Germany (elke.waesch@rz.hu-berlin.de); 4Vernadsky Institute of Geochemistry and Analytical Chemistry, Kosygin Str. 19, Moscow, Russia (nazarov@geokhi.ru). In memory of Marty Prinz who was supposed to provide us with his expertise but was not permitted to do so.

Introduction: D’Orbigny, Buenos Aires Province, Argentina, is the sixth and by far the largest angrite known. Its bulk chemical and mineral chemical compositions, rare gas abundances and oxygen and rare gas isotope compositions fit the compositional ranges known from angrites [1]. D’Orbigny, however, is peculiar with respect to three features: the abundance of shelled hollow spheres, the presence of abundant open druses and the abundant presence of glasses [1, 2]. Thanks to the co-operation of the owner of D’Orbigny we had the opportunity to investigate in detail and in an unusual way this unusual rock. Here is a brief preliminary report.

Shape of the stone: D’Orbigny, as it was found, had a somewhat unusual shape (Fig.1). As a typical oriented-flight stone it had a front shield (~ 35 cm long) which gently sloped from the stagnation center and which was covered by regmaglypts and a dark brown fusion crust. The back side had a much smaller diameter (~ 20 cm) and consisted of a concave, round, pan-like indentation with a marked large (2 cm) and several smaller, round open vugs. It was also covered by fusion crust, much less than the front side, and had also less developed regmaglypts. The front and back shields were curved in a semi-parallel way and were intergrown on one side with the opposite side opening like a clam. The space between the plates was filled by a highly porous lithology, very rich in open druses and hollow spheres, partly filled by caliche.

Textures and structure: In an attempt to avoid too much contamination we decided to part the meteorite by breaking. Hammer with or without chisel turned out not to be suitable because the porous portion of the rock was physically week and tended to crumble. Consequently, the parting was done with a heavy hydraulic press, the smaller parting with a laboratory hydraulic press. The front and back shield lithology is a dense, medium to coarse–grained, sub-ophitic basaltic rock (see Fig. in [1]. It consists mainly of anorthite, augite and olivine. Anorthite forms hollow and hopper – like plates which enclose olivine and augite and which are intergrown with olivine and partly enclosed by large augite crystals. Olivine and augite are compositionally zoned from Fo20 to kirschsteinite and from En27Fs22Wo51 to En1Fs47Wo53, respectively [1]. Thisshield lithology contains also occasional hollow spheres ("vugs", see below), small, almost closed druses with augite and anorthite crystals and some large (cm) olivine xenocrysts(?). The hollow spheres are unevenly distributed throughout D’Orbigny, present in all parts, but most common in the porous inner portion. They are between a few mm and 2.5 cm in diameter. The shell is a dense granular intergrowth of fine-grained anorthite and olivine with occasionally some augite at the outside (Fig. 2). The grains tend to be oriented perpendicular to the surface. The inner side of the shell is fairly smooth but has a roughness suggestive of replicating a former solid filling with granular texture (shell fills former indentations at grain boundaries, see Fig.1 in [2]). The surface is commonly lined with a thin cover of sulphides. Druses are omnipresent in the porous portion of D’Orbigny. They are irregular, open spaces up to 3+ cm in elongation into which perfectly crystallized augites of prismatic habit and anorthite plates protrude. Druses border to walls consisting of a coarse-grained, porous, ophitic basaltic rock rich in anorthite and augite and poor in olivine. Druses commonly also border to shells of hollow spheres with the protruding crystals originating at the shell’s surface (Fig.3). The porous part of D’Orbigny contains many large olivine crystals and also some polycrystalline olivinite (Fig.5). The shapes of these olivinites are very complex and rather suggestive of irregular pore space than of fragmental shapes. Also common in the porous part are glasses filling in part druse space, interstitial space between olivine, augite, or anorthite grains and filling former hollow spheres [2].

Discussion and conclusion: The shape of D’Orbigny, its structure with highly porous lithologies alternating with denser ones and its mineralogical heterogeneity strongly suggest that the rock was not formed by a simple crystallisation of a basaltic melt. The mushroom – like aspect of D’Orbigny and the alternating texturally different layers are suggestive of a directional growth structure rather than of an igneous or metamorphic rock fragment. The abundantly present druses give clear evidence for a pneumatolytic process and the gradations from druses to less porous, coarsegrained, ophitic basaltic rock suggest growth of the whole entity under similar conditions. The hollow spheres appear to be the oldest building unit. Solid spheres of unknown composition (we may speculate on CaS or nitrides) were covered by anorthite + olivine at the earliest stage of rock formation. The solid core of the spheres was subsequently lost. It is possible that this phase became unstable, presumably during the late, highly oxidizing event that created

Lunar and Planetary Science XXXII (2001) 1737.pdf D'ORBIGNY: GENESIS OF ANGRITES: G. KURAT et al. the Fe-augite and kirschsteinite rims. The decomposing phase may also have supplied the Ca necessary for the formation of kirschsteinite. The latest event appears to have been the precipitation of glasses into some of the open pore spaces. The origin of the glass and the fact that some spaces were filled, but others not, remains a mystery, as does the origin of olivinite, which likely was formed at an early developing stage, before the very strong Fe-Ca metasomatism occurred. Plenty of work remains to be done.

References: [1] Kurat G et al., this volume; [2] Varela M.E et al., this volume.

Acknowledgements: We thank the owner of the meteorite for his co-operation. The study was supported by FWF in Austria ( P-13975-GEO, G.K., P.I.), the Austrian Academy of Sciences and CONICET, Argentina.

Lunar and Planetary Science XXXII (2001) 1737.pdf

 

 

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