DAG 868
Class
:
Ureilite
(unshocked diamond rich)
Ureilites containing diamonds were always thought to have been formed by impact shocks.
Dar al Gani 868 Libya
A new type of Ureilite ( DaG 868 ) has been discovered and was published now.
The features of DaG 868 seem to suggest that reconsiderations about the origin and building history of ureilites are necessary now: The Ureilite is containing fine-grained diamonds but olivines appear to be unshocked (show no evidence of undulose extinction!) - Graphite found in pigeonite crystals in DaG 868 however contain diamonds! This suggests that the diamonds have not(!) been formed by shock impact or chemical vapor deposition and raises major questions about building history of ureilites. Catalytic transformation of graphite to diamond will be a mechanism to be considered for this new ureilite type. Such DaG 868 falls in an own group of “ weakly shocked diamonds “with” the presence of diamonds” with ALH 78019 and 78262.
A 40.03 g stone was found on 13th of April of 2000 by anonymous finder in DaG in the Central Saharan desert.
Classification and mineralogy (H. Takeda,
Chiba and M. Ohtsuki UTok): the stone has a typical texture
for ureilites consisting of anhedral to subhedral olivine and pigeonite
(0.9 – 3 mm across); carbonaceous material forms rims
around some grains or, in some cases, veins; one plate of graphite
occurs within a large pigeonite crystal;
olivines appear to be unshocked compared
to most ureilites, and have homogeneous cores (Fa20.6) and reduced rims (Fa8) with high CaO contents (0.25-0.38 wt.%); pigeonite, Fs16.8Wo7.5. X-ray diffraction and reflected light microscopy (T. Nakamura and T. Nakamuta, KyuU) confirm the presence of fine-grained diamonds in a graphite lath. Specimens: main mass with anonymous finder; type specimens, 6.33 g, NSMT; 1.8 g,
KyuU.
The section shows an aggregate of anhedral to subhedral grains (0.9-3 mm across) of olivine and pyroxene. The largest olivine crystal reaches up to 5.8×3.2 mm. Some grains are rimmed with carbonaceous material, but carbon veins distributed in more restricted regions. Trace amounts of nickel-iron originally present have been largely weathered to limonite.
Microprobe analyses (H. Takeda, Chiba Inst. of Tech.; T. Ishii and M. Ohtsuki, Ocean Res. Inst., Univ. of Tokyo) show olivine of uniform core composition (Fa20.6) with reverse zoning towards more Mg-rich rims (Fa8), and with notable high CaO content (0.25-0.38 wt. %). The pyroxene has pigeonitic composition Wo7.5En75.7Fs16.8. A plate of graphite was grown within a large pyroxene crystal. The meteorite is a ureilite. Olivine grains do not show undulose extinction and appears to be nearly unshocked compared to most ureilites. However, the presence of fine-grained diamond in a lath of graphite has been confirmed by reflected light optical observation and X-ray diffraction method (T. Nakamura and T. Nakamuta, Univ. of Kyusyu). Specimens; main mass, at anonymous finder; type specimen, 6.33 g National Sci. Museum, Tokyo; 1.8 g Kyusyu Univ..
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