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LUN A – Anorthositic Highland Breccias
Synonyms: feldspatic breccias, lunar highland breccias
General: The meteorites of this group are designated LUN A for their lunar origin, and their anorthositic (feldspatic) compositions. Originally, they were coarse-grained plutonic rocks, forming the ancient highlands that dominate both sides of the Moon. Over time, these rocks were thoroughly granulated, heavily brecciated, and partially melted by impacts, forming the anorthositic breccias we know today.
Description: Light to dark grey rocks with a heavily brecciated interior, usually revealing a typical breccia-in-breccia texture, and angular white clasts. Due to long terrestrial residence time, and a pronounced weathering many of these lunaites are devoid of crust. On more fresh samples, the fusion crust is olive-green, and often vesiculated.
Mineralogy: As can be derived from their name, all members of this group are anorthositic breccias, consisting primarily of Ca-rich plagioclase, i.e., anorthositic clasts. Other clasts typically include impact-melts, and lithic clasts of troctolitic, noritic, or gabbro-noritic compositions. Lunar anorthositic highland breccias are subdivided into different types of breccias, such as regolith breccias, fragmental breccias, granulitic breccias, or impact-melt breccias – designations bearing witness to the complex formation histories of these highly evolved rocks.
Formation history: As stated above, all lunar anorthositic breccias were originally derived from the coarse-grained plutonites forming the ancient lunar surface, and large parts of the terrains that are now known as the lunar highlands. Over billions of years, these rocks were thoroughly granulated, brecciated, and partially melted by impacts, leading to the formation of the major subtypes of lunar anorthositic rocks. The lunar surface is covered by a thick regolith layer, and consequently, most lunaites are regolith breccias. These rocks consist of abundant white clasts of anorthositic rocks and minor dark clasts of highland basalts, combined with various mineral and glass fragments, and mixed with a dark matrix of solidified rock powder. Impact-melt breccias display marks of severe shock-metamorphism, partial melting, and recrystallization, suggesting that they are products of larger impact events. The Fragmental breccias are composed of anorthositic rock fragments and other, mostly felsic clasts set in a fine-grained matrix of pyroxene and olivine. They lack regolith components, and represent deeper layers of the lunar surface.
Origin: Planetary. Comparisons to the lunar samples that have been returned by the Apollo and Luna missions have shown that the meteorites of this group are genuine samples of the Earth’s satellite, the Moon. Have a look at our introductory section about the LUN group for further details. All lunar anorthositic breccias were derived of the lunar highlands, and most are thought to have their origin on the far side of the Moon, dominated by highland terrains.
Members: The majority of all known lunar meteorites belong to this group, comprising about 20 distinct members. Typical regolith breccias, available to the private collectors market, are the first Saharan lunaite DaG 262, or Dhofar 025 from Oman. Impact-melt breccias include the largest lunaite found so far, DaG 400, and the beautiful NWA 482, the only white lunaite in our collections. Fragmental breccias are rare, and represented by just a few Antarctic finds, Dhofar 280, and its respective pairings, as well as by the small DaG 996 from Libya. But the rarest members of the LUN A group are the granulitic breccias with only two members known, Dhofar 733, and Dhofar 026. Our own team recovered 12 stones paired to the latter find, Dhofar 457-468, and we offer selected samples of this most intriguing, and very fresh lunar granulitic breccia for sale.