[meteorite-list] Pallasite vs. Mesosiderite

From: Michael Gilmer <michael_w_gilmer_at_meteoritecentral.com>
Date: Sat, 3 Jan 2009 09:25:11 -0800 (PST)
Message-ID: <337924.64992.qm_at_web58407.mail.re3.yahoo.com>

Elton,

Well said. That is a very coherent explanation of the differences
between the two. With your permission only, I'd like to repost this
on the Cloudy Nights meteorite list. If this is OK with you, email
me offlist. :)

Best regards and clear skies,

MikeG


.........................................................
Michael Gilmer (Louisiana, USA)
Member of the Meteoritical Society.
Member of the Bayou Region Stargazers Network.
Websites - http://www.galactic-stone.com and http://www.glassthrower.com
MySpace - http://www.myspace.com/fine_meteorites_4_sale
..........................................................


Message: 2
Date: Fri, 2 Jan 2009 23:03:39 -0800 (PST)
From: Mr EMan <mstreman53 at yahoo.com>
Subject: [meteorite-list] Pallasite vs. Mesosiderite
To: metlist <meteorite-list at meteoritecentral.com>
Message-ID: <262076.5974.qm at web55208.mail.re4.yahoo.com>
Content-Type: text/plain; charset=us-ascii

This is a lost draft regarding the question about pallesites vs mesosiderites in the for what it is worth category.

These two types of meteorites were trasditionally lumped together for a very long long time as "stony-irons". We know now they have little in common chemically nor in point of origin.

Two examples that can appear similar are Huckitta; a pallasite and Vaca Muerta; a mesosiderite. Given that they were widely available stony irons that happened to superficially resemble each other owing to extensive weathering. This perpetuated the misconception that they were related, IMO.

Pallasites contain the mineral assemblage magnesium iron silicate with the formula (Mg,Fe)2SiO4.aka olivine wich is a mixture of the minerals forresterite and fayalite. In fact a pallasite vs mesosiderite distinction is the silicate in pallasites are olivine (Mg,Fe)2SiO4, where the mesosiderite's pyroxene silicates are mostly XY(Si,Al)2O6

I do not know for sure I am fairly certain that notuing more than a trace of olivine--neither massive or "pallasite-like" olivine crystals have been found in mesosiderites.

All mesosiderites have been linked to a single parent body totally disrupted both with extensively intermixed strata. Mesosiderites are an assemblage of iron, eucrite, diogenite, impact melt breccia, howardite soup--you name it. By disrupted I mean really comingled with lots of surface and mantle material shot through the center and into the other side, flash melting, biblical porportions of chaos crumbled up silicates with iron chunks for flavor mixing.

Pallasites represent a less mixed, none-the-less "disrupted" parent body-- specifically, material from the mantle core boundary*. The olivine within the metal portions was emplaced in the iron via a yet to be confirmed process. Possible via a cumulate condensation or a impact imparted sloshing of a magma chamber in contact with the molten core. Because there are Widmanstatten patterns in pallasites we know that whatever the process, it wasn't the one that excavated the meteorites from the core of the parent body because rapid cooling would not allow those patterns to form.

Some recent finds show entire sections void of olivine crystals all together ( hence "siderite" portions). I haven't looked it up recently but unlike a single parent in mesosiderites there are 7-13 separate parent bodies sampled in the worlds sampling of pallasites.

AS mentioned before, very weathered pallesites can supreficially resemble mesosiderites such as is the case with Huckitta. Almost everyone owns some Huckitta which dosen't look at all like a typical pallasite. Much of the Huckitta in private collections is not the pristeen olivine metal mix we know so well but oxiadized metal and hydrated silicates. It looks like a meso in many appearance respects. Couple this with the availability of Vaca Muerta and in absence of anything else for comparrison and it is easy to see why anyone would assume they were very related.

Elton
* there is a theory about iron pools/pockets not at the core of asteroids but those are hard to explain rotationaly etc.





      
Received on Sat 03 Jan 2009 12:25:11 PM PST