Antarctic Meteorite Newsletter 29,1 - Petrographic Descriptions

Petrographic Descriptions Go to Pet. Desc. Page 1 of 2

Sample No.: LAP 031173;
LAP 04642

Location: LaPaz Ice Field

Field No.: 16105; 17406

Dimensions (cm): 1.5x1.0x0.5
1.0x1.0x0.5

Weight (g): 1.360; 1.196

Meteorite Type: H Chondrite (Impact Melt)

Macroscopic Description: Kathleen McBride
The exteriors are covered with brown/black fusion crust with oxidation halos. The interiors are rusty brown with a brittle texture.

Thin Section (,2) Description: Tim McCoy and Linda Welzenbach
These sections are so similar that a single description suffices. The meteorites exhibit a microcrystalline silicate matrix including rounded metal-sulfide blebs reaching 20 microns and relict grains, chondrules and clasts reaching up to a few mm. Mafic silicate compositions (Fa₁₉, Fs₁₇) indicate they are of H chondrite parentage and textures suggest they are impact melt breccias.

Cross-Polarized Light Plane-Polarized Light Reflected Light

Cross-Polarized Light Reflected Light

Sample No.:	LAP 031214
Location:	LaPaz Ice Field
Field No.:	16820
Dimensions (cm):	3.0x2.0x2.0
Weight (g):	9.504
Meteorite Type:	CM 1-2 Chondrite

Macroscopic Description: Kathleen McBride
Thick purple-black fusion crust covers 40% of the exterior. Polygonal fractures are visible. The interior is a dark gray to black matrix with an oxidation rind.

Thin Section (,2) Description: Tim McCoy and Linda Welzenbach
The sections consist of a few isolated mineral grains in a black matrix; rare metal and sulfide grains are present. Olivine compositions are Fa_1-41. The meteorite is a CM chondrite and is intermediate between type 1 and type 2.

Plane-Polarized Light Reflected Light Reflected Light

Sample No.:	LAP 031280
Location:	LaPaz Ice Field
Field No.:	16046
Dimensions (cm):	4.0x2.75x1.0
Weight (g):	17.752
Meteorite Type:	Eucrite (Brecciated)

Macroscopic Description: Kathleen McBride
Shiny, rough brown/black fusion crust covers 50% of this achondrite's exterior. The interior is a sandy textured, tan colored matrix with gray inclusions.

Thin Section (,2) Description: Tim McCoy and Linda Welzenbach
The meteorite is a dominated by a fine-grained (50-100 micron grain size) basaltic host with coarser-grained areas that may be clasts incorporated prior to metamorphism. Pyroxene is exsolved on a 1-20 micron scale. Orthopyroxene analyses range to Fs₆₀Wo₅ and augite to Fs₃₂Wo₄₀. Plagioclase is An₈₈Or_0.5. The Fe/Mn ratio of the pyroxene is ~30. The meteorite is a eucrite.

Cross-Polarized Light Plane-Polarized Light Cross-Polarized Light Plane-Polarized Light

Sample No.:	LAP 031323
Location:	LaPaz Ice Field
Field No.:	16647
Dimensions (cm):	1.5x1.5x1.0
Weight (g):	3.125
Meteorite Type:	Acapulcoite

Macroscopic Description: Kathleen McBride
50% of the exterior has brown/black fusion crust. The rusty, granular textured interior has a high metal content.

Thin Section (,2) Description: Tim McCoy and Linda Welzenbach
The section consists of an equigranular mixture of olivine (Fa₁₃), orthopyroxene (Fs₁₂Wo₂), clinopyroxene (Fs₅Wo₄₃), plagioclase (An₁₅Or₄), metal and troilite. Mafic grain sizes are ~100 microns, triple junctions are common and plagioclase is interstitial to and includes mafic silicates. The rock is an acapulcoite, but similar in texture to the intermediate members of the acapulcoite-lodranite clan EET 84302 and GRA 95209.

Cross-Polarized Light Plane-Polarized Light Cross-Polarized Light Plane-Polarized Light

Sample No.:	CMS 04016
Location:	Cumulus Hills
Field No.:	14674
Dimensions (cm):	5.0x4.0x3.0
Weight (g):	162.069
Meteorite Type:	L3 Chondrite

Macroscopic Description: Kathleen McBride
The exterior and interior of this meteorite is rusty with a high metal content.

Thin Section (,2) Description: Tim McCoy and Valerie Reynolds

Cross-Polarized Light	Plane-Polarized Light

The section exhibits numerous well-defined chondrules (up to 1.5 mm) in a black matrix of fine-grained silicates, metal and troilite. Weak shock effects are present. Polysynthetically twinned pyroxene is abundant. The meteorite is moderately weathered. Silicates are unequilibrated; olivines range from Fa_1-26, with most grains Fa_20-26, and pyroxenes from Fs_5-12. The meteorite is an L3 chondrite (estimated subtype 3.8).

Sample No.:	LAP 04840
Location:	LaPaz Ice Field
Field No.:	17198
Dimensions (cm):	5.0x3.0x1.75
Weight (g):	50.405
Meteorite Type:	R Chondrite

Macroscopic Description: Kathleen McBride
Rough, black fusion crust with polygonal fractures is present on the exterior surface. The interior is a fine grained crystalline matrix with black and white crystalline inclusions. The meteorite was very hard with no rusting or apparent metal.

Thin Section (,4) Description: Tim McCoy
The section is texturally heterogeneous, containing relict chondrules up to 1 mm, isolated mineral grains of 100-200 microns and microcrystalline areas reaching 1 mm with mafic silicate grain sizes of 5-10 microns. Shock effects are pervasive, particularly in plagioclase. The rock consists of FeO-rich olivine (Fa₃₈) and orthopyroxene (Fs₃₀Wo₁). Plagioclase is An₇Or₃. A brown, strongly pleochroic mineral comprises ~15% of the section. Microprobe analyses suggest hornblende with approximately 48% SiO₂, 3% Na₂O, 6% Al₂O₃, 16.5% MgO, 11% FeO, 10% CaO and < 0.5% TiO₂, MnO and K₂O. Low totals and the strong pleochroism may be due to structurally bound OH, as no Cl was noted in EDS analyses. Opaques include troilite, pentlandite and chromite. The meteorite is an R chondrite of petrologic type 6.

Oxygen isotope analysis laser fluorination: D. Rumble
The analysis of two silicate-rich fractions yielded δ¹⁸Ο=+4.28, δ¹⁷Ο=+4.37, and δ¹⁸Ο=+4.76, δ¹⁷Ο=+5.76, similar to other R-chondrites.

Cross-Polarized Light Plane-Polarized Light Cross-Polarized Light Plane-Polarized Light Reflected Light

Sample No.:	LAP 04843
Location:	LaPaz Ice Field
Field No.:	17006
Dimensions (cm):	5.0x5.0x2.5
Weight (g):	108.639
Meteorite Type:	CV3 Chondrite

Macroscopic Description: Kathleen McBride
The exterior has rough, black fusion crust with purple shiny areas. Vugs of plucked areas are visible. The muddy gray matrix has white clasts with evaporites. This meteorite was very hard.

Thin Section (,4) Description: Tim McCoy and Linda Welzenbach
The section exhibits large chondrules (up to 3 mm) and CAIs in a dark matrix. Olivines range from Fa_1-46 and a single pyroxene is Fs₁. The meteorite is an oxidized CV3.

Cross-Polarized Light	Plane-Polarized Light	Cross-Polarized Light	Plane-Polarized Light

Sample No.:	LAP 04844
Location:	LaPaz Ice Field
Field No.:	17074
Dimensions (cm):	2.0x2.0x1.5
Weight (g):	6.831
Meteorite Type:	Diogenite

Macroscopic Description: Kathleen McBride
The exterior has some patchy brown/black fusion crust. The tan to yellowish fine grained matrix has green and brown clasts of various sizes.

Thin Section (,2) Description: Tim McCoy and Linda Welzenbach

Cross-Polarized Light	Plane-Polarized Light

The section shows a groundmass of coarse (up to 1.5 mm) comminuted pyroxene. Orthopyroxene has a composition of Fs₂₃Wo₂. The meteorite is a diogenite and is probably paired with the LAP 91900 pairing group.

Sample No.:	MAC 04930
Location:	MacAlpine Hills
Field No.:	15086
Dimensions (cm):	0.5x0.7x0.4
Weight (g):	0.880
Meteorite Type:	H3 Chondrite

Macroscopic and Thin Section (,2) Description: Tim McCoy and Linda Welzenbach

Cross-Polarized Light	Plane-Polarized Light

This pea sized rusty ordinary chondrite exhibits numerous small chondrules (up to 1 mm) in a black matrix with irregular metal and troilite. The meteorite is highly weathered. Silicates are unequilibrated; olivines range from Fa_2-24 and pyroxenes from Fs_3-12. The meteorite is an H3 chondrite (estimated subtype 3.6).

Sample No.:	MAC 041169
Location:	MacAlpine Hills
Field No.:	15371
Dimensions (cm):	1.75x1.5x0.75
Weight (g):	3.187
Meteorite Type:	Eucrite (Unbrecciated)

Macroscopic Description: Kathleen McBride
40% rough, black fusion crust covers the exterior of this meteorite. The interior is a coarse grained black and white matrix with rusty areas.

Thin Section (, 3) Description: Tim McCoy

Cross-Polarized Light	Plane-Polarized Light

The meteorite is a medium-grained (100-200 micron grain size), unbrecciated basalt with numerous late-stage interstitial melt pockets that can reach several hundreds of microns. Mineral compositions are relatively homogeneous with orthopyroxene (Fs_57-64Wo_2-5), with 1-5 micron lamellae of augite (Fs₃₂Wo₃₉), and plagioclase (An₈₉Or_0.5). The Fe/Mn ratio of the pyroxene is ~30. The meteorite is an unbrecciated eucrite.

Sample No.:	MAC 041269
Location:	MacAlpine Hills
Field No.:	15820
Dimensions (cm):	2.0x1.5x1.0
Weight (g):	4.509
Meteorite Type:	Howardite

Macroscopic Description: Kathleen McBride
50% of the exterior surface is covered with black shiny (like glass) fusion crust. The interior is a black and gray, fine grained matrix with black and white inclusions. Very minor amount of rust is present.

Thin Section (,2) Description: Tim McCoy and Linda Welzenbach

Cross-Polarized Light	Plane-Polarized Light

The section shows a groundmass of comminuted pyroxene and plagioclase (up to 0.5 mm) with fine- to coarse-grained basaltic clasts ranging up to 5 mm. Pyroxene ranges from pigeonite composition (possibly finely exsolved) of Fs₅₆Wo₆ to augite of Fs₂₈Wo₄₀. Plagioclase is An₈₉Or_0.5. The meteorite is a howardite.

Go to Pet. Desc. Page 1 of 2

Sample No.:	LAP 031173; LAP 04642
Location:	LaPaz Ice Field
Field No.:	16105; 17406
Dimensions (cm):	1.5x1.0x0.5 1.0x1.0x0.5
Weight (g):	1.360; 1.196
Meteorite Type:	H Chondrite (Impact Melt)