Primitive Achondrites  Guidelines

Acapulcoites  Chart
Acapulcoites have average grain sizes of 150 to 230μm with Rare  relict chondrules .
Rare but easily seen grains of chrome-diopside are found in Acapulcoites and Lodranites .
Metal FeNi 5 to 14 %
Triolite  3 to 6 %
Olivine Fo95.8  20 to 30%
Orthopyroxene En93.5-87.4  40%  (Ca-poor pryoxene )
Plagioclase An15 to 12
Ca-pyroxene Cr-diopside
Kamacite is ubiquitous in all but the most reduced specimens with olivine Fo95.8
Oxygen Isotopic tests
Acapulcoites and Lodranites share many similarites including I/O but
as you can see grain size is quit different !
Acapulcoites were heated above the Fe,Ni-FeS eutectic
But below the silicate solidus


Brachinites   Chart 
 
Brachinites have an average grain size of 200 to 1000μm but with NO relict chondrules !
Olivine Fo65-70
Orthopyroxene En74-72  (Ca-poor pryoxene ) ONLY TRACE AMOUNTS !!
Ca-pyroxene Cr-augite
Metal FeNi ONLY TRACE AMOUNTS and the Ni portion 19-55 wt%
Note the sample of Eagles Nest a 1.2g fragment is slightly magnetic !
Brachinites have a granular fabric dominated by olivine grains that meet in triple junctions
which rarely may be prismatic and display preferred orientation .
Oxygen Isotopic tests 

 

Lodranites  Chart
 Lodranites have an average grain size of 340 to 700μm but with NO relict chondrules !
Rare but easily seen grains of chrome-diopside are found in Acapulcoites and Lodranites . 
Metal FeNi 0.2 to 20 %
Triolite 0.1 to 5.3%
Olivine Fo97-87  83 to 30%
Orthopyroxene En96-86  6 to 50%   (Ca-poor pryoxene )
Plagioclase An31-15
Ca-pyroxene Cr-diopside

Oxygen Isotopic tests
Acapulcoites and Lodranites share many similarites including I/O but
as you can see grain size is quit different !
Lodranites were more strongly heated than Acapulcoites .
Some were heated above the silicate solidus 1150 C causing fractionation of
the ( plagioclase-Ca-Pyroxene ) liquid .
The metal appears "lumpy" at times .


Winonaties  Chart
Winonaties have an average grain size of 55-230 < 1000 but with rare recrystallized relict chondrules !
Metal FeNi 0.1 to 12.5 %

Triolite 1 to 19%
Graphite 0 to 0.6%
Schreibersite 0 to 1%
Olivine Fo100-95  15 to 30%

Orthopyroxene En100-91  25 to 45%   (Ca-poor pryoxene )
Plagioclase An25-7
Ca-pyroxene Cr-diopside
Graphite may rim inclusions or individual grain .
Winonaties are shock stage S1 yet the olivines and pyroxenes may be recrystallized .


Primitive achondrites are those meteorites whose bulk compositions are approximately chondritic, but have been texturally modified by partial melting or metamorphic recrystallization. Because primitive achondrites are very different from other meteorites, but have similarities within themselves, they will be discussed here as a diverse group. A fundamental distinction between the acapulcoite and lodranite classes is that lodranites are relatively coarse-grained (0.5 to 1.0 mm) and acapulcoites are finer-grained (0.2 to 0.4). However, they are not easily distinguishable on casual observation; nor are they distinguishable using oxygen isotopes (figure below). 


 
 

The following sections present specimen illustrations and descriptions of differentiated achondrites, which includes the HED clan (howardites, eucrites, and diogenites), angrites, aubrites, and ureilites. A summary of the diagnostic mineralogic characteristics of all achondrites is given in the table below (modified after Table 8.1 in Hutchison, 2004, Meteorites: A Petrologic, Chemical and Isotopic Synthesis, Cambridge University Press).

 Differentiated
 Achondrites
Acapulcoite & Lodranite 1AB iron silicate & Winonaite  Ureilite Brachinite Aubrite Eucrite Diogenite Howardite Angrite
Olivine/Pyroxene <=1 <1 >1 >>1 <<1 <<1 <<1 <<1 1
Olivine Fa3-14 Fa1-8 Fa5-25 Fa30-35 Fa0 - Fa27-35 Fa8-89 Fa10-100
Olivine Fe/Mn                  
Orthopyroxene En86-97 En91-99 En80-90 tr En100 tr En67-77 variable -
Ca-pyroxene En51Wo44  min En75Wo15  min En50Wo50 variable tr variable Wo>50
Plagioclase An12-31 An8-25 - An22-32 tr: An2-8 An60-98 An60-91 An60-98 An100
Silica - - - - - tr tr tr -
Kamacite  min  min  min -  min tr-min tr tr tr
Taenite  min  min - tr tr - - tr -
Troilite  min  min tr  min tr tr tr tr tr

Notes:  min = minor, <5 vol. %; tr = trace, <0.5 vol. %

 

O/I