Achondrites Guidelines

Angrites,  example  D'Orbigny    Angra dos Reis
Angrites,  are unique in having large amounts of Ti-rich augite (fassaite).
Fassaite is uncommon in terrestrial and areroidal rocks, but more common in lunar rocks.
This pyroxene is  millimeters in size and ranges from 90 vol.% in Angra dos Reis
(above) to 30 vol. % in SAH 99555.
In addition to  fassaite, angrites contain various amounts of olivine, a Ca-rich olivine
(kirschsteinite), plagioclase, troilite, Ti-magnetite, NiFe metal, and apatite.
One angrite, SAH99555, contains vesicles (gas bubbles) that are the
largest observed in an asteroidal meteorite .
Angrites are unbrecciated and unshocked !
Angrites have atomic Ca/Al ratios >1 and have high U/Pb  and Low Rb/Sr ratios .
Angrites have abundant and unusually Ca-rich diopside-hedenbergite pyroxene with highTi and Al ( fassaite )
Pyroxene.......20-24 ( exception Angta dos Reis  93 )  Wo52-55En28-33Fs12-20 to pure hedenbergite
Plagioclase....33-46 
( exception Angta dos Reis  none )
Olivine...........42-29 
( exception Angta dos Reis  3 )
Olivine in angrites is Ca-rich and ranges to kirchsteinite , FeCaSiO4

Silica..............None
Chromite........Trace       
 ( exception Angta dos Reis  2 )
The spinel in angrites is close to hercynite (FeAl2O4) , not chromite

Ilmenite..........Trace  Titanomagnetite , not Ilmenite occurs in angrites
Fe,Ni metal....Trace
Triolite............Trace
Phosphate.......Trace   but found in mm size grains
Fe/Mn.............88 to 93
Ca/Al...............1.1 to 2.2
D'Orbigny has almost pure anorthite and strongly zoned pyroxene Wo52En28Fs20 and olivine Fo64 to Fa83La17
Angra dos Reis has almost pure homogenous pyroxene Wo55En32Fs12 and olivine Fo52Fa46La2

 

Aubrites   example Pena Blanca Springs
Aubrites are enstatite achondrites that consist principally of nearly Fe-free enstatite with minor amounts of
NiFe metal, troilite, plagioclase, olivine or diopside.
They are igneous in origin and crystallized in an extremely reducing (oxygen-poor) magma that promoted
the formation of small amounts of rare to unique minerals, for example, oldhamite (CaS), which is  not found in terrestrial rocks
Aubrites share many similarities to EH and EL chondrites .
1. Both have FeO-free enstatite as there most abundant silicate .
2. Being highly reduced such that Ca and Cr occur as sulfides
3. Both having O/I ratios that plot on the terrestrial fractionation line


Enstatite..........................( 75-98 ) Wo 0.1-1.2 Fs <0.12
Low-Ca clinoenstatite .....None to Wo 0.6 Fs >0.05
Diopside .........................(0.2-8 ) Wo40-46 Fs<0.22  to none
Forsterite ........................( 0.3-10 ) Fo>99.9
Plagioclase ......................( 0.3-16 ) An2-8 An18 Ab80: An0.4Ab88
Fe,Ni metal .....................( tr-3.3 ) 0.1-2.4 wt% Si
Triolite ............................( tr-2.9 ) .06-5.7 wt% Ti & ( Cr possibly )
Schreibersite ...................( <.4)  up to 10..3 wt% Ni
weathered opaques.......... none to (8)

  

Aubrites are breccias with clasts that may have been derived from different
depths within their parent body .

 

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. %