o6 emucm3 _2__ the Hc-value of an by goodbaby

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									           HIGH MAGNETIC COERCIVITY O F METEORITES CONTAINING
           THE ORDERED F e N i (TETRATAENITE)                              A S THE MAJOR


           T a k e s i NAGATA, National Institute of P o l a r Research; Tokyo 173.


       T h e n a t u r a l remanent magnetization (NRM) of ALHA-77260 ( ~ 6chondrite and             )
S t . Sdverin ( L L ~ )          chondrite is extremely s t a b l e f o r the AF-demagnetization t e s t ,
as i l l u s t r a t e d i n F i g . 1 , w h e r e t h e intensity of r e s i d u a l NRM a f t e r AF-demagneti-
zing up to 1000 Oe peak is still about a half of that of the original NRM. T h e
d i r e c t i o n of NRM of both m e t e o r i t e s @so is v e r y s t a b l e , being confined to the
total r a n g e of 30" d u r i n g t h e whole p r o c e s s of AF-demagnetization. T h e magnetic
c o e r c i v e f o r c e s (H,) of ALHA-77260 and S t . S d v e r i n are 180 Oe and 500 Oe
r e s p e c t i v e l y , which are unusually l a r g e as an L o r L L chondrite.

       It h a s been r e p o r t e d , on the o t h e r hand,
that t h e metallic component of St. S Q v e r i n
c o n s i s t s of 4-05 kamacite, 51% of q r d e r e d
                          of                                                    1.2
F e N i (tetrataenite) and 9% of o r d i n a r y dis-                      -
                                                                           I
                                                                           a
o r d e r e d taenite i n volume (1). S i n c e t h e                      z     1 1                 5T.SEVERIN     .
o r d e r e d F e N i c r y s t a l is highly anisotropic,
optically a s well as magnetically, t h e magneto-
                                                                           -
                                                                           4
                                                                           A
                                                                           5    ,a
                                                                           Z,
c r y s t a l l i n e anisotropy constant K1 being about
3 . 2 ~ o6 emu/cm3 (2), t h e Hc-value of an
           1                                                               5     6


o r d e r e d F e N i c r y s t a l should be v e r y l a r g e ,
                                                                           z
                                                                           Zz
                                                                           2     & '
i. e . H , = ~ K/ ~ ~ = 4 . 9 x 1 0 O e , w h e r e Js=l .3x103
                       ~                   3                               4
                                                                           3
                                                                           0
emu/cm3. T h e o b s e r v e d high stability of NRM                       m    -2

and a l a r g e value of Hc of St. S Q v e r i n c a n be                  -
                                                                           W
                                                                           a
r e a s o n a b l y well i n t e r p r e t e d on t h e b a s i s of co-         oO      Zoo   400     600   800   1000   oe.
existence of an o r d e r e d F e N i p h a s e and a                                  AF- DEMAGNET~ZING FIELD
kamacite phase.

     ALHA-77260 a l s o contains c l e a r t a e n i t e
domains of about 59wt76 i n Ni-content, which
is identified to an o r d e r e d F e N i p h a s e (3), i n                             Fig. 1
addition to a s m a l l e r amount (about 13% of
metal) of kamacite of 5 . 5 ~ 7 6          Ni. T h e thermomagnetic c u r v e s of ALHA-77260 a r e
shown i n Fig.2, w h e r e t h e second-run thermomagnetic c u r v e s indicate that t h i s
chondrite a f t e r heating t o 8 0 0 contains 3.14wt76 of a taenite p h a s e of 59W% Ni,
                                                ~ ~
0.20wt76 of a taenite p h a s e of 36wt% Ni and 0.48wt5 of a kamacite p h a s e of 5.5wt$
   .
Ni T h e first-run heating thermomagnetic c u r v e i n d i c a t e s , however, t h a t t h e
original unheated metallic component c o n s i s t s of a kamacite p h a s e and a n o r d e r e d
F e N i p h a s e , which is t r a n s f o r m e d t o an o r d i n a r y taenite p h a s e by t h e first-run
heating p r o c e d u r e . T h e o b s e r v e d high stability of NRM and a comparatively l a r g e
value of Hc of ALHA-77260 a l s o will be due t o t h e p e s e n c e of o r d e r e d F e N i p h a s e
together with kamacites             .
      S i m i l a r c h a r a c t e r i s t i c s of thermomagnetic c u r v e s have been found i n S a n t a
C a t h e r i n a ataxite which contains about 50% of o r d e r e d F e N i p h a s e (4) and ALHA-
7721 9 messociderite which contains about 10% of o r d e r e d F e N i p h a s e (3).




   O Lunar and Planetary Institute                 Provided by the NASA Astrophysics Data System
             Ml3TEORITES CONTAINING THE ORDERED F e N i

                                   Nagata T.


       T h e highly s t a b l e NRM component p o s s e s s e d
by the o r d e r e d F e N i p h a s e in meteorites would
have been a c q u i r e d as a chemical remanent                                                ALHA 7 7 2 6 0
                                                                                                F I R S T RUN
magnetization (CRM) during a long time p r o c e s s
of a formation of t h e o r d e r e d F e N i c r y s t a l
s t r u c t u r e by v e r y slow diffusions of Ni-atoms
through the t a e n i t e c r y s t a l l a t t i c e a t low
t e m p e r a t u r e s (53 2 0 ~ i~ )
                                   n the p r e s e n c e of a
magnetic field. If s o , paleomagnetic s t u d i e s of
meteorites containing t h e o r d e r e d F e N i p h a s e
as t h e major ferromagnetic constituent will
have to be re-examined from the newly found                                                     SECOND RUN

viewpoint. It h a s been known that the intensity
of CRM a l s o is approximately proportional t o
a magnetic field applied during t h e chemical
change p r o c e s s . S i n c e an experimental
simulation of the acquisition of CRM of t h e
o r d e r e d F e N i p h a s e may be p r a c t i c a l l y im-
possible, a c e r t a i n substitutional experimental                                    T U I I ~ ~ ~ R E
p r o c e d u r e will have to be considered i n o r d e r
t o estimate t h e paleointensity f o r t h e o r d e r e d
F e N i phase.                                                                        Fig. 2

       o n l y a p r a c t i c a l l y possible method at p r e s e n t will be a comparison of
c h a r a c t e r i s t i c s of ARM and i t s AF-demagnetization with those of AF-demagneti-
zation of NRM on an assumption that t h e acquisition of p a r t i a l CRM is p r o p o r -
tional t o that of p a r t i a l ARM with r e s p e c t to t h e microscopic magnetic c o e r c i v e
f o r c e . F o r a h i g h e r r a n g e of microscopic magnetic c o e r c i v e f o r c e , i. e . l a r g e r
than 400 Oe, t h e paleointensity (h?) is thus estimated as h p 4 . 14/& Oe f o r
ALHA-77260 and                     =0.40/jQ0e f o r S t . S b v e r i n , w h e r e f,denotes t h e propor-
tionality                                                                 he
                                    of ARM t o CRM. ~ h e j ~ v a l u a s not yet been known f o r t h e
o r d e r e d F e N i p h a s e . In t h e c a s e of CRM produced by a chemical change from
d F e z 0 3 t o FegO4, f c is about 10.

References:
                      ,                       ,
(1) Danon R. b. S c o r z e l l i R. B. S o u z a Azavedo I. and Christophe-Michel-
    Ldvy M., (1 979) N a t u r e 281, p. 469-471               .
(2) NQel L., P a u l e v e J . , Pauthenet R . , L a u g i e r J. and Dautreppe P
    J. Appl. P h y s . , 35, p. 73-76.
                                                                                                     ., (1964)
(3) C l a r k e R . S . Jr. and S c o t t E.R.D.,         (1980) Amer. Mineral., 65, p. 624-
    630.
                 .                   .
(4) Danon J , S c o r z e l l i R , S o u z a Azavedo I. , Curvello W               ., Albertwen J .F .
                               ,
    and Knudsen J .M. (1979) Nature 277, p 283-284;            .




         O Lunar and Planetary Institute              Provided by the NASA Astrophysics Data System

								
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