[meteorite-list] magnetic lecture

From: rochette <rochette_at_meteoritecentral.com>
Date: Thu Apr 22 10:13:10 2004
Message-ID: <v04003a00bad01dd033f8_at_[80.12.145.133]>

Dear list


instead of making partial repeated specific comments on magnetism I
thought I could try to summarize all my previous posts on the subject
into a small "lecture"(I confess I teach magnetism). For those that get
irritated by the very idea of being lectured, stop now!

For those with a good physics background you can find approximative and
inexact points but I tried to keep it simple!

Finally for Tet, I repost the recipy to distinguish sulfides from real
metal grains if they are a few mm wide: test a freshly polished surface
using an ohmeter (circuit controller available in any warehouse).

*******

Basic definitions


<fontfamily><param>Times</param><bigger><bigger>Magnetization (M) is a
property of matter. It can be induced (Mi) by an external magnetic
field (H) or else it can be remanent (Mr), i.e. present without
external field. Magnetic susceptibility K is the ratio Mi/H.

A magnetized object creates itself a magnetic field in its surrounding,
rapidly decreasing with distance (as the inverse of the cube). This
gradient, applied on another positively magnetized object is
responsible for an attraction. A (permanent) magnet is an object
carrying a strong remanence, thus able to magnetize (induce a Mi) a
nearby material, and thus attract it. So one call a material " magnetic
" if it is attracted by a magnet. A magnetic material can itself be (or
become) a magnet, depending on its composition, grain size, etc, and on
its history in terms of magnetic field exposure. Natural materials,
being exposed to very weak field, do not develop a strong remanence,
unless striken by a lightning. However when exposed to a large field
(such as available at the contact of a magnet) it can develop a strong
remanence. An easy way to see this is to take a natural magnetic rock
(for example a basalt or a meteorite not previously exposed to a
magnet) and observe the deviation it makes on a compass when put close
to the needle. Then magnetize the rock by touching it with a strong
magnet and repeat the compass experiment.

        The notion of active (the magnet) and passive (the attracted material)
object is practical in a sense but not very general and a bit
misleading: once magnetized by the field of the magnet (or of a strong
external field) the " passive " object does exert a magnetic pull on
the magnet or on other " passive " materials. If you put two identical
"passive" object (e.g. pure Fe metal) into a homogeneous (thus not able
by itself to exert any attraction) field created by a solenoid, they
become both "active".


Magnetic or non magnetic?


The popular definition of " magnetic caracter " (being attracted by a
magnet) is also misleading as it depends on the strength of a magnet.
Every material containing even minute amount of magnetic elements
(Fe,Ni,Co,Cr,Mn,and rare earths) has a positive susceptibility
(paramagnetism) and is thus attracted by a magnet. Every other
materials are repelled, i.e. have a negative susceptibility
(diamagnetism).

A very strong magnet can lift an usual paramagnetic minerals such as
olivine, pyroxene, etc. Troilite could be also put in that category.
However these paramagnetic minerals are not enough magnetized by a
small hand magnet to show a visible attraction.

More strongly magnetic (ferromagnetic) minerals are FeNi alloys (the
typical metal in meteorites), as well as cohenite and schreibersite
(found in E chondrites) and magnetite (more typical of oxidzed C
chondrites) ; below (with 10-100 times smaller magnetization but still
detectable by the " magnet test ") can be listed pyrrhotite (a sulfide
related to troilite) and hematite. The sulfide family is very complex
(e.g. pyrite has a very weak susceptibility 100 times less than
troilite, and 10000 less than pyrrhotite).

The susceptibility of a meteorite depends thus on the amount and nature
of the Fe-Ni bearing minerals it contains (other magnetic elements are
negligible compared to Fe-Ni). One observes about three orders of
magnitude variation from a purely paramagnetic meteorite
(olivine-pyroxene bearing, like Chassigny) to a metal rich meteorite
(H, E, acapulcoite,Š). Still, Chassigny is 100 times more magnetic than
a terrestrial limestone.


The magnet test


The magnet test place a threshold that transform a rather continuous
palette of susceptibility (see my " magnetic chart " available on
request) into a yes or no answer (attraction or not). So it critically
depends on the type of magnet used, the way the sample is shaped and
placed with respect to the magnet, and how the yes is appreciated (does
the sample has to really stick to the magnet or is a small movement of
a suspended sample approached by a magnet sufficient ?). The now
popular strong rare earth magnet are able to give positive answer for
any type of meteorite (except a few achondrites) but also for a wide
variety of terrestrial rocks (basalts, some granite and metamorphic
rocks). If one use a weaker ferrite magnet, terrestrial material will
become negative but also a lot of interesting meteorites (SNC, lunar,
HED, angrite, some CV and CM, R, even some LL6).

Moreover the magnet test destroys the natural magnetization of the
tested meteorites, which are subsequently of no use for specific
scientific investigation (called paleomagnetism).

Magnetic classification of meteorites can thus be much more efficiently
performed using a measurement of magnetic susceptibility : it preserves
natural remanence and is able to be much more specific than the magnet
test. Such measurements are easily performed with a hand probe
(documentation available on request).

</bigger></bigger></fontfamily>


Pierre
Received on Sat 26 Apr 2003 07:28:18 AM PDT