Spectral abscissa

2014-01-16T11:39:24Z

130.233.85.49: corrected "sometines" -> "sometimes"

'''Magnetic mineralogy''' is the study of the [[Magnetism|magnetic]] properties of [[mineral]]s. The contribution of a mineral to the total magnetism of a rock depends strongly on the type of magnetic order or disorder. Magnetically disordered minerals ([[diamagnet]]s and [[paramagnet]]s) contribute a weak magnetism and have no [[remanence]]. The more important minerals for [[rock magnetism]] are the minerals that can be magnetically ordered, at least at some temperatures. These are the [[ferromagnets]], [[ferrimagnets]] and certain kinds of [[antiferromagnets]]. These minerals have a much stronger response to the field and can have a remanence.

== Weakly magnetic minerals ==
=== Non-iron-bearing minerals ===

Most minerals with no iron content are diamagnetic.<ref name=Dunlop>{{Harvnb|Dunlop|Özdemir|1991}}</ref> Some such minerals may have a significant positive [[magnetic susceptibility]], for example [[Serpentine group|serpentine]],<ref name=Hunt>{{Harvnb|Hunt et al.|1995}}</ref> but this is because the minerals have inclusions containing strongly magnetic minerals such as [[magnetite]]. The susceptibility of such minerals is negative and small (Table 1).

{| class="wikitable collapsible collapsed"
|+ Table 1. Susceptibilities of non-iron-bearing minerals <ref name=Hunt/>
|-
! Mineral !! Volume susceptibility at room temperature <math>\times 10^{-6}</math> (SI)
|-
| [[graphite]] || -80 to -200
|-
| [[calcite]] || -7.5 to -39
|-
| [[anhydrite]] || -14 to -60
|-
| [[gypsum]] || -13 to -29
|-
| [[ice]] || -9
|-
| [[orthoclase]] || -13 to -17
|-
| [[magnesite]] || -15
|-
| [[forsterite]] || -12
|-
| [[halite]] || -10 to -16
|-
| [[galena]] || -33
|-
| [[quartz]] || -13 to -17
|-
| [[celestine]] || -16 to -18
|-
| [[sphalerite]] || -31 to -750
|}

=== Iron-bearing paramagnetic minerals ===

Most iron-bearing [[Carbonate minerals|carbonates]] and [[Silicate minerals|silicates]] are paramagnetic at all temperatures.<ref name=Dunlop/> Some [[Sulfide minerals|sulfides]] are paramagnetic, but some are strongly magnetic (see below). In addition, many of the strongly magnetic minerals discussed below are paramagnetic above a critical temperature (the [[Curie temperature]] or [[Néel temperature]]). In Table 2 are given susceptibilities for some iron-bearing minerals. The susceptibilities are positive and an order of magnitude or more larger than diamagnetic susceptibilities.

{| class="wikitable collapsible collapsed"
|+ Table 2. Susceptibilities of some paramagnetic minerals <ref name=Hunt/>
|-
! Mineral !! Volume susceptibility <math>\times 10^{-6}</math> (SI)
|-
| [[garnet]] || 2,700
|-
| [[illite]] || 410
|-
| [[montmorillonite]] || 330-350
|-
| [[biotite]] || 1,500-2,900
|-
| [[siderite]] || 1,300-11,000
|-
| [[chromite]] || 3,000-120,000
|-
| [[orthopyroxene]] || 1,500-1,800
|-
| [[fayalite]] || 5,500
|-
| [[olivine]] || 1,600
|-
| [[jacobsite]] || 25,000
|-
| [[franklinite]] || 450,000
|}

== Strongly magnetic minerals ==

=== Iron-titanium oxides ===

Many of the most important magnetic minerals on Earth are oxides of [[iron]] and [[titanium]]. Their compositions are conveniently represented on a [[ternary plot]] with axes corresponding to the proportions of {{Titanium}}4+, {{Iron}}2+, and {{Iron}}3+. Important regions on the diagram include the ''titanomagnetites'', which form a line of compositions {{Chemical formula|Fe|3-x|Ti|x|O|4}} for {{math|''x''}} between 0 and 1. At the {{math|''x''=0}} end is [[magnetite]], while the {{math|''x''=1}} composition is [[ulvöspinel]]. The titanomagnetites have an [[Spinel#The spinel structure|inverse spinel]] crystal structure and at high temperatures are a [[solid solution]] series. Crystals formed from titanomagnetites by cation-deficient oxidation are called ''titanomaghemites'', an important example of which is [[maghemite]]. Another series, the ''titanohematites'', have [[hematite]] and [[ilmenite]] as their end members, and so are also called ''hemoilmenites''.<ref name=Dunlop/> The crystal structure of hematite is [[Trigonal crystal system|trigonal]]-[[Hexagonal crystal system|hexagonal]]. It has the same composition as [[maghemite]]; to distinguish between them, their chemical formulae are generally given as γ{{Chemical formula|Fe|2|O|3}} for hematite and α{{Chemical formula|Fe|2|O|3}} for maghemite.

=== Iron sulfides ===

The other important class of strongly magnetic minerals is the [[iron sulfide]]s, particularly [[greigite]] and [[pyrrhotite]].

== See also ==
*[[Magnetochemistry]]
== Notes ==
{{Reflist}}

== References ==
{{Refbegin}}
*{{cite book
|last1 = Dunlop
|first1 = David J.
|last2 = Özdemir
|first2 = Özden
|title = Rock magnetism: Fundamentals and Frontiers
|publisher = [[Cambridge Univ. Press]]
|year = 1997
|isbn = 0-521-32514-5
}}
*{{Citation
| last = Hunt
| first = Christopher P.
| last2 = Moskowitz
| first2 = Bruce P.
| contribution = Magnetic properties of rocks and minerals
| year = 1995
| editor-last = Ahrens
| editor-first = T. J.
| title = Rock Physics and Phase Relations: A Handbook of Physical Constants
| publication-place = Washington, DC
| publisher = American Geophysical Union
| volume = 3
| pages = 189–204
}}
{{Refend}}

[[Category:Geomagnetism]]

formulasearchengine - User contributions [en]

Spectral abscissa