Dear Peter

Thanks for the link, but unfortunately there is no optical data for akageneite
on this site or on any of the links from this, and even basic information like
crystal system varies between the sites - most sites list it as 
monoclinic, but
Mineralienatlas has it as tetragonal.

I was hoping that Vagn Buchwald might respond, since he has published on this
mineral. The email address that I found for him on the web doesn't work - do
you have a current one?

The more general point that I would draw from this, for 
archaeometallurgists who
work on the extractive metallurgy of iron, is that the correct 
identification of
iron oxides and hydroxides is a fundamental precondition to correct
interpretation of  archaeological specimens. The fact that there is no
systematic compilation of the optical properties of iron minerals found 
in ores
and slags should therefore be a matter of concern to us. The general 
decline of
interest in optical mineralogy among geologists means that we can't hope for
them to come up with one - archaeometallurgists and conservation scientists
need to do it. Raman microscopy is great, but really not yet a viable
alternative for most archaeomatallurgists - access to instrumentation, and the
expense and inadequacy of commercial libraries of spectra, will mean that
polarized light microscopy (both transmitted and reflected) will 
continue to be
the basic tool for most archaeometallurgists.

I have my own criteria for identifying iron minerals, pulled from the 
literature
and my own experience over the last 20 years, and will be glad to share them,
but they are not complete. I just this week confirmed iscorite in a sample for
the first time (and am wondering how often I have overlooked it in the
past...). Some archaeometallurgists may wonder why we need to make these fine
distinctions, but the motive is not mineralogical. Most partially-reduced ores
that I have examined in thin section - and many slags too - are altered to a
large degree by corrosion, and it is essential to take this into account when
trying to reconstruct the mechanism of reduction or ore to iron. The approach
most commonly used at present - scanning electron microscopy - is just not
adequate for this task. Nor is optical examination in reflected PPL only; one
must use both transmitted and reflected XPL as well.

With best regards
Dave

P.S. in respect to the high prices and poor coverage of existing commercial
libraries of Raman spectra, members of this list may be interested in the work
of my colleague Bob Downs, in the Department of Geosciences at the University
of Arizona. The 2009 Mars space flight will carry a miniature Raman
spectrometer, and Downs has a large group of researchers racing to collect new
Raman spectra from over 4000 minerals for a database to interpret the spectra
that the spacecraft will send back to earth. Since this work is funded 
by NASA,
the entire library of spectra will be available to everyone over the 
web without
charge.

See http://www.geo.arizona.edu/xtal/group/index.htm, and links from there, for
further details.



Quoting Peter Kresten <[log in to unmask]>:

> Hello Dave,
> Try http://webmineral.com, there you will find data on most minerals.
> Peter
>
>
>> From: [log in to unmask]
>> Reply-To: Arch-Metals Group <[log in to unmask]>
>> To: [log in to unmask]
>> Subject: Akaganeite
>> Date: Sat, 23 Dec 2006 14:52:36 -0700
>>
>> Does anyone know the optical characteristics of akaganeite (beta-FeO(OH)) -
>> colour, bireflectance, anisotopy, extinction, internal reflections?  I can't
>> find them listed in the usual reference books for ore microscopy.
>>
>> I'm looking at some heavily corroded iron smelting slags and partly 
>> reduced ores
>> from Senegal. Fissures and pores within these are filled with masses or
>> concentric laminae of tiny crystals that are bright orange in 
>> transmitted PPL,
>> distinctly bireflectant from pale to darker grey in reflected PPL 
>> (in air), and
>> very strikingly anisotropic from dark grey to pale grey-blue in 
>> reflected XPL
>> (in air). Internal reflections are orange-red. The same material is 
>> also often
>> present in corrosion jackets around slags and partly-reduced materials.
>>
>> I'm very familiar with haematite, lepidocrocite and goethite in 
>> ores, but this
>> material doesn't look like any of these.
>>
>> Any guidance?
>>
>> Dave Killick
>
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