<?xml version="1.0" encoding="ISO-8859-1"?><article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<front>
<journal-meta>
<journal-id>1026-8774</journal-id>
<journal-title><![CDATA[Revista mexicana de ciencias geológicas]]></journal-title>
<abbrev-journal-title><![CDATA[Rev. mex. cienc. geol]]></abbrev-journal-title>
<issn>1026-8774</issn>
<publisher>
<publisher-name><![CDATA[Universidad Nacional Autónoma de México, Instituto de Geología]]></publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id>S1026-87742005000300391</article-id>
<title-group>
<article-title xml:lang="es"><![CDATA[Estudio mineralógico de la desestabilización de ópalos mexicanos]]></article-title>
<article-title xml:lang="en"><![CDATA[Mineralogic study of the destabilization of Mexican opals]]></article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname><![CDATA[Aguilar-Reyes]]></surname>
<given-names><![CDATA[Bertha Oliva]]></given-names>
</name>
<xref ref-type="aff" rid="Aff"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname><![CDATA[Ostrooumov]]></surname>
<given-names><![CDATA[Mikhail]]></given-names>
</name>
<xref ref-type="aff" rid="Aff"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname><![CDATA[Fritsch]]></surname>
<given-names><![CDATA[Emmanuel]]></given-names>
</name>
<xref ref-type="aff" rid="Aff"/>
</contrib>
</contrib-group>
<aff id="Af1">
<institution><![CDATA[,Université de Nantes Institut des Matériaux Jean Rouxel Laboratoire de Physique Cristalline]]></institution>
<addr-line><![CDATA[ ]]></addr-line>
<country>France</country>
</aff>
<aff id="Af2">
<institution><![CDATA[,Universidad Michoacana de San Nicolás de Hidalgo Instituto de Investigaciones Metalúrgicas Departamento de Geología y Mineralogía]]></institution>
<addr-line><![CDATA[ ]]></addr-line>
<country>Mexico</country>
</aff>
<pub-date pub-type="pub">
<day>00</day>
<month>00</month>
<year>2005</year>
</pub-date>
<pub-date pub-type="epub">
<day>00</day>
<month>00</month>
<year>2005</year>
</pub-date>
<volume>22</volume>
<numero>3</numero>
<fpage>391</fpage>
<lpage>400</lpage>
<copyright-statement/>
<copyright-year/>
<self-uri xlink:href="http://www.scielo.org.mx/scielo.php?script=sci_arttext&amp;pid=S1026-87742005000300391&amp;lng=en&amp;nrm=iso"></self-uri><self-uri xlink:href="http://www.scielo.org.mx/scielo.php?script=sci_abstract&amp;pid=S1026-87742005000300391&amp;lng=en&amp;nrm=iso"></self-uri><self-uri xlink:href="http://www.scielo.org.mx/scielo.php?script=sci_pdf&amp;pid=S1026-87742005000300391&amp;lng=en&amp;nrm=iso"></self-uri><abstract abstract-type="short" xml:lang="es"><p><![CDATA[RESUMEN Este trabajo presenta los resultados del primer estudio mineralógico del fenómeno de desestabi-lización por blanqueamiento en ópalos mexicanos de origen volcánico. En los espectros Raman de las muestras de ópalo estudiadas se observan bandas características de grupos moleculares de H2O en dos regiones diferentes: alrededor de 3,200 cm-1 para la parte no afectada por la desestabilización (de color naranja) y alrededor de 2,900 cm-1 para la parte desestabilizada (de color blanco). La forma e intensidad de las bandas presentes a 960 cm-1 (grupos Si-OH) y 1,600 cm-1 (grupos moleculares de H2O) constatan esta modificación debida al agua. El análisis térmico diferencial (ATD/ATG) mostró que el contenido total de agua es de 6 a 85% menor en la parte desestabilizada que en la parte naranja inalterada; el agua presente en la parte de color blanco es también más móvil, ya que su eliminación comienza por debajo de los 100 °C. Asimismo, mediante la medición de la superficie específica (método BET) se pudo constatar que, sistemáticamente, la superficie específica es entre 2 y 30 veces más grande para la parte desestabili-zada. La microscopia electrónica de barrido (MEB) ha permitido verificar diferencias microestructurales entre las dos partes del ópalo, observándose que en la parte desestabilizada ocurre una descompactación del material. Los resultados obtenidos muestran que la causa de la desestabilización está relacionada con una transformación estructural que involucra la formación de cristobalita de recristalización y la incorporación de grupos H2O moleculares a la estructura de los ópalos volcánicos.]]></p></abstract>
<abstract abstract-type="short" xml:lang="en"><p><![CDATA[ABSTRACT This paper presents the first mineralogical study of the destabilization phenomena by whitening in Mexican opals. The Raman spectra of the opal samples are characterized by the well-resolved bands assigned to molecular H2O, at 3,200 cm-1 and at 2,900 cm-1 for the unaltered (orange) and for the desta-bilized (whitened) parts, respectively. The shape and intensity of the bands at 960 cm-1 (groups Si-OH) and 1,600 cm-1, confirm this modification due to molecular H2O. Water contents, measured by thermal differential analyses (ATD/ATG) decrease in the whitened part (between 6 and 85%). Molecular H2O present in the destabilized part is more mobile than in the unaltered areas, since its elimination begins below 100 °C. The specific surface (BET method) is always larger in the destabilized whitened part (up to 30 times). Scanning electronic microscopy (SEM) revealed microstructural differences between both parts of the opal: the unaltered (orange) part is more compacted that the destabilized (white) part. These results suggest that the opal instability is the result of structural changes that include the recristalization to cristobalite and the incorporation of molecular H2O in the structure of volcanic opals.]]></p></abstract>
<kwd-group>
<kwd lng="es"><![CDATA[ópalo]]></kwd>
<kwd lng="es"><![CDATA[desestabilización]]></kwd>
<kwd lng="es"><![CDATA[espectrometría Raman]]></kwd>
<kwd lng="es"><![CDATA[agua molecular]]></kwd>
<kwd lng="en"><![CDATA[Opal]]></kwd>
<kwd lng="en"><![CDATA[destabilization]]></kwd>
<kwd lng="en"><![CDATA[Raman spectrometry]]></kwd>
<kwd lng="en"><![CDATA[molecular water]]></kwd>
</kwd-group>
</article-meta>
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