Common gem opal: An investigation of micro- to nano-structure

Research areas:

• Surfaces planétaires (2011 - 2016)

Year:

2008

Authors:

• Eloise Gaillou
• Emmanuel Fritsch
• Bertha Aguilar-Reyes
• Benjamin Rondeau
• Jeffrey Post
• Alain Barreau
• Mikhail Ostroumov

Journal:

AMERICAN MINERALOGIST

Volume:

93

Number:

11-12

Pages:

1865-1873

Month:

NOV-DEC

ISSN:

0003-004X

BibTex:

@article{WOS:000261072500016, author = "Eloise Gaillou and Emmanuel Fritsch and Bertha Aguilar-Reyes and Benjamin Rondeau and Jeffrey Post and Alain Barreau and Mikhail Ostroumov", abstract = "The microstructure of nearly 200 common gem opal-A and opal-CT samples from worldwide localities was investigated using scanning electron microscopy (SEM). These opals do not show play-of-color, but are valued in the gem market for their intrinsic body color. Common opal-AG and opal-CT are primarily built from nanograins that average similar to 25 run in diameter. Only opal-AN has a texture similar to that of glass. In opal-AG, nanograins arrange into spheres that have successive concentric layers, or in some cases, radial structures. Common opal does not diffract light because its spheres exhibit a range of sizes, are imperfectly shaped, are too large or too small, or are not well ordered. Opal-AG spheres are typically cemented by non-ordered nanograins, which likely result from late stage fluid deposition. In opal-CT, nanograins have different degrees of ordering, ranging from none (aggregation of individual nanograins), to an intermediate stage in which they form tablets or platelets, to the formation of lepispheres. When the structure is built of lepispheres, they are generally cemented by non-ordered nanograins. The degree of nanograin ordering may depend on the growth or deposition rate imposed by the properties of the gel from which opal settles, presumably, fast for non-ordered nanograin structures in opal-CT to slow for the concentric arrangement of nanograins in the spheres of opal-AG.", doi = "10.2138/am.2008.2518", extra = "PICL", issn = "0003-004X", journal = "AMERICAN MINERALOGIST", month = "NOV-DEC", number = "11-12", pages = "1865-1873", title = "{C}ommon gem opal: {A}n investigation of micro- to nano-structure", volume = "93", year = "2008", }

Abstract:

The microstructure of nearly 200 common gem opal-A and opal-CT samples
from worldwide localities was investigated using scanning electron
microscopy (SEM). These opals do not show play-of-color, but are valued
in the gem market for their intrinsic body color. Common opal-AG and
opal-CT are primarily built from nanograins that average similar to 25
run in diameter. Only opal-AN has a texture similar to that of glass. In
opal-AG, nanograins arrange into spheres that have successive concentric
layers, or in some cases, radial structures. Common opal does not
diffract light because its spheres exhibit a range of sizes, are
imperfectly shaped, are too large or too small, or are not well ordered.
Opal-AG spheres are typically cemented by non-ordered nanograins, which
likely result from late stage fluid deposition. In opal-CT, nanograins
have different degrees of ordering, ranging from none (aggregation of
individual nanograins), to an intermediate stage in which they form
tablets or platelets, to the formation of lepispheres. When the
structure is built of lepispheres, they are generally cemented by
non-ordered nanograins. The degree of nanograin ordering may depend on
the growth or deposition rate imposed by the properties of the gel from
which opal settles, presumably, fast for non-ordered nanograin
structures in opal-CT to slow for the concentric arrangement of
nanograins in the spheres of opal-AG.