OPAL
Opal is hydrous silica (SiO2·nH2O).
Technically, opal is not a mineral because it lacks a crystalline
structure. Opal is supposed to be called a mineraloid, but most
geologists don't mind calling it a mineral. Opal is made up of extremely
tiny spheres (colloids)
that can be seen with a scanning electron microscope (SEM).
Gem-quality opal, or precious opal, has a wonderful
rainbow play of colors (opalescence). This play of color is the
result of light being diffracted by planes of voids between large areas of
regularly packed, same-sized opal colloids. Different opalescent colors
are produced by colloids of differing sizes. If individual colloids are
larger than 140 x 10-6 mm in size, purple & blue & green
colors are produced. Once colloids get as large as about 240 x 10-6
mm, red color is seen (Carr et al., 1979).
Not all opals have the famous play of colors,
however. Ordinary opal has a wax-like luster & is often milky
whitish with no visible color play at all. Opal is moderately hard (H = 5
to 6), has a white streak, and has conchoidal fracture.
Several groups of organisms make skeletons of opaline
silica, for example hexactinellid sponges, diatoms, radiolarians, silicoflagellates,
and ebridians. Some organisms incorporate
opal into their tissues, for example horsetails/scouring rushes and sawgrass. Sometimes, fossils are preserved in opal or
precious opal.
Common opal (4.2 cm across) from Nevada. The milky white color is usually
attributed to an abundance of microscopic inclusions.
Cacholong opal from Tampa Bay, Florida, USA. Cacholong opal is
a non-lustrous, microporous variety of common opal. Cacholong opal will
stick to your tongue, a consequence of its microporous and hygroscopic nature
(public display, Field Museum of Natural History, Chicago, Illinois, USA).
Jelly opal in volcanic breccia from Mexico.
Opal
in vesicular basalt from Humboldt County, Nevada, USA. (CMNH 11028, Cleveland
Museum of Natural History, Cleveland, Ohio, USA)
Bacon opal (a.k.a. candy stripe opal) from the Thomas Range of Utah, USA.
Precious opal in matrix (2.2 cm across) from Queretaro State, Mexico.
Black opal (polished) from Lightning Ridge, northern New South Wales, eastern
Australia. Lightning Ridge produces most of the world's precious black
opal from fluvial sediments of the Finch Clay (upper Lower Cretaceous).
Precious opal in matrix (Field Museum of Natural History public display, Chicago,
Illinois, USA).
Precious opal from Western Australia. (CSM public display, Colorado School of
Mines Geology Museum, Golden, Colorado, USA)
Precious opal in matrix from Australia. Specimen owned by Stan & Pris
Woollams.
Precious opal vein intruding sandstone fragment (6.6 cm across) (above & below).
Locality:
Andamooka Opal Fields, erosive edge of the Stuart Range Plateau, west of
northern Lake Torrens, South Australia
Stratigraphy of host rock: Bulldog Shale, Marree Subgroup, Aptian Stage, upper
Lower Cretaceous
Precious opal from Andamooka, South Australia (CIS 212-125, Cranbrook Institute of
Science collection, Bloomfield Hills, Michigan, USA).
Precious opal from Andamooka, South Australia (CIS 212-133, Cranbrook Institute of
Science collection, Bloomfield Hills, Michigan, USA).
Precious opal from Queensland, Australia (CIS 212-148, Cranbrook Institute of
Science collection, Bloomfield Hills, Michigan, USA).
Precious opal from Queensland, Australia (CIS 212-152, Cranbrook Institute of
Science collection, Bloomfield Hills, Michigan, USA).
Precious opal from Virgin Valley, Humboldt County, Nevada, USA (CIS 212-10, Cranbrook
Institute of Science collection, Bloomfield Hills, Michigan, USA).
Precious opal (3.3 cm across) from the Yita Ridge-Guy Meda Wihe River-Gift Ridge
area of Menz Gishe District, northeastern Shewa (Shoa) Province,
northeast-central Ethiopia. The opal there occurs in nodules in an Early
to Middle Miocene-aged rhyolitic welded tuff (Alaji Rhyolite Formation, a.k.a.
Amba Alaji Rhyolites, a.k.a. Aliyu Amba Ignimbrites).
Ethiopian opal has been identified as opal-CT, which
consists of extremely tiny cristobalite-tridymite aggregates called
leptospheres.
The Australian opals (see above) are composed of
opal-A, which consists of amorphous, hydrous silica spheres (colloids).
Opal
(polished) from the Coober Pedy Opal Field of South Australia (CSM # A83.301,
Colorado School of Mines Geology Museum, Golden, Colorado, USA).
Opal
(polished) (Colorado School of Mines Geology Museum, Golden, Colorado, USA).
Black opal (polished, 87 carats) (public display, Field Museum of Natural
History, Chicago, Illinois, USA)
Black opal (polished) from Australia. (public display, Field Museum of
Natural History, Chicago, Illinois, USA)