The word 'texture' refers to the detailed structure and makeup
of all kinds of materials, such as textile, rocks and many others.
The pictures in this chapter illustrate that textures in minerals
and rocks are very diverse and may be formed by a variety of
processes, like crystallization and sedimentation, but also
through pressure, deformation or geochemical processes. Many
pictures in this paragraph are of 'thin sections', wafer-thin (10 to
40 micron) slices of rock, which are glued on to glass and prepared with special skill.Thin sections are one of the fundamental
means for analyzing rocks and minerals, and geologists routinely
examine them with a polarizing microscope. The inspection is
carried out with the help of various optical means and with
transmitted light that mayor may not be polarized. When the
light is polarized, crystals often take on striking colors, due to
birefringence of the light beams.
The actual sizes of the thin sections shown in this paragraph and
the next are in the range 10 to 50 mm. and not mentioned separately in the texts.
CARNALLITE Carnallite is a rock salt and as such belongs to
a group of sediments referred to as evaporites. Evaporites form
part of marine sedimentary sequences and are precipitated by
the drying up of isolated parts of continental seas during dry
and warm climates. A present day example is the Dead Sea of
Israel. In thin section and polarized light, the tightly interlocked
salt crystals take on attractive colors.
BISCHOFITE Bischofite is a magnesium containing rock salt,
like the carnallite of the previous plate. These two pictures are
of salts mined, by slurry pumping, from the deeper subsurface in
the north of the Netherlands. The thin sections here photographed are rather exceptional, since a special technique is
required for their preparation and preservation.
BORDERLINE The fine, darkish material in this thin section is
basalt, a magma that has solidified at, or near, the earth's surface.
When ascending to the surface from the deep underground,
such magmas may bring along bits and pieces of other rocks.
Some of these pieces do not melt, such as the chunk of peridotite on the right hand side of the picture, in which the olivine
crystals display their characteristic, lively colors of interference.
USCO I ES Muscovite is a micaceous mineral occurring
frequently in plutonic and metamorphic rocks. It is the mineral
TEXTURE
112
that often gives the sparkle to granite and gneiss or appears as
tiny stacks of wafers in other plutonic rocks (see Aquamarin,
page 54). In thin section and polarized light, micas display fading
colors of birefringence and a characteristic 'goose-flesh' texture.
PER DOTITE Peridotite is a 'plutonic' rock. It is formed at
great depth, where the earth's crust changes into the semi-solid
mantle. It is assumed that most of the earth's mass, notably the
mantle, has a peridotitic composition, but this has never been
proven (the earth's interior is simply too deep and too hot for
sampling, we know less of it than we do of the surface of the
Moon or Mars). Many meteorites, Le. those of the stony variety
(the other variety is metallic), also have the same composition,
which suggests that peridotite is a key building material of the
whole solar system. The colorful olivine that makes up the bulk
of peridotite, has been partly serpentinized, which means that it
has been altered, along the grain boundaries, into the water rich
mineral serpentine. This accounts for that craquelee look of an
old oil painting.
T G
EYE Tiger-eye is a fibrous mineral, belonging to the silicate group. It is well known for it's silky, golden gloss and is
often cut and polished into cabochons and trinkets. Here, in thin
section, its appearance is more that of a sanguineous chunk of
meat.
YLONITE MOSAIC Shown here in thin section is
mylonite, a thinly laminated, metamorphic rock. Fine-textured
seams of mica are intercalated with thin bands of quartz. The
quartz grains have undergone recrystallization and have thus
been rearranged into a close-fitting mosaic.
ORNBLENDE Hornblende belongs to a group of minerals
called the amphiboles, which occur in igneous as well as metamorphic rocks. In this thin section exposure the delicate texture inside the crystals is accentuated by the subtle gradations
in the colors of birefringence.
E Perthite, sometimes incorrectly referred to as a
feldspar mineral, is actually a texture of fine, interspersedlayers
occurring in alkali feldspars. This lamellar texture results from a
separation of sodium- and potash-feldspars during cooling and
solidification of the mineral.Thin section, polarized light.
of all kinds of materials, such as textile, rocks and many others.
The pictures in this chapter illustrate that textures in minerals
and rocks are very diverse and may be formed by a variety of
processes, like crystallization and sedimentation, but also
through pressure, deformation or geochemical processes. Many
pictures in this paragraph are of 'thin sections', wafer-thin (10 to
40 micron) slices of rock, which are glued on to glass and prepared with special skill.Thin sections are one of the fundamental
means for analyzing rocks and minerals, and geologists routinely
examine them with a polarizing microscope. The inspection is
carried out with the help of various optical means and with
transmitted light that mayor may not be polarized. When the
light is polarized, crystals often take on striking colors, due to
birefringence of the light beams.
The actual sizes of the thin sections shown in this paragraph and
the next are in the range 10 to 50 mm. and not mentioned separately in the texts.
CARNALLITE Carnallite is a rock salt and as such belongs to
a group of sediments referred to as evaporites. Evaporites form
part of marine sedimentary sequences and are precipitated by
the drying up of isolated parts of continental seas during dry
and warm climates. A present day example is the Dead Sea of
Israel. In thin section and polarized light, the tightly interlocked
salt crystals take on attractive colors.
BISCHOFITE Bischofite is a magnesium containing rock salt,
like the carnallite of the previous plate. These two pictures are
of salts mined, by slurry pumping, from the deeper subsurface in
the north of the Netherlands. The thin sections here photographed are rather exceptional, since a special technique is
required for their preparation and preservation.
BORDERLINE The fine, darkish material in this thin section is
basalt, a magma that has solidified at, or near, the earth's surface.
When ascending to the surface from the deep underground,
such magmas may bring along bits and pieces of other rocks.
Some of these pieces do not melt, such as the chunk of peridotite on the right hand side of the picture, in which the olivine
crystals display their characteristic, lively colors of interference.
USCO I ES Muscovite is a micaceous mineral occurring
frequently in plutonic and metamorphic rocks. It is the mineral
TEXTURE
112
that often gives the sparkle to granite and gneiss or appears as
tiny stacks of wafers in other plutonic rocks (see Aquamarin,
page 54). In thin section and polarized light, micas display fading
colors of birefringence and a characteristic 'goose-flesh' texture.
PER DOTITE Peridotite is a 'plutonic' rock. It is formed at
great depth, where the earth's crust changes into the semi-solid
mantle. It is assumed that most of the earth's mass, notably the
mantle, has a peridotitic composition, but this has never been
proven (the earth's interior is simply too deep and too hot for
sampling, we know less of it than we do of the surface of the
Moon or Mars). Many meteorites, Le. those of the stony variety
(the other variety is metallic), also have the same composition,
which suggests that peridotite is a key building material of the
whole solar system. The colorful olivine that makes up the bulk
of peridotite, has been partly serpentinized, which means that it
has been altered, along the grain boundaries, into the water rich
mineral serpentine. This accounts for that craquelee look of an
old oil painting.
T G
EYE Tiger-eye is a fibrous mineral, belonging to the silicate group. It is well known for it's silky, golden gloss and is
often cut and polished into cabochons and trinkets. Here, in thin
section, its appearance is more that of a sanguineous chunk of
meat.
YLONITE MOSAIC Shown here in thin section is
mylonite, a thinly laminated, metamorphic rock. Fine-textured
seams of mica are intercalated with thin bands of quartz. The
quartz grains have undergone recrystallization and have thus
been rearranged into a close-fitting mosaic.
ORNBLENDE Hornblende belongs to a group of minerals
called the amphiboles, which occur in igneous as well as metamorphic rocks. In this thin section exposure the delicate texture inside the crystals is accentuated by the subtle gradations
in the colors of birefringence.
E Perthite, sometimes incorrectly referred to as a
feldspar mineral, is actually a texture of fine, interspersedlayers
occurring in alkali feldspars. This lamellar texture results from a
separation of sodium- and potash-feldspars during cooling and
solidification of the mineral.Thin section, polarized light.