BIF (see under Folded BIF) is more or less a result of the same
forces and conditions that are at work in forming the largescale grabens. Actual size 25 x 40 mm.
MARAMA BA In this polished slab of BIF, the soft greens
and reds of jasper, the metallic glow of hematite, and the golden
fibers of tiger-eye evoke the likeness of an alien landscape,
remote and otherworldly. Actual size 90 x 120 mm.
APEX In this polished slab of BIF, from Western Australia, a
thin seam of hematite has been deformed and fragmented into
a crested form by the same stresses that generated the fibrous
texture of the surrounding tiger-eye. Actual size 75 x 100 mm.
BIOTI
IN 0
P RO E E A flake of biotite, a
micaceous mineral, has been buckled up, like a pennant in the
wind, by internal stress forces inside a holocrystalline mass of
orthopyroxene. The image is of a thin section of a rock sample
from the Bushveld complex, South Africa. The colors result
from polarization of the transient light used to make the photograph.
'Cleavage' is an advanced
stage of microstructural faulting and deformation in a rock,
whereby the rock in question often takes on a micaceous, finely foliated fabric. Crenulation cleavage, as seen in this thin section, results from successive phases of such deformation.
SNO
L
T This thin section demonstrates the
rotational deformation of a rigid object, in this case a large garnet crystal, that is suspended in a fine grained, more readily
deforming rock.Thin section, polarized light.
PRESS R FRIN E Pressure fringes, also called pressure
shadows, are a type of microstructure formed by intense shear
forces in deep-seated rocks. This example shows a darkish
brown spherule of pyrite in a schistose host-rock. The pyritic
spherule rolled along but remained rigid while the schist was
deformed. Quartz, formed in the 'wake' of the rotating
spherule, was stretched out into a fibrous and curved shape
that looks much like an old-fashioned airplane propeller. This
thin section is prepared from Archaean rock in the Yilgarn
region, Western Australia, which is some 2.5 billion years old!
WAVY CAL ITEThis thin section shows a detail of a calcite
vein precipitated inside a fracture in a rock, from hot fiuids,
deep in the earth. Deformational stress during the precipitation of the calcite brought about the curved, fibrous shape of
the crystals.
129
forces and conditions that are at work in forming the largescale grabens. Actual size 25 x 40 mm.
MARAMA BA In this polished slab of BIF, the soft greens
and reds of jasper, the metallic glow of hematite, and the golden
fibers of tiger-eye evoke the likeness of an alien landscape,
remote and otherworldly. Actual size 90 x 120 mm.
APEX In this polished slab of BIF, from Western Australia, a
thin seam of hematite has been deformed and fragmented into
a crested form by the same stresses that generated the fibrous
texture of the surrounding tiger-eye. Actual size 75 x 100 mm.
BIOTI
IN 0
P RO E E A flake of biotite, a
micaceous mineral, has been buckled up, like a pennant in the
wind, by internal stress forces inside a holocrystalline mass of
orthopyroxene. The image is of a thin section of a rock sample
from the Bushveld complex, South Africa. The colors result
from polarization of the transient light used to make the photograph.
'Cleavage' is an advanced
stage of microstructural faulting and deformation in a rock,
whereby the rock in question often takes on a micaceous, finely foliated fabric. Crenulation cleavage, as seen in this thin section, results from successive phases of such deformation.
SNO
L
T This thin section demonstrates the
rotational deformation of a rigid object, in this case a large garnet crystal, that is suspended in a fine grained, more readily
deforming rock.Thin section, polarized light.
PRESS R FRIN E Pressure fringes, also called pressure
shadows, are a type of microstructure formed by intense shear
forces in deep-seated rocks. This example shows a darkish
brown spherule of pyrite in a schistose host-rock. The pyritic
spherule rolled along but remained rigid while the schist was
deformed. Quartz, formed in the 'wake' of the rotating
spherule, was stretched out into a fibrous and curved shape
that looks much like an old-fashioned airplane propeller. This
thin section is prepared from Archaean rock in the Yilgarn
region, Western Australia, which is some 2.5 billion years old!
WAVY CAL ITEThis thin section shows a detail of a calcite
vein precipitated inside a fracture in a rock, from hot fiuids,
deep in the earth. Deformational stress during the precipitation of the calcite brought about the curved, fibrous shape of
the crystals.
129