Geoscience Reference
In-Depth Information
1cm
1cm
1cm
Bt
Ep
Ms
Ky
Hbl
Grt
Chl
St
9
(
(
(
1cm
1cm
1cm
Bt
Ky
Act
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(
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Gln
Grt
Crd
Amph
Omph
And
Px
Tlc
Crd
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(
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Figure 9.8
Examples of common metamorphic minerals. (a) Typical rounded crystals of dark red garnet (Grt),
wrapped by a tectonic fabric. Dark prisms with pointed ends are staurolite crystals (St), appearing black due to
graphite inclusions. The crystal shape is a better diagnostic feature than colour. Staurolite is characteristic of
iron-rich metasediments. (b) Biotite (Bt) and muscovite (Ms) are common as shiny, elastic fl akes defi ning the
foliation, as in this gneiss. Bright blue grains are small kyanite (Ky) crystals. (c) Three green minerals are
associated with pale pink, Ca-garnet in this metabasic rock. The characteristically yellowish green mineral is
epidote (Ep), which commonly occurs as aggregates. The dark green (almost black) mineral is the amphibole
hornblende (Hbl). The other bluish-green mineral is chlorite (Chl), common in fi ne-grained, low-grade rocks. (d)
The bladed shape of kyanite (Ky) is a reliable diagnostic feature. However, the striking blue colour displayed in
this kyanite-biotite schist is commonly pale or absent in other examples of the mineral. (e) Fibrous masses of
sillimanite with a pearly sheen are intergrown here with quartz, but often occur with biotite. (f) Prismatic
amphibole crystals may show rhombic cross-sections in well-formed crystals. Amphiboles are widespread, and
abundant in many mafi c rocks. This dark green amphibole is actinolite (Act), intergrown with white mica.
(g) Pyroxene generally appears duller than amphibole, and may show square or rectangular cross-sections.
Pyroxene is characteristic of high-grade rocks. In this sheared gabbro, olive-brown pyroxene crystals (Px) have
been altered during deformation to dark coronas of amphibole (Amph), forming eye-shaped features known as
augen. (h) Cordierite (Crd) is usually found as indistinct ovoid blobs as here, looking almost like grease-spots or
raindrops. Contrast these with the sharp outlines of andalusite (And) porphyroblasts in this hornfels. (i) Talc is
so soft that it feels soapy. Here, talc (Tlc) forms ovoid pseudomorphs with a pearly sheen after olivine in a
metagabbro, with thin garnet (Grt) coronas (pink), dark glaucophane (Na-amphibole; Gln), bright green
omphacite (Na-pyroxene; Omph) and a pale groundmass of altered plagioclase.
