In thin section, it shows high relief with a characteristic green - brown - yellow pleochroism. Optic sign and 2V angle cover a wide range and not very useful in the distinction of hornblende. Basaltic Hornblende also called Oxy-hornblende - is a dark brown to reddish brown variety of hornblende that results from oxidation during crystallization of basalts, andesites, dacites, and rhyolites. It usually has a dark reaction rim that consists of opaque oxide, and is characteristically pleochroic in yellow to brown to reddish brown colors.
Anthophyllite - does not occur in igneous rocks, but is a constituent of metamorphic rocks. Cummingtonite - Grunerite - is more common in metamorphosed igneous rocks where members of the series occur with hornblende.
It has been found in siliceous volcanic rocks as well. Cummingtonite is optically positive, while grunerite is optically negative. Members of this series can be distinguished from orthorhombic Anthophyllite by the inclined extinction of the monoclinic Cummingtonite-Grunerite series, and can be distinguished from tremolite and actinolite by the higher refractive indices and higher birefringence of the Cummingtonite Grunerite series.
Glaucophane - Riebeckite - Glaucophane is a common mineral in blueschist facies metamorphic rocks that result from low temperature, high pressure metamorphism along ancient subduction zones. Riebeckite is found in alkali granites, syenites, and peralkaline rhyolites. Glaucophane is easily distinguished from the other amphiboles by its characteristic blue-lavender pleochroism.
Glaucophane is length slow, whereas Riebeckite is length fast. Arfvedsonite - occurs most commonly in peralkaline volcanic rocks and alkaline plutonic igneous rocks, where it typically occurs with the sodic pyroxene aegerine. Its blue green to yellow green pleochroism distinguish it from the other amphiboles. The chart below, also found in your lab assignments, summarizes the properties used to distinguish the amphiboles.
Examples of questions on this material that could be asked on an exam. Inosilicates Pyroxenes and Amphiboles. The single chain silicates have a basic structural unit consisting of linked SiO 4 tetrahedra that each share 2 of their oxygens in such a way as to build long chains of SiO 4. The basic structural group is thus Si 2 O 6 with an Si:O ratio of The most important inosilicates are the pyroxenes. The same thing happens to a rock when it is heated enough.
Of course, it takes a lot of heat to melt a rock. It takes temperatures between and 1, degrees Celsius 1, and 2, degrees Fahrenheit to melt a rock, turning it into a substance called magma molten rock. Heat, pressure and chemical fluids are the three factors that contribute to metamorphism.
Heat can come from the compressive force of gravity, friction between tectonic plates or radioactivity. Pressure builds as rock is buried, and more rock piles on top of it. Direct pressure from tectonic plates also occurs. There are two major kinds of metamorphism: regional and contact. Regional metamorphism.
Most metamorphic rocks are the result of regional metamorphism also called dynamothermal metamorphism. These rocks were typically exposed to tectonic forces and associated high pressures and temperatures. How do you tell the difference between hornblende and augite? Is enstatite and Orthopyroxene? How can you tell the difference between clinopyroxene and orthopyroxene? Is hornblende an amphibole? Is feldspar a quartz? What Colour is amphibole? Which silicate is called amphibole?
How can you tell the difference between pyroxene and amphibole? What does amphibole look like? Is Garnet an amphibole? Hornblende, calcium-rich amphibole mineral that is monoclinic in crystal structure. Hornblende has highly reflective cleavages whereas augite cleavages are dull. Hornblende tends to form elongate rectangular crystals whereas augite crystals tend to be blocky. This shows the typical very dark green color of augite as well as its typically mediocre cleavage. General considerations.
Amphiboles are found principally in metamorphic and igneous rocks. They occur in many metamorphic rocks, especially those derived from mafic igneous rocks those containing dark-coloured ferromagnesian minerals and siliceous dolomites.
Identification: Typically, amphiboles form as long prismatic crystals, radiating sprays and fibrous aggregates. They are generally dark coloured though their colours can range from colourless to white, green, brown, black, blue or lavender. This property is related to composition, particularly iron content. It is used as paving stones and as a veneer or facing on buildings both for interior and exterior use. It is also used as crushed stone for the usual crushed stone applications such as road and railroad bed construction.
In this application it is used locally, near the source of the amphibolite. Wollastonite is a calcium inosilicate mineral. Although it is a single-chain silicate, wollastonite is not a pyroxene.
Wollastonite have a different type of infinite-chain structure, with three tetrahedra per unit cell arranged parallel to y Fig. Olivine is commonly recognized by it high retardation, distinctive fracturing, lack of cleavage, and alteration to serpentine. Colorless to olive green in thin section. Hardness Simple twinning is relatively common. Crystal habit and cleavage distinguish hornblende from dark-colored pyroxenes.
Is a common mineral occuring in both igneous and metamorphic rocks. It characteristically forms near other amphiboles due to the complete solid solution. The key difference between muscovite and biotite is that muscovite contains mainly potassium and aluminium, while biotite mainly contains potassium and magnesium. Muscovite and biotite are phyllosilicate minerals. It is found in many intrusive igneous rocks ranging in composition from granites to diorite to gabbros to syenites.
It occurs as phenocrysts in several varieties of extrusive igneous rocks, such an andesite. Orthorhombic pyroxenes differ from monoclinic pyroxenes in that they have parallel extinction.
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