vibration direction of lower polarizer (blue tourmaline is darkest w/ c-axis 2) The colour of amphibole is green, brown or black in hand specimen and green or brown in thin section. The minerals of the amphibole group crystallize in the orthorhombic, monoclinic, and triclinic systems, but the crystals of the different species are closely similar in many respects. Thin Section: Plane light: Moderate to high positive relief. 2nd to 4th order with highest interference colors in thin section in upper first or lower second order. From Broken Hill, New South Wales, Australia. Chlorite (Green, low interference colours) Its primary use might be as a mineral specimen. Chemically they form a group parallel to the pyroxene group, being silicates with calcium, magnesium, and ferrous iron as important bases, and also with manganese and the alkalis. Download: small … Brownish varieties have X=greenish-yelow/brown, Y=yellowish to reddish brown and Z=grey to dark brown. Pleochroic in various shades of green and brown. Amphibole has two cleavages running parallel to the length of the crystal. To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. Pyroxene (Two cleavages at 90 o, higher extinction angle, typically paler coloured), Common alteration product: Chlorite (pale green). However, hornblende is the most plentiful mineral in a rock known as amphibolite which has a huge number of uses. Note: grains showing two well-developed cleavages may be tricky to find, Note: extinction angles measured in elongate crystals typically <30o. Blue pleochroism in thin Green varieties usually have X= light yellow green, Y=green or grey-green and Z=dark green. Cleavages at 56 and 124 degrees which form a distinctive diamond shape in cross section. At Monte Somma and Vesuvius, Campania, Italy. Metabasic rocks may contain large amounts of amphibole at moderate grades = Amphibolites. The common varieties are tschermakitic and magnesio- and ferro-hornblende. In PPL a thin section of Hornblende ranges from yellow -green to dark brown. Crystals are slender and prismatic. Take the Amphibole test Add tags Comment Rate. Because of their relative instability to chemical weathering at the Earth’s surface, amphiboles make up only a minor constituent in most sedimentary rocks. Also partings on {100} and {001}. glaucophane. Glaucophane - Na-rich found stable at low T and high P (blueschists) amphiboles by distinct blue color in hand sample. A few historic localities for well-crystallized material include: Cite this article as: Geology Science. Crystal habit and cleavage distinguish hornblende from dark-colored pyroxenes. Glaucophane also has a parallel extinction when viewed under Exhibiting an extensive range of possible cation substitutions, amphiboles crystallize in both igneous and metamorphic rocks with a broad range of bulk chemical compositions. gray or black. Sample 11AS01A is an amphibole and biotite-rich tonalitic gneiss (S1, L 1) intruded by leucocratic veins; both are ptygmatically folded. Stronger pleochroism correlates to higher iron contents. The prismatic cleavage angle of amphiboles is about 56° and 124°, while the pyroxene cleavage angle is about 87° and 93°. Darkest when c-axis parallel to colorless, pale blue, yellow; Y= lavender-blue, bluish green; Z= blue, [online] Available at: https://geologyscience.com/minerals/amphibole/ [29th November 2020 ], Economically Important Metal Concentrations in Earth’s Crust, Porphyry Deposits: General characteristics and modeling, https://geologyscience.com/minerals/amphibole/, White, colorless – (brittle, often leaves cleavage debris behind instead of a streak), Two directions intersecting at 124 and 56 degrees, 2.9 to 3.5 (varies depending upon composition). The mineral hornblende has only a few makes use of. The amphiboles, however, contain hydroxyl. These chains are bonded to octahedral strips consisting of three regular octahedral sites (M1, M2, M3) and one larger 6- to 8-fold site (M4). Certain molecules that are present in some varieties contain aluminum and ferric iron. There is no twinning in Common mineral in Granites, Diorites, Metabasics, Calc-silicates, Rock-forming minerals associated with Amphibole: Plagioclase. Hornblende (thin section) View Description. {110} perfect – intersect at 56 and 124 degrees. It is differentiated from pyroxenes by cleavage and from other amphiboles by color. 3) Plagioclase is the prevalent light-coloured constituent, the quantity of quartz or epidote or scapolite should be lower than that of plagioclase. Commonly replaces: Clinopyroxene, Complete or partial alteration at lower temperature associated with influx of fluids, Amphibole forms typically well-shaped crystals in rocks with abundant quartz or plagioclase. The more strongly coloured minerals are often relatively Fe-rich or rich other transition metal ions (e.g. Biotite (One cleavage, straight extinction), Thus elongate sections through amphibole crystals show a single cleavage, cross sections show two cleavages intersecting at 120 o. Amphibole. Home Rocks and Minerals Hornblende (thin section) Reference URL Share . Hornblende – Fe-Al-rich stable at high T (igneous rocks and high T metamorphic rocks), Typically paler amphiboles contain less Fe and more Mg. Return to top greenish blue, violet, Measured: 10° to 80°, Calculated: Simple twinning is relatively common. Deeply coloured minerals in hand specimen are likely to be also coloured in thin section, although typically the colour is much stronger in hand specimen than thin section. 62° to 84°, nα = 1.606 – 1.637 nβ = 1.615 – In the USA, from Franklin and Sterling Hill, Ogdensburg, Sussex Co., New Jersey; from Edwards, Pierrepont, and Gouverneur, St. Lawrence Co., New York. However amphibole-rich rocks rarely contain a strong cleavage in comparison to mica-rich lithologies. The amphibole structure consists of doubled (Si 4 O 11) 6- chains running parallel to c-axis (Fig.4). amphibole cleavage. Prismatic crystal that can be, but is not necessarily, elongated. A thin section of 11AS01A captures the contact between a leucocratic vein and the gneiss (Fig. Return to introduction. section/grain mount distinguishes from other amphiboles. Distiguishing factors are the lack of birds eye extinction and the two distinct cleavages. Glaucophane has It is reduce for use as size stone. The amphiboles and pyroxenes closely resemble one another and are distinguished by cleavage. Actinolite – Fe-rich stable at moderate T (moderate T metamorphic rocks) Note: Regional metamorphism may cause elongate amphiboles to align in similar orientation. Note: twinning in amphiboles is relatively unusual. perpendicular to vibration direction of polarizer). Ideal crystal forms are elongate with The International Mineralogical association presently classifies amphiboles as a mineral supergroup, inside which might be businesses and several subgroups. Lavender blue, blue, dark blue, The highest excellent pieces are reduce, polished, and sold under the name “black granite” for use as building going through, ground tiles, counter tops, and other architectural makes use of. Hornblende is easly confused with biotite. 6 C). Key optical features of amphibole in thin section: Green (more rarely Brown, Colourless or Blue) Two cleavages (intersect at 120 o) Pleochroic Moderate interference colours Inclined extinction (typically 30 o) Common minerals that might be confused with amphibole and may occur in similar rocks: cross polars. Amphiboles may be inexperienced, black, colorless, white, yellow, blue, or brown. Colorless to pale green to deep green in thin sectoin with varying degrees of pleochroism. Distinguished from other