Pseudomorphs: The False Form Minerals

Andy Holt

http://www.emporia.edu/earthsci/amber/go336/holt


Table of Contents
  • Introduction
  • List of Minerals and Their Pseudomorphic Mineral Counterparts
    • Specific pseudomorph examples include:
  • References
  • Disclamer

  • Introduction

    The term pseudomorph comes from two Greek words. The words are pseudes which means false, and morphe which means shape or form. Pseudomorphs are minerals that have been chemically altered in some way and are new minerals which can have a new crystalline structure, but they still retain the shape of the original mineral. These changes occur when the mineral is reduced, oxidized, elements are added, or when elements are completely replaced. It is also possible for a mineral to pseudomorph into a new mineral and then turn back into the original mineral. There are many different pseudomorphic minerals in the world and only a few will be briefly examined.


    List of Minerals and Their Pseudomorphic Mineral Counterparts

    Please note that this list is far from conclusive and it is only meant to give an idea of how many pseudomorphic minerals are on Earth.

    Alpha-quartz to Beta-quartz
    Pyrite to Hematite
    Aragonite to Calcite
    Pyrite to Limonite
    Aragonite to Copper
    Pyrrhotite to Marcasite
    Aragonite to Cuprite
    Pyrrhotite to Wurtzite
    Azurite to Malachite
    Siderite to Pyrite
    Chalcopyrite to Stannite
    Stibnite to Stibiconite
    Garnet to Limonite
    Tungstite to Ferberite
    Marcasite to Hematite
    Paravauxite to Sigloite
    Pyrite to Geothite
    Wulfenite to Vanadinite

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    Alpha-quartz to Beta-quartz

    Each pseudomorph has changes in its physical properties due to the different crystal structure. Beta-quartz is different because it is a pseudomorph of alpha-quartz (regular quartz) and the chemical composition is the same; however, the crystal system is different. For this change from alpha- to beta-quartz to take place, an increase in energy, namely heat, is required. Beta-quartz is in the hexagonal crystal system while regular quartz is trigonal or rhombohedral subdivision of hexagonal. The differences do not stop there. Beta-quartz is more symmetrical, has a lower index of refraction (1.54 vs. 1.55), and is less dense (2.53 vs. 2.65).
    Regular quartz image taken from The Mineral Quartz, http://www.galleries.com/minerals/silicate/quartz/quartz.htm

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    Aragonite to Calcite


    Aragonite image taken from The Mineral Aragonite,
    http://mineral.galleries.com/minerals/
    carbonat/aragonit/aragonit.htm
    When aragonite pseudomorphs into calcite it is similar to the beta-quartz and alpha-quartz in that the chemical composition is the same, but the internal crystalline structure is different. Aragonite is in the orthorhombic crystal system where as calcite is trigonal. Aragonite's hardness goes down from a 3.5-4 to a 3 when it pseudomorphs into calcite. The density also lessens from 2.95 in aragonite to 2.72 in calcite. The cleavage changes as it should with a change in crystal systems. Aragonite has prismatic cleavage and calcite, rhombohedral or cleaves in three directions and forms rhombohedrons. Aragonite is only singly refractive where as calcite exhibits double refraction.

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    Azurite to Malachite

    Azurite image taken from The Mineral Azurite,
    http://mineral.galleries.com/minerals/carbonat/
    azurite/azurite.htm


    Malachite image taken from The Mineral Malachite,
    http://mineral.galleries.com/minerals/
    carbonat/malachit/malachit.htm

    Azurite and malachite are carbonates like aragonite and calcite. Azurite and malachite differ in the type of copper ion each mineral has. Azurite has a plus 2 copper cation and malachite has a plus 3 copper cation. One similarity that these two copper bearing minerals have in common is that they are both in the monoclinic crystal system. This is where the similarities end. Azurite is typically found in a tabular habit, and malachite is usually botryoidal or fiberous. In general, malachite has a higher hardness than azurite (4 vs. 3.5-4). Azurite has a lower specific gravity, 3.8, than malachite, 4. The reason that the azurite changes in color from blue to green when morphing into malachite is because the copper in the mineral is being oxidized. Oxidation is the same reason that the Statue of Liberty in New York Bay is green and not the reddish-orange look exhibited by elemental copper.


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    Pyrite to Goethite


    Pyrite image taken
    from The Mineral Pyrite, http://mineral.galleries.com/
    minerals/sulfides/pyrite/
    pyrite.htm

    These two minerals, pyrite and goethite, have almost nothing in common except that they both have iron as their cations and they are both opaque. Pyrite is a sulfide, whereas goethite is a hydroxide. Pyrite falls into the isometric crystal system and goethite is orthorhombic. Pyrite is also has a higher specific gravity (5.1 vs. 4.3) and has a higher hardness ( 6-6.5 vs. 5-5.5) than the pseudmorphic mineral that replaces the original sulfur ions with oxygen and hydrogen. Goethite typically has a dull luster where as true to metal compounds pyrite's luster is metallic. The striations seen in the pyrite picture to the left would be visible on a pseudomorphic goethite due to the fact that pseudomorphs keep the same outward appearance as the initial mineral.


    Goethite image taken from
    http://nsminerals.atspace.com/
    Bridgeville.html
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    Pyrite to Limonite

    Again we have the mineral pyrite, only this example is of it morphing into limonite instead of goethite. Again we have a sulfide changing into a hydroxide and they are both opaque minerals. The pyrite will fracture concoidally unlike the limonite which crumbles. Limonite does not have a defined chemical composition, rather it is the name typically reserved for a iron hydroxide mineral that has not be fully identified. This affects limonite's hardness and specific gravity. The hardness varies for limonite from 4-5.5 which is less than pyrite's hardness of 6-6.5. The specific gravity of limonite is somewhere between 2.9 to 4.3. The streaks of the two mineral also vary. Pyrite's streak is a greenish-black and limonite's is yellowish-brown.
    Pyrite image (left) taken from The Mineral Pyrite, http://mineral.galleries.com/minerals/sulfides/
    pyrite/pyrite.htm

    Limonite image (right) taken from The Mineral Limonite, http://mineral.galleries.com/minerals/oxides/
    limonite/limonite.htm

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    References

    Books

    Holden, Martin., Mathez, Edmond A. ed., 1991. The Encyclopedia of Gemstones and Minerals. Friedman Group.

    Tennissen, Anthony C., 1974., Nature of Earth Minerals. Prentice Hall.

    Wenk, Hans-Rudolf, Andrei Bulakh. 2004, Minerals: Their Constitution and Origin. Cambridge.

    Internet

    Amethyst Galleries, Inc .2006. Amethyst Galleries' Mineral Gallery. http://mineral.galleries.com/default.htm. Nov. 26, 2006.

    Glendale Community College. Glendale Community College Earth Science Image Archive. http://www.gc.maricopa.edu/earthsci/imagearchive/pseudomorphs.htm. Nov. 26, 2006.

    Hyrsl, Jaroslav., Alfred Petrov. Nov. 1998. Pseudomorphs from Bolivia: A Review. Rocks and Minerals. http://www.findarticles.com/p/articles/mi_m0GDX/is_1998_Nov/ai_53356001/pg_1 . Nov. 26, 2006.

    Mineralogy of Nova Scotia, Canada. Bridgeville Iron Mines. http://nsminerals.atspace.com/Bridgeville.html. Nov. 26, 2006


    Disclamer

    I am a student at Emporia State University. The goal of this assignment was to learn how to and create a webpage as well as to expand my knowledge on a topic regarding mineralogy, in this case pseudomorphic minerals. This webpage was created for GO 336, Mineralogy.

    This webpage was created on November 26, 2006.

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    Comments or questions? Email Andrew Holt.

    Return to Student Webpages, www.emporia.edu/earthsci/amber/go336/webpages.htm.

    Copyright 2006 Andrew Holt.