Pseudomorphs: The False Form Minerals
Andy Holt
http://www.emporia.edu/earthsci/amber/go336/holt
Table of Contents
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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, 
Malachite image taken from The Mineral Malachite, |
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 |
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/ |
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.