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Roger Weller, geology instructor

wellerr@cochise.edu

zircons
by Clarissa Madrid
Physical Geology
Spring 2016
  
 
 

Zircon: The Oldest Mineral on Earth

 

            The oldest mineral found on Earth dates back to be around 4.374 billion years old, give or take six million years (1). These minerals were found in Australia on a mountain range called Jack Hills (see Fig. 1, 2, and 3). To be considered a mineral, the material has to meet the following five criteria: “1.) an element or a chemical compound, 2.) naturally occurring, 3.) a crystalline structure, 4.) inorganic with no carbon (exceptions are carbonates and pure carbon), and 5.) distinct physical properties” (Weller).  The minerals called zircon are a part of the crystal family and contains a crystalline structure (see Fig. 4). Crystals are minerals whose

…internal, geometrical repeating pattern of atoms is consistent throughout the entire structure; the object may also have flat surfaces called crystal faces that reflect the internal arrangement of atoms. A crystalline structure is where the atoms within a crystal are all aligned in a geometrical, repeating three dimensional pattern. (Weller)

 

                       
figure 1












figure 2

                                                                                 

   
figure 3
 

   


                       
figure 4

                                                                           

 

The zircon crystals were not found as a whole, but in sandstone according to one source (see Fig. 5) (2), and in another source in granitic rocks, such as granodiorite rocks (see Fig. 6) (1). The zircon crystals are resistant to chemical weathering so they may be found in many different igneous and metamorphic rocks (see Fig. 7, it is merely an example (not found in Jack Hills) of Red-black Zircon crystals in contrasting white marble matrix with black Biotite scattered throughout).
 

Sandstone is a clastic sedimentary rock predominantly made up of sand grains (mainly quartz and feldspar) and is usually formed by a lithification process. Lithification is a natural procedure of loose sediments transforming to solid rock either by cementation and/or compaction (Weller). Granodiorite rocks are coarse-grained, intermediate, igneous rocks that contain more plagioclase than orthoclase, and are composed midway of granite and diorite igneous rocks (Weller). “Igneous rocks are rocks formed as magma cools and solidifies. Coarse grained is a term that describes an igneous rocks texture in which the mineral grains in the rock are visible to the naked eye. Plagioclase feldspars are the calcium and sodium rich feldspars. Orthoclase feldspar is a potassium feldspar. Diorite is a coarse grained igneous rock containing mostly albite (white) feldspar and hornblende. Granite is a coarse grained igneous rock made up of feldspar, mica, and quartz” (Weller).
 

https://classconnection.s3.amazonaws.com/128/flashcards/765128/jpg/quartz_sandstone1331010912836.jpg                 http://skywalker.cochise.edu/wellerr/vrtglg/ca-whitneygranite/6trip-whitney9.jpg                   Zircon Crystal in Matrix from Bulbin, Gilgit District, Northern Areas, Pakistan

(Figure 5)                                             (Figure 6)                                             (Figure 7)

 

A study called “Bowen’s reaction series” was conducted to describe how these igneous rocks were formed in the process of the cooling of the magma. Three different series came out of Bowen’s reaction series called the continuous series, the dis-continuous series, and the residual series. The continuous series is described as

…”how the composition of developing feldspar crystals changes as the magma cools. The first plagioclase feldspar crystals to form at high temperatures are calcium-rich (anorthite). The last crystals to form are sodium-rich (albite). During the entire crystallization process the already formed feldspar crystals chemically react with the magma. Bowen's reaction discontinuous series describes the sequence in which the ferromagnesian crystallize as magma cools. The first member of the discontinuous series to form is olivine, followed by pyroxene, then amphibole, and finally biotite. Bowen's reaction series of residual, low temperature silicates are the last silicate minerals to crystallize out of a cooling magma. First is muscovite (white mica), then quartz, and last is orthoclase feldspar.” (Weller)
 

The zircon crystals were formed by a crystallization process in either the cooling of magma or metamorphism. The chemistry makeup of a zircon crystal is ZrSiO4. The zircon crystals found at Jack Hills contained uranium. Uranium holds a radioactive element which allows the scientist to conduct a more accurate test for dating of the crystal. A Radioactive mineral is “a substance that contains atoms whose nuclei are unstable, which causes it to slowly decay and emit radiation. Radioactive minerals take an extremely long time to decay if kept in proper settings” (8). Over a certain amount of time, the uranium turns into another element (its paramorph element) called lead (2). Paramorph is a “pseudomorph involving two minerals with an identical composition but different crystal structures. The original mineral forms, but conditions then cause it to be unstable, so it transforms into the other mineral with the same chemical structure while retaining the original crystal shape” (10).
 

The change from uranium to lead occurs when the radioactive material becomes unstable and “cannot withstand their composition and begins to decay” (10). Unstable refers to a “mineral with a crystal structure that is unable to exist at the current temperature or environment. When certain minerals become unstable, they transform into their paramorphs” (8). “The half-life of the radioactive uranium 238 isotope is 4.5 billion years, which makes it useful for dating extremely old materials” (2). “Radioactive half-life is the amount of time it takes for half of the atoms of a specific radioactive isotope to decay. An isotope is a variety of a chemical element, identified by the number of protons plus neutrons in the atomic nucleus” (R.W).   
 

The age of the zircon crystals give scientist a deeper insight to when the cooling of the Earth occurred. The evidence from the age suggest that the Earth cooled fairly quickly from its lava formation, within about 160 million years; after the theory of the mars size collision with Earth, which had formed the Earth’s moon (see Fig. 8 it is a timeline showing when the cooling of the Earth took place and when the formation of the Jacks Hills zircon crystal was formed) (2).

 

(Figure 8)http://i.livescience.com/images/i/000/062/887/original/Timeline.jpeg?1393022721

 

Zircon crystals are also used as gemstones, and is also the birthstone for December. It is known for its various colors of blue, green, orange, brown, red, yellow, purple, and clear colors. These zircons can also contain radioactivity. According to an article they undergo a metamiction process which destroys the crystal structure (10). The definition of metamict crystals are crystalline minerals that lose their crystal structure due to radioactive destruction of the radioactive elements within them (10). They are then heat treated to stabilize them for the use as a gemstone (10). The heat treatment is said to destroy the radioactive element within (10). Heat treatment is also used to enhance the transparency of the crystal (10). It can also change the entire color of a crystal depending on the heat treatment that’s given (10). They can be found in Cambodia, Burma (Myanmar), Sri Lanka (Ceylon), Australia, Tanzania, Mozambique, Madagascar, Nigeria, Pakistan, and Afghanistan (10).
 

 

Yellow Zircon   Brownish-Orange Zircon   Golden-Yellow Zircon  Bright Blue Zircon   Reddish-Purple Zircon   White Zircon

References
 

 

 

 

 

 

1.      http://www.livescience.com/43584-earth-oldest-rock-jack-hills-zircon.html

 

2.      http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/cs_zircon_chronolgy.html

 

3.      (Figure 1) Where Australia is located on a world map. http://ontheworldmap.com/australia/australia-location-map.jpg

 

4.      (Figure 2) Where Jack Hills is located in Australia. http://d366w3m5tf0813.cloudfront.net/wp-content/uploads/Jack_Hills_map.jpg

 

5.      (Figure 3) Jack Hills, Australia, where rocks were found to contain the oldest known minerals on Earth, a 4.4 billion-year-old zircon.
Credit: John Valley, University of Wisconsin
http://www.livescience.com/43584-earth-oldest-rock-jack-hills-zircon.html

 

6.      (Figure 4) Cathodoluminescence image of a 400-μm Jack Hills zircon.
Credit: John Valley, University of Wisconsin
http://www.livescience.com/43584-earth-oldest-rock-jack-hills-zircon.html

 

7.      (Figure 5) A sandstone rock. https://classconnection.s3.amazonaws.com/128/flashcards/765128/jpg/quartz_sandstone1331010912836.jpg   

 

8.      (Figure 7) An example of a zircon crystal in the raw. http://www.minerals.net/GemStoneInTheRoughImages/zircon-matrix-pakistan-g.jpg

 

9.      (Figure 8) Timeline showing when the zircon crystals were formed in Jack Hills.

            http://i.livescience.com/images/i/000/062/887/original/Timeline.jpeg?1393022721

 

10.  http://www.minerals.net/gemstone/zircon_gemstone.aspx

 

11.  The different colored zircon crystal gemstones shown in order from right to left.

http://www.minerals.net/GemStoneImages/zircon-yellow-gem-232236a.jpg

http://www.minerals.net/GemStoneImages/zircon-gem-165527a.jpg

http://www.minerals.net/GemStoneImages/zircon-gem-164813a.jpg

http://www.minerals.net/GemStoneImages/zircon-blue-gem-266974a.jpg

http://www.minerals.net/GemStoneImages/zircon-gem-171684a.jpg

http://www.minerals.net/GemStoneImages/zircon-gem-276598a.jpg

 

12.  (Figure 6) and all Weller in text citations on vocabulary words are credited to the Cochise College physical geology professor Roger Weller and can be found on his web site at: http://skywalker.cochise.edu/wellerr/GLGP-illvocab/GLGP-I.htm