(Fe) Fi Fo...Rust? EarthCache
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In a little corner of eastern Columbia, you’ll find a most interesting sight that few likely see. Looking upon a section of exposed bedrock, one can see evidence of a valuable mineral used the world over in many applications. That mineral is iron. How did it get there? How do we even know it’s there, anyway? Before we answer those questions, let’s learn about iron and its role in geology.
What is iron, exactly?
Iron is a chemical element with the symbol Fe (Latin: ferrum) and atomic number 26. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.
Iron exists in a wide range of oxidation states, from −2 to +6, although +2 and +3 are the most common. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen and water. Fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion.
Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is relatively soft, but is unobtainable by smelting because it is significantly hardened and strengthened by impurities, in particular carbon, from the smelting process. A certain proportion of carbon (between 0.002% and 2.1%) produces steel, which may be up to 1000 times harder than pure iron. Steels and iron alloys formed with other metals (alloy steels) are by far the most common industrial metals because they have a great range of desirable properties and iron-bearing rock is abundant.
Iron also plays an important role in biology, forming complexes with molecular oxygen in hemoglobin and myoglobin; these two compounds are common oxygen transport proteins. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals. A human male of average height has about 4 grams of iron in his body, a female about 3.5 grams. This iron is distributed throughout the body in hemoglobin, tissues, muscles, bone marrow, blood proteins, enzymes, ferritin, hemosiderin, and transport in plasma.
Where do we find iron?
Most iron is found right beneath your feet in the Earth’s crust, particularly in sedimentary rocks. Iron-rich sedimentary rocks are sedimentary rocks which contain at least 15% iron. Most sedimentary rocks, however, contain iron in varying degrees. The majority of these rocks were deposited during three specific geologic time periods: The Precambrian (3.8 billion years ago to 570 million years ago), the early Paleozoic (570 to 410 million years ago), and the middle to late Mesozoic (205 to 66 million years ago). Despite the multiple periods of in which they are deposited, they make up a very small portion of the total sedimentary record.
Iron-rich sedimentary rocks have economic use for the iron ore contained within. Iron deposits have been located on all major continents with the exception of Antarctica. They are a major source of iron and are mined for commercial use. The main iron ores are from the oxide group consisting of hematite, goethite, and magnetite. The carbonate siderite is also typically mined. A productive belt of iron formations is known as an iron range.
What are the main types of iron formations?
Ironstones
Ironstones consist of 15% iron or more in composition. This is necessary for the rock to be considered an iron-rich sedimentary rock. Generally, they are from the Phanerozoic which means that they range in age from the present to approximately 540 million years ago.
They can contain iron minerals from the following groups: oxides, carbonates, and silicates. Some examples of minerals in iron-rich rocks containing oxides are limonite, hematite, and magnetite. An example of a mineral in iron-rich rock containing carbonates is siderite and an example of minerals in an iron-rich rock containing silicate is chamosite.
Ironstones typically exhibit these features:
• They are often interbedded with limestones, shales, and fine-grained sandstones.
• They are typically nonbanded, however they can be very coarsely banded on occasion.
• The components of the rock range in size from sand to mud, but do not contain a lot of silica.
• They are not laminated and sometimes contain ooids. Ooids can be a distinct characteristic though they are not normally a main component of ironstones. Within ironstones, ooids are made up of iron silicates and/or iron oxides and sometimes occur in alternating laminae.
• They normally contain fossil debris and sometimes the fossils are partly or entirely replaced by iron minerals. A good example of this is pyritization.
• They are smaller in size and less likely to be deformed or metamorphosed than iron formations. The term iron ball is occasionally used to describe an ironstone nodule.
Iron formations
Iron formations must be at least 15% iron in composition, just like ironstones and all iron-rich sedimentary rocks. However, iron formations are mainly Precambrian in age which means that they are 4.6 billion to 590 million years old, so they are much older than ironstones.
Iron formations usually follow these characteristics:
• They're well banded and the bands can be anywhere from a few millimeters to tens of meters thick.
• The layers have very distinct banded successions that are made up of iron rich layers that alternate with layers of chert.
• They are often associated with dolomite, quartz-rich sandstone, and black shale.
• They sometimes grade locally into chert or dolomite.
• They can have many different textures that resemble limestone. Some of these textures are micritic, pelleted, intraclastic, peloidal, oolitic, pisolitic, and stromatolitic.
• In low-grade iron formations, there are different dominant minerals dependent on the different types of facies. The dominant minerals in the oxide facies are magnetite and hematite. The dominant minerals in the silicate facies are greenalite, minnesotaite, and glauconite. The dominant mineral in the carbonate facies is siderite. The dominant mineral in the sulfide facies is pyrite.
• Most iron formations are deformed or metamorphosed simply due to their incredibly old age, but they still retain their unique distinctive chemical composition; even at high metamorphic grades. The higher the grade, the more metamorphosed it is. Low grade rocks may only be compacted while high grade rocks often can not be identified.
• They often contain a mixture of banded iron formations and granular iron formations. Iron formations can be divided into subdivisions known as: banded iron formations (BIFs) and granular iron formations (GIFs).
The above classification scheme is the most commonly used and accepted, though sometimes an older system is used which divides iron-rich sedimentary rocks into three categories: bog iron deposits, ironstones, and iron formations. A bog-iron deposit is iron that formed in a bog or swamp through the process of oxidation.
How was iron deposited into the rock?
There are four facies types associated with iron-rich sedimentary rocks: oxide, silicate, carbonate, and sulfide facies. These facies correspond to the water depth in a specific marine environment. Oxide facies are precipitated under the most oxidizing conditions. Silicate and carbonate facies are precipitated under intermediate redox conditions. Sulfide facies are precipitated under the most reducing conditions. There is a lack of iron-rich sedimentary rocks in shallow waters which leads to the conclusion that the depositional environment ranges from the continental shelf and upper continental slope to the abyssal plain.
How does iron chemically react to the environment?
Commonly, the presence of iron is determined to be within a rock due to certain colorations from oxidation. Oxidation is the loss of electrons from an element. Oxidation can occur from bacteria or by chemical oxidation. This often happens when ferrous ions come into contact with water (due to dissolved oxygen within surface waters) and a water-mineral reaction occurs.
On to the Earthcache!
By now, you should have a good understanding of what iron is, how it’s formed and its uses. Let’s apply what you’ve learned! Two distinct sections of rust-colored rock stand before you, indicating that very nearby, iron-rich rock resides. Look at these bands and refer the the material you’ve just read to answer the following questions. Failure to answer the questions and then sending them to the cache owner will result in the finder’s log being quietly deleted.
1. On average, how wide (in feet) are the two streaks of oxidized iron before you?
2. Looking at the rock face, you can easily tell where it’s coming from. About how high is the rock layer from which the oxidized iron is emanating?
3. Describe the process of oxidization.
4. How much iron does the average man and woman have in their body?
5. In what three periods of Earth’s history were the majority of iron-rich rocks formed?
A picture of yourself posted at the site would be nice, but it’s not required.
Happy caching!
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