Cosmic Connection to the Elements
The chemical elements are all around us, and are part of us. The composition of the Earth, and the chemistry that governs the Earth and its biology are rooted in these elements.
The elements have their ultimate origins in cosmic events. Further, different elements come from a variety of different events. So the elements that make up life itself reflect a variety of events that take place in the Universe. The hydrogen found in water and hydrocarbons was formed in the moments after the Big Bang. Carbon, the basis for all terrestrial life, was formed in small stars. Elements of lower abundance in living organisms but essential to our biology, such as calcium and iron, were formed in large stars. Heavier elements important to our environment, such as gold, were formed in the explosive power of supernovae. And light elements used in our technology were formed via cosmic rays. The solar nebula, from which our solar system was formed, was seeded with these elements, and they were present at the Earth’s formation. Our very existence is connected to these elements, and to their cosmic origin.
“To make an apple pie from scratch, you must first invent the universe.”
-Carl Sagan
Elements in the Earth’s crust
Almost 99% of the minerals making up the Earth’s crust are made up of just eight elements. Most of these elements are found combined with other elements as compounds. Minerals are elements or compounds that occur naturally in the Earth’s crust. Rocks are mixtures formed of minerals. Just as elements are the building blocks of minerals, so minerals form the building blocks of rocks. Table 1 gives some information about the most common elements in the Earth’s crust in order of abundance.
Element name / Symbol / Percentage by weight of the Earth’s crustOxygen / O / 47
Silicon / Si / 28
Aluminium / Al / 8
Iron / Fe / 5
Calcium / Ca / 3.5
Sodium / Na / 3
Potassium / K / 2.5
Magnesium / Mg / 2
All other elements / 1
Table 1 The elements in the Earth’s crust
A mineral can be defined as a naturally occurring inorganic solid that possesses an orderly internal structure and a definite chemical composition. There are some substances, like opal, which have the appearance of a mineral but lack any definite internal structure, are sometimes called "mineraloids". Lutgens and Tarbuck give the following list of essential characteristics of a "mineral":
- It must occur naturally.
- It must be inorganic
- It must be a solid
- It must possess an orderly internal structure, that is, its atoms must be arranged in a definite pattern.
- It must have a definite chemical composition that may vary within specified limits."
The most common minerals are the silicates, as one would guess by looking at the abundances of the elements in the Earth's crust, but there is a great variety of minerals. Minerals are classified in many ways, including hardness, optical properties, crystal structure, etc. Shipman, et al. comment that over 2000 minerals have been found in the Earth's crust, but that about 20 of them are common and fewer than 10 account for over 90% of the crust by mass.
Non-silicates constitute less than 10% of the Earth's crust. The most common non-silicates are the carbonates, the oxides, and the sulfides. There are also naturally occurring phosphates and salts. Carbon is found in both graphite and diamond form. Some minerals are valued as gems because of their hardness, color and beauty.
Rocks are classified by mineral and chemical composition, by the texture of the constituent particles and by the processes that formed them. These indicators separate rocks into igneous, sedimentary and metamorphic. They are further classified according to particle size. The transformation of one rock type to another is described by the geological model called the rock cycle.
Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth's crust (example granite), while volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta (examples pumice and basalt).
Sedimentary rocks are formed by deposition of either clastic sediments, organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65%; sandstones 20 to 25% and carbonate rocks 10 to 15%.
Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals.
The three classes of rocks — the igneous, the sedimentary and the metamorphic — are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points (more or less arbitrary) in a continuously graduated series.
Questions
1. Where do Elements have their origins?
2. When was the Hydrogen found in water formed?
3. Where was the Carbon in all life formed?
4. How were heavier elements formed?
5. How many elements make up almost 99% of Elements in Earth’s crust?
6. What are minerals?
7. What are the building blocks of minerals? What about rocks?
8. What are the 3 most abundant elements in the Crust?
9. List the 5 characteristics of a mineral.
10. What are 3 ways that minerals are classified?
11. How are rocks classified?
12. What are the 3 types of rocks?
13. What do we call the transformation of one rock type to another?
14. What is the difference between an extrusive and intrusive igneous rock?
15. Where do sedimentary rocks form?
16. How are metamorphic rocks formed?
17. These rock types are classified further, but are there really any distinct boundaries between one type of igneous rock and another?