Pegs 101

Welcome to Pegmatology 101. My name is Professor Rabbit. I will be teaching this course on the Basics of Pegmatology. I will be discussing the what, when, why and how of pegmatites.

I have traveled all over the world studying pegmatites with my colleagues at the University of New Orleans. I will be sharing some of my knowledge and photographs from my travels. I hope you find my course interesting and informative. Please feel free to e-mail me any topics you would like to see addressed in this class. E-Mail – Professor Rabbit

Our first lesson will be a general introduction into Pegmatology.

“Class pay attention!”

What is a pegmatite?

How do you define a rock with diverse mineralogy, variable textural features, complex structural units, and exotic geochemistry?  In part, because of these qualities, pegmatites are difficult to define briefly and accurately. They have been described as unique, striking, bizarre, erratic, confusing, puzzling, unusual, to name just a few.  So why do scientist study pegmatites?  Because they are unique, striking, bizarre, erratic, confusing, puzzling, and unusual.

The word pegmatite was first used in print (1813) by A. Brogniart who ascribed the term (apparently a classroom usage) to L’Abbe Hauy. The term was used to describe a rock composed of “feldspar lamellae and quartz” otherwise known as the synonym, graphic granite as we see in the photo below.

In 1845, W. Haidinger was apparently the first to use the word pegmatite to describe “coarse-grained, feldspar-rich granites”.  However, in 1849, A. Delesse used the word pegmatite to also include rocks of very large grains which consisted of orthoclase, quartz and silvery mica, and which occur so commonly in the form of dikes, small stocks and nests in other rocks.  Our present-day use of the word follows the basic idea of Delesse, but also include the caveat that they be of igneous origin.  In most cases, the igneous rocks are of granitic composition, although other compositions (e.g., granodioritic, gabbroic) may exist. Many others include as part of the definition, aspects about the mineralogy, texture, structure, chemistry and mode of occurrence.  Taking this approach, we can define pegmatites as such: Pegmatite, in the strictest sense, is a textural term used to describe exceptionally coarse- to gigantic-grained igneous rocks.  They occur as tabular dikes, sill, lenses or veins near the margins of plutons.  They tend to have zoned or layered structures, extremely variable texture of mineral aggregates and some of them are enriched in rare elements. The shape and size of pegmatites vary greatly from linear, tabular bodies with straight edges to bulbous and irregular masses to turnip-shaped bodies. Pegmatites may be several meters long and less than 1 meter thick or as much as 3000 meters long to 700 meters wide.

Below is an illustration of a tabular dyke with straight edges.

Below we see a pegmatite that has irregular contacts with a fine-grained granite.

World map of pegmatite localities.

Where are pegmatites found? 

Pegmatites are widely distributed in the earth’s crust and are found on all continents (yes…even Antarctica.).  They are most abundant in mountain chains and on stable shield areas (like the Canadian Shield).  They are typically associated with large granite bodies often distributed along their margins, but are also found within them.

How old are pegmatites?

Pegmatites are almost as old as the earth’s crust.  Pegmatites of Precambrian age (2.8 to 1.0 billion years) are the most abundant and widespread.  These are generally found in the stable shields of Canada, Greenland, Russia and similar geologic environments.  In contrast, some of the youngest pegmatites (roughly 20 to 5 million years) are found in the Himalaya mountains of Pakistan and Nepal. 

Why are pegmatites important?

Granitic pegmatites are important sources of rare-elements, such as beryllium, niobium, tantalum, tin, lithium, rubidium, cesium and gallium; industrial minerals; gems and mineral specimens.  When present in economic quantities, these rare-elements may be extracted for use in a wide range of technological applications, such as lightweight alloys, nuclear engineering and electronics (beryllium); ceramics, pharmaceutical products, lubricants, smelting of aluminum ore and lithium-batteries (lithium); electronic capacitors, jet engines and prosthetic devices (tantalum); magnetohydrodynamic electric generators, biological and medical research (cesium); and integrated circuits and light-emitting laser diodes (gallium). 

Although alternative geologic sources are available, pegmatites remain a primary source of some rare-metals, and for this reason, our understanding of the economic potential and pegmatite-generating process must be constantly upgraded.

The industrial minerals, feldspar and quartz, are extracted from pegmatite deposits for use by the glass and ceramic industries, while mica is used in construction materials and insulation. 

Some of the world’s best-known gem material is obtained from pegmatite deposits.  Varieties of beryl (aquamarine, golden, morganite), spodumene (kunzite, hiddenite) and tourmaline (pink, green and multi-colored elbaite), as well as garnet and topaz are all valued precious stones originating from pegmatites. 

How large can minerals grow?

One of the features which attract many people to pegmatites, for the first time, are the abundance of minerals that are found in them. There have been about 550 different minerals found in pegmatites. Pegmatite minerals are typically much larger than most minerals in other rocks.  For example, the minerals in granites are typically on the order of a few millimeters across whereas similar minerals in pegmatites may be several centimeters or meters across.  The purple tourmaline on the right, “The Rocket”, illustrates how large some minerals can grow. The largest known crystal in a pegmatite, a spodumene from South Dakota, measured almost 13 meters (42 feet) long.

How to become a “pegmatologist”

pegmatologist is someone who studies pegmatites.  This person is generally a scientist, but may also be an amateur who LOVES collecting pegmatite minerals and will do just about anything to learn more about them.  Do you want to be a pegmatologist?  It’s easy.  Go into the field, look at a lot of pegmatites, collect their minerals, examine the pegs closely, and read…A LOT!   Unfortunately, that’s the biggest problem for the amateur…a lack of easily obtainable reading material.  During our many discussions with mineral collectors, miners and non-scientists, it became increasingly clear that an easily understandable, non-technical book on pegmatites was needed.  Sadly, a book of this sort doesn’t exist (to my knowledge).  We are privilege to have the services of Professor Rabbit who will be teaching the newcomer or beginner, basic info in the field of “pegmatology”.  This course will not answer all the questions a beginner might have, but it’s a start.  We hope it stimulates a deeper interest in the subject and provides a brief, yet intriguing insight into the complex and dynamic world of pegmatites. On the “Student Opportunities” page we have information on Internships and Graduate Studies available for anyone interested in pursuing pegmatite studies.

Selected Reading

Starving for more information on pegmatites?  Here’s a selected list of books and articles that should help satisfy your cravings…or make you hungry for more. The Pegmatology Reading List includes books and articles that the reader might find useful in their quest to better understand pegmatites.  The list includes review articles by prominent scientists and short course handbooks.  Some of these references may be difficult to acquire.  Check your local University Library for assistance.