Rock & Gem
PEGMATITES: Nature’s Jewel Box
Granite Pegmatite Deposits Yield Gem Riches
Among the most beautiful and eye-catching mineral specimens exhibited at mineral shows are the granite pegmatite minerals. This includes lovely crystallized tourmalines, bright colorful topaz, richly-hued garnets, gemmy beryl varieties like delicate pink morganite and crystals of sparkling chrysoberyl. With them are matrix pieces like perfectly crystallized quartz, feldspar and mica. Often minor rare species occur with them as well.
The pegmatite minerals we so admire are the rare remains of huge granite masses that originate at least 30 kilometers deep in the earth’s crust. Not every granite pegmatite holds gem pockets but those that do yield rare and beautiful crystal groups found on every continent.
We see so many of these wonderful pegmatite minerals we may think gem pegmatites are fairly common. Far from it. For centuries we have made it a practice of finding and mining them.
These choice gems form only when the last chemical vestiges of an enormous molten granite mass under great pressure cools so crystals can form. The fluid pressure also forces the molten mass to slowly move toward the earth’s surface forcing the molten mass into any weakened horizontal boundary between existing rock layers. Once exposed near the earth’s surface we see them as lighter layers and deposits seemingly thrust between darker layers of granite and other igneous rocks. Humankind has made a concerted effort to seek them out now that we knew of the treasures they hold. Nature helps us by weathering out crystal evidence that tell us where to dig.
As a mineral collector who spent years collecting specimens from Connecticut’s gem pegmatite deposits, I’ve always wondered how crystals in gem pockets formed.Over the next 70 years more and more information has come to light as people like John Sinkankas, Richard Jahns and especially Dave London of the University of Oklahoma studied the gem pegmatite problem. These
experts, among others, have delved successfully into the mysteries and secrets of these rare gem deposits.
I was lucky to meet Jahns during his teaching lifetime, worked with Sinkankas days on end learning about California’s pegmatites and got to know David Johnson at Arizona State University and enjoy his writings and lectures in pegmatites. His recent works are very revealing and well worth reading if you are interested in pegmatites. Several articles and his book, “Pegmatites” are superb and contain detailed studies of the subject.
Sinkankas made countless studies and collecting trips to the Southern California and Baja California, Mexico pegmatite mines. Jahns studied the pegmatites of New England and wrote major works for the federal government on those pegmatite deposits. Johnson studied Arizona’s pegmatites and went on to develop, in depth, pegmatite research at the university level. From their writings, and others, I have found answers to the pegmatite questions I had in the 1950s.
Gem pegmatite deposits are possible because those rare minor chemical remnants of huge molten granite batholiths end up at the very end of a pegmatite’s journey and develop the complex gem crystals we seek. The granites in the earth’s crust vary according to their chemistry. The most common type are fine grained and composed mainly of quartz, feldspar some mica and very minor other minerals. The crystals are very small and basically shapeless. This type granite yields no gem crystals but is quarried for building stone, counter tops and even tombs.
How slowly a granite mass cools affects grain size and the possibility of gem pockets. The more slowly it cools grain size gets larger and may show some crystal forms. Other factors in particular the watery fluids of the granite come into play. The fluids tend to become richer in rare mineral matter as the common mineral crystals like quartz and feldspar leave the fluid and crystallize.This brings about a change in the fluid and it becomes more viscous. London suggests the remaining fluid becomes almost gel-like and the dissolved gases in the fluid develop bubbles which can account for the pockets observed when opening a pegmatite.
This brings up a misconception collectors seem to have. We refer to the openings in a pegmatite as ‘pockets’ because of their crystal content. However, the crystal pockets we collect in the Pike’s Peak and other areas are referred to as ‘miarolitic cavities’ - a name taken from a pegmatite district in Italy. They are, in fact, pegmatites are different as the Pike’s Peak pockets are well known for smoky quartz and microcline/amazonite crystals and not gem tourmaline and other gem crystals.
Miarolitic cavities only differ in their content
from gem pegmatites in their chemistry and resulting crystal content - but all are pegmatites.
The watery or gel-like fluids of an intruding granite mass is mainly a silica fluid so quartz and feldspar form first as they crystallize at high temperatures. But those same fluids also are carrying a lot of elements in trace amounts that stay fluid later and only crystallize at lower temperatures. These trace elements are critical in determining the final gem crystals we seek in gem pegmatite pockets. These minor (but so important) elements in the final solution of the pegmatite include uranium, lithium, sodium, calcium, cesium, beryllium, fluorine, rhubidium, boron, niobium, tantalum, and even traces of metals like chromium, iron and manganese.
These uncommon elements are vital to forming the gem crystals we seek in pegmatite pockets. One reason they persist in solution until the end is they do not easily become part of the silicon minerals quartz and feldspar, but readily form their own species like tourmaline in the later forming gem crystal pockets.
The constant fluid pressure causes a rising granite to move horizontally between weaker rock layers, carrying its load of rare elements with it. Such narrowing space and movement causes the hot mass to slowly cool and crystallization of quartz and feldspar begins. With cooling comes a slow drop in pressure. The dissolved gases I mentioned earlier can expand, which can then exert pressure on the hot surrounding rock, literally creating pockets in which crystals can later develop.
The pockets are not empty chambers as the gases and fluids accumulate giving the minerals in solution an opportunity to accumulate and crystallize sequentially. Some elements - like lithium and fluorine - remain in solution much longer than other elements like silicon. This means the final vestiges of the fluids from the once-huge granite mass end up with areas of concentration for these rare elements to form gem crystals like beryl, tourmaline, topaz, and a variety of species. The trace metal elements are not concentrated enough to form their own crystals. So they end up as
trace elements in crystals that are forming. Elements such as manganese, iron, vanadium, and chromium end up as chromophores in the forming gem crystals giving them the beautiful colors we enjoy.
This explains why watermelon elbaite is pink in the center thanks to manganese as its forming and green on the perimeter thanks to traces of iron atoms during final crystallization adding the contrasting green “rind” of the crystal.
Over the years there has been some disagreement over what has been referred to as “pocket explosion.” When some gem pockets are opened they contain lovely gem crystals. But instead of being attached to the pocket walls where they formed, they are broken and scattered throughout the pocket filling, usually a feldspar clay. After the crystals develop, it’s obvious that something happens to not only break some crystals from their bases, but even scatter fragmented crystals in some cases.
Proof of this is well documented. I’ve dug in crystal pockets where the lovely crystals are not where they formed. Thank goodness the pocket clay is there to cushion their movement. Some crystals are found many inches away from their base which is still attached to the pocket wall. In extreme cases pieces of a fractured crystal have been found scattered all the way across the pocket. That’s a pretty good reason to think some sort of explosion or pressure burst happened in the pocket after crystallization.
A practical example of this are the elbaite crystals being recovered from the Pederniera mine in Brazil. The tourmaline crystals from here can be as much as a foot long or more, very slender and often in diverging sprays of jaunty angles, obviously fragile and very fragmented when the pockets are opened. These are among some of the most beautiful gem tourmalines found in recent years and when they are whole are in high demand.
When you see these amazing tourmaline crystal sprays on display, you have to wonder how such long and quite delicate prisms could have survived intact. They did not!When a gem pocket is opened, miners carefully remove every section of the elbaite crystals, with every scrap including the crystal base still attached to the pocket wall. Then the fun begins. Each piece from a pocket is carefully cleaned and laid out on a table and, like a huge jigsaw puzzle, the crystals are put back together piece by piece - a complex task of marching broken end to broken end.
It helps that today’s cements are powerful and nearly invisible so you can’t see the repairs. Years ago we were limited in what glues we could use and the glues were often easy to spot. Today, all sorts of cements and glues are available and many are practically invisible. The cements are used to assemble and return to its matrix where formed. Such work on crystals today is common knowledge and reputable dealers discuss this practice freely. What had been torn apart, perhaps by a pocket explosion, is returned to its matrix as a whole crystal where it formed.
Nature’s creation is then whole again and ready for display.
This brings up the question of how these giant crystals are broken, not violently but fractured from their base and broken into sections so nicely they can be put back together. But what can cause a pocket explosion in what seems to be a sealed chamber? Ground movement may be a contributing factor.
When the final stages of a granite pegmatite finally settle down and create pockets of crystals, the solution is still hot, something around 700 degrees or hotter. Trapped in a pegmatite flow within the crust, water still under pressure can’t convert to steam and remains fluid. Imagine what the water in a gem pocket will do if a sudden drop in pressure happens as the pocket develops a fault. Water converts to explosive steam and more fragile crystals will fracture, yet survive undamaged in pocket clay and can be reassembled.
Do all gem pockets explode? No, but when they do we now have the means to retrieve Mother Nature’s creations. When you next see gorgeous pegmatite gems remember the hazardous journey they made just to please you.