The LCT Pegmatite and the Production of Gemstones

Beryl crystals on schorl from a vug in a pegmatite
Photo by Rob Laninsky

As can be seen from the list if gemstones found in pegmatites most of them come from LCT pegmatites.  The major components of these pegmatites beside the usual feldspar, quartz and mica are Lithium, Cesium and Tantalum hence LCT pegmatites.  The three primary pegmatites grade into each other according to depth and pressure.  The simple pegmatite is the closest to the body of intrusive granite having the highest pressure and temperature.  As the temperature and pressure decreases the simple pegmatite grades into the LCT pegmatite, and finally grades into a NYF pegmatite.  The NYF pegmatite contains the elements neodymium, yttrium and fluorine hence its name NYF.

The simple pegmatite does not contain miarolitic cavities usually, but as the pegmatite grades into the LCT phase the cavities become more prevalent, and in a NYF pegmatite they are quite common.  It is in these cavities where the gems form usually as flawless crystals under the right conditions.  It is estimated that only about 10% of these cavities produce flawless gems.  These cavities are usually termed “vugs” when they are inside a pegmatite.

Topaz crystal on quartz from a pegmatite
Photo by Rob Lavinsky

If the vug is cracked or broken by earth movements it will lose its pressure causing the crystals to shatter so they are found as fragments on the floor of the vug.  Another common feature of vugs is allowing more hydrothermal fluid to enter causing this additional fluid to etch the minerals present, and sometimes completely removing them.  A late stage in the formation of a vug is the production of clay that coats the crystals with a thin layer.  Unless the integrity of the vug has been breached this clay layer is composed of leftovers from the process of pegmatization. If the vug has been breached the clay is clay that has been washed into the pegmatite from surface sources.  In either case the clay is usually kaolin.

Red tourmaline crystals with quartz and lipidolite on calcite
Photo by Rob Lavinsky

Even though the gem forming minerals are located in vugs in the center zone of the pegmatite the same minerals forming the pegmatite are found locked within the body of the pegmatite in this case the stones contain so many imperfections and fractures to render them useless as gems for the most part.  These minerals can include many of those used as gems, but in this case they can be used as ores for several important metals such as lithium, cesium and niobium and tantalum.

Most of the LCT pegmatites display zoning with each zone carrying a specific suite of minerals.  Some of these pegmatites can have as many as seven different zones. The center zone is composed mainly of quartz, and is also the zone containing the vugs.

The gem containing vugs can range in size from a few centimeters to the size of a large room.  Some of these large vugs can contain several million dollars worth of gemstones as some of the monsters in the Pala district of southern California or the monster found by two hunters in Newry, Maine that yielded $8 million worth of gem grade tourmaline.  Other fabulous finds of pegmatite gems are found all over the world.  Some of the most prominent have been found recently in Afghanistan and Pakistan that have produced many valuable gemstones.  

A list of the Gemstones found in Pegmatites

Triphylite found in a cavity in pegmatite in Newry, Maine. This is a rare gemstone
Photo by Rob Lavinsky

The most common gemstones found in pegmatites are beryl, topaz and tourmaline according to the experts there are over 100 other minerals that are found in pegmatites that are used as gemstones. The following list is of some of the many other gemstones that are found in pegmatites. Finding all the gemstones listed here will probably keep you busy for the rest of your life.

The other gemstones found in pegmatites

Name              Color               Mineral           Abundance      Pegmatite Type

achroite            c                      tourmaline        rare                  LCT

albite                g                      feldspar            common           NYF & LCT

amazonite         g                      feldspar            common           NYF > LCT    

amblygonite      y – c                amblygonite      rare                  LCT

aquamarine       b-g                  beryl                 common           LCT

beryllonite         c, pl & y         beryllonite         very rare           LCT

brazilianite        y-g                  brazilianite         very rare           LCT

chrysoberyl       g – y               chrysoberyl        very rare           LCT

danburite          c, y                  danburite            rare                  LCT

elbaite              c, p, g, b           tourmaline         common           LCT

euclase             b – g                euclase               very rare           LCT

fluoapatite         b, p, pur, g      apatite                common           NYF < LCT

goshenite          c                      beryl                  common           NYF < LCT    

hambergite        c, pl, y             hambergite        very rare           NYF & LCT

heliodor            y                      beryl                  rare                  NYF & LCT

hiddenite           g – y                 spodumene       very rare           LCT

hydroxlherderite  c, pl, y, b       hydroxyl “         very rare           LCT

indicolite           bl                     tourmaline         common           LCT

jeremejevite      b                      jeremejevite        very rare          LCT

kunzite              p                      beryl                   rare                 LCT

lazulite              b                      lazulite                rare                 LCT

lepidolite           pur – p             mica                 common           LCT

liddicoatite        p – r                 tourmaline        very rare           LCT

londonite          y – c                 londonite          very rare           LCT

manganotantalite  r                    m-tantalite        very rare           LCT

microlite           y, c-br              microlite            rare                  LCT

montebrasite     c, pl, p-y          montebrasite      rare                  LCT

morganite         p – o                beryl                  rare                  LCT

oligoclase         c                      plagioclase        common           NYF & LCT

petalite             c                      petalite               rare                  LCT

pezzottaite        r                       beryl                 very rare           LCT

phenakite          c, p, y               phenekite          rare                  LCT

pollucite            c                      Pollucite           rare                  LCT

quartz               c, p, sm, pur     quartz               common          NYF & LCT

rhodozite          y-c                   rhodozite          very rare           LCT

rossmanite        p-r, c, g            tourmaline        very rare           LCT

rubelite p-c                               tourmailine       very rare           LCT

sanidine            y, c                   feldspar           common           LCT

simpsonite        y, o                  simpsonite        very rare           LCT

spessertine        o                      garnet               common           LCT

spodumene       c-g                   spodumene       common           LCT

stibiotantalite     y                     stibiotantalite     very rare          LCT

topaz                b-c-p                topaz                 common          NYF & LCT

triplite               r, br, p             triplite               common          NYF

triphyllite           b-g                 triphyllite           rare                  LCT

verdelite           g                      tourmaline         rare                  LCT

zircon               c, p, br, g         zircon                common          NYF & LCT

Abbreviations: c, colorless; g, green; y, yellow; b, blue; p, pink; r, red; o, orange;

br, brown; pur, purple; sm, smokey; pl, pale.

After V.B. Simmons Gem bearing Pegmatites in Geology of Gem Deposits, Editor Lee A. Groat, Yukon Geological Survey

This list is incomplete, but they are always finding new minerals in pegmatites and some of them are gemstones. Some of them are so rare that only one example is known. The hunt goes on!

The Different Types of Pegmatites

Pegmatite in mica schist.
Photo by Arlette

If we seem to be staying on the subject of pegmatites it is because they are very important in the production of gemstones. There are over 100 different types of gems that are produced from pegmatites. Most of these pegmatites are chemically the same as granite, and the term actually refers to their texture rather than their composition. There are three recognized varieties of granitic pegmatite, and one that is recognized in gabbro. All pegmatites containing crystals that are larger than 2 cm in diameter and can reach truly gigantic proportions. There is a feldspar mine in the Ural Mountains of Russia that has been producing feldspar from any single crystal for many years.

Pegmatites were classified in the mid-20th century by Buddington with a depth zone classification of granitic rocks in 1959. Cerny’s revision of 1991 is the most commonly when used today for the classification of pegmatites. Basically these classifications are based on the depth at which pegmatites were emplaced, metamorphic grade as well as minor element content. Under this classification Abyssal pegmatites are the highest grade that have been in place that either high or low pressure. Muscovite grade pegmatites were emplaced in high-pressure, but a lower temperature. Rare element pegmatites were emplaced at even lower pressures and temperature. In this classification the miarolitic pegmatites having large cavities were emplaced at shallow depths. The most gemstones are found in the miarolitic pegmatites within the enclosed cavities.

Beryl with microcline found in a pegmatite cavity.
Photo by Rob Lavinsky

The simple pegmatite is composed of feldspar, quartz and lesser amounts of muscovite mica. This is the simplest type of pegmatite that is found that can range from tiny veins a few millimeters wide to great masses of pegmatite that can cover several acres, and be meters wide. For the most part simple pegmatites have a very uniform composition, and little variance in the size of the crystals from one side to the other. Approximately 80% of all pegmatites found in nature are of this type. There are very few gemstones found in the simple pegmatites. Most of them contain white colored feldspar and milky quartz with sometimes bands of muscovite mica parallel to the sides of the vein. Under the above classification this type of pegmatite would fall under the classification of being an abyssal pegmatite.

Red tourmaline on lepidolite and quartz from a pegmatite cavity.
Photo by Rob Lavinsky

The rare element class of pegmatites is further divided into the sub classifications LCT and NYF. LCT pegmatites contain lithium, cesium and tantalum. NYf stands for a niobium, yttrium and fluorine. These are perhaps the rarest types of pegmatite. Most of the gemstones bearing pegmatites are of the LCT variety. The closer they form to the surface the more apt they are to have miarolitic cavities containing gemstones.

How a Pegmatite Works

A pegmatite vein in a roadcut.
Photo by Woodlouper

The next several articles that are going to be published in this blog concern a very special type of rock formation that is called pegmatites. Another name that is sometimes used for this rock is giant granite because the large sized crystals of different minerals that are found in them. The most common minerals in a pegmatite are feldspar, mica and quartz with quartz in many pegmatites making up almost 50% of the total minerals present.

Although they are described as veins of pegmatite most of them are tabular shape masses of rock that are injected as a granitic magma into pre-existing cracks in the country rock, or sometimes they also form large ovals that are more or less deformed by earth movements that occur at the same time they are being deposited deep in the earth.

A pegmatite mine.
Photo by Pfuss

The crystals are found in a pegmatite are the result of very slow cooling that occurs in the depths of the earth's crust several miles beneath the surface. The pegmatites that are now exposed at the surface have all had several miles of rock eroded away so we can now see them at the surface.

Although most pegmatites are at home in gneiss or schist some of them are actually found in granite intrusions that have taken a long time to cool so that the pegmatite grades from granite. That only pertains to some of pegmatites, it is theorized that changes in the chemical composition can cause some pegmatites to be formed in a very short time; sometimes a matter of days and still have large sized crystals.

Closeup of a pegmatite
Photo by Mark A. Wilson

Unless a pegmatite is within a granitic intrusion they are all outliers of such an intrusion. The further a pegmatite is from the parent intrusion the more likely it is to have rare minerals in its composition. There are three main types of pegmatites of granitic composition and the fourth that is found in more mafic rocks such as gabbro. The further they are from the granitic or gabbro like intrusion the more apt they are to contain the proper chemical ingredients to form gemstones.

There are generally three major minerals and under the right conditions can form gemstones, they are beryl, topaz and tourmaline. There are also more than 100 other minerals that under the right conditions can also form gemstones. Sometimes even the body of the pegmatite itself is code is cabochons if it is especially attractive. Graphic granite that is found in some pegmatite makes especially attractive 

cabochons.

The rare mineral "bazzite" found in a pegmatite cavity. This is an analog of beryl containing the rare earth element scandium.
Photo by Rob Lavinsky 

Many pegmatites display different zones that are apt to contain different suites of minerals. The outer portion of a pegmatite displays very fine crystals and is often termed an aplite. There are several different shells that a pegmatite can display numbering up to seven. The inner core of all zoned pegmatites is usually quartz and in the inner core is where you usually find cavities that are filled with flawless crystals of various gemstones. Although it is unusual there are many instances where a single large cavity can produce several million dollars worth of gemstones.

The Tools of the Trade for Rockhounding by Raymond A. Kukkee, Prospector

Kakabeka Falls Ontario a Rockhound's paradise
Photo by Hans Jurgen-Hubner

Raymond Kukkee has been a hard-rock prospector in Northern Ontario for over thirty years, and the following article is built on his experiences while prospecting.  What he has to say about the tools needed to be a prospector is just as valid for Rockhounding. 

What you should take with you on a prospecting outing is always a good question. It's always better to have something and NOT need it, than to be 60 miles out on the trail wishing you did bring it along.
    
 Here's a list of what I usually take along on my day trips.  Remember I 'm going out in my truck and might be  dragging  a 4WD Quad along to save the feet too!

Even for a 2 hour walking trip, I take most of  the following tools and other helpful items,  but once out in the field  at times,  I may leave some  items  in the truck, depending on the outing:


•    A jeweler's loupe,  I use a 25x triplet
•    Topographic  and geology maps of area to be explored
•    Prospector's license*
•    Pad and pencils. Take extra pencils, they get lost easily. A compact  waterproof paper notebook is handy
•    A compass  (liquid-filled, good quality)  I use a Silva  or Brunton surveying type  c/w signaling mirror
•    A GPS unit capable of mapping,  projecting and tracking way points.    My present GPS is a Garmin GPSmap76CX, which is reliable and runs for a couple of days on 2 AA batteries.
•    Safety glasses for bashing rocks.  We only have ONE set of eyes so take care of them!
•     A compact first aid kit
•    A belt-carried hunting knife -6"   and / or a multi-tool type belt knife
•    A prospecting "pick" hammer. (In this area we have a special tool called a "Tweedie" tool named after Ron Tweedie, the inventor and maker.  It is a 36" short-handled   hammer with a 3" flat blade opposite that can be used both  for whacking rocks and grubbing, to remove moss/dirt for stripping.  (In prospecting that is convenient, it saves you bringing along a wide pick.)   At times it might be the only hammer you carry.
•    A sharp cold chisel  ½" x 12" long
•    A 4-lb short- steel- handled  "crack hammer"  unbreakable (Estwing) for serious sample removal.
•    Some paper to wrap special delicate specimens
•    Plastic sample bags 6-10  mil  8x14"  are good,  but  you can use recycled plastic bags of any type. I take along heavy recycled 1-liter plastic milk bags when available.*
•    A red or orange wax pencil for marking stakes or posts.
•    Fluorescent flagging tape to mark sample locations
•    A black marker to write ON the flagging tape
•    A short-handled ox-head axe--for cutting claim posts.   If cutting claim lines, I also  take along a
•    Swedish brush axe which is the bow design.
•    A folding shovel for the back pack, and a long-handled shovel for the truck.
•    A very compact emergency 3"x4"x 2"fishing outfit, a few hooks, line, sinkers, and a small lure.   Maybe a small collapsible fishing rod!
•    Wide-brimmed floppy hat that will cover the ears.
•    Full face mosquito net that fits over the floppy hat for extreme insect conditions.
•    Fly dope.   Various types, it must work on blackflies, mosquitoes, and deer flies.  But is generally USELESS for those tiny noseeum sandflies in hot weather
•    Extra socks to keep the feet dry.
•    Heavy boots that have high ankle protection. Waterproof boots are best
•    Rain gear--lightweight rubber but full body coverage including a hood--or the  Gortex type
•    Long-legged pants,  NOT shorts.
•    Long-sleeved shirt and jacket
•    Leather gloves, or good quality cotton gloves with leather facing
•    Eye-glasses.  I always take a spare pair!
•    Sunglasses
•    Sunscreen: 40SPF minimum.
•    A compact, folding dome tent.
•    In some areas, a snake bite kit is a good idea.
•    For SOME locations, belt-carried  Bear pepper spray is important.
•    Aspirin or Tylenol.
•    Medications if you need them.
•    Drinking WATER:  NEVER  allow yourself to become dehydrated.
•    Matches. Use safety matches, protected to keep dry, or bring along a Fire Starter (magnesium) bar.
•    Food:
•    Dry Trail mix for snacks.   Oranges are good too, and keep well
•    Lunch!    The best part of prospecting out in the bush!  I almost always take cheese sandwiches going on day trips into remote areas.    Take food that will still be good for a week even if it's squashed, melted, or beaten up. Dehydrated type foods are good for long trips too.
•    **  NOTE:   NEVER take meat sandwiches or chicken with mayonnaise out into hot weather, they can be unsafe to eat after a few hours in summer heat!
•    Hot Coffee or Tea in a thermos for cold days.
•    A small single-burner propane stove can be handy!
 
    
I guess that’s about all you  can carry, but then there’s a spare key for the vehicle,  spare gasoline, a couple of quarts of oil, some tools, a Jack-all, a couple of spare tires, and walkie-talkies if you have them.

Don’t forget that 8-lb sledgehammer for serious work, a compact bow saw for firewood -- and your driver’s license.

I guess that’s about all you should need at this point.   I hope you’re having FUN!  If I think of anything else, I’ll let you know!     Stay safe!

Raymond  

* Needed in Canada!