Lab 2. Igneous Rocks and the Gems Produced from Them

A) Introduction to Igneous Rocks

Igneous: Gems can occur in any type of rock. In the next few exercises we will look at all three types of rock, igneous, sedimentary, and metamorphic. Each forms in a different way. Rocks formed from cooling melted (molten) earth materials are called igneous rocks (rocks full of fire). Gems formed in these rocks crystallize as the molten rock cools (in most cases). The molten material is called magma below the surface and lava when it flows on the surface.

The igneous rocks are primary and formed the crust as the earth cooled. They still are being formed today both above and below ground. The melts are mainly composed of silicon and oxygen. These two elements bond together making the back bone of a group of minerals called the silicates. Along with silicon and oxygen (these two element make up 76% of the crust), aluminum, iron, calcium, sodium, magnesium, and calcium make up about 99% of the earth’s crust by weight. So in a melt, these elements usually predominate and make up most of the rock forming minerals (dozen or so common minerals). Though they are abundant and important not all that many gems are silicates, most common of these are emerald (beryl group), quartz, garnets, and tourmaline. A lot of gems, even in of silicate family, have exotic elements. These exotic elements, such as beryllium, boron, lithium, chromium, vanadium, and zirconium need to be concentrated. So gem formation often requires unusual geologic circumstances.

Sometimes water-rich hot fluids generated from the cooling rock separate from the main magma and bring with them the exotic elements that do not easily fit into the common silicate mineral structures (that is ferromagnesian silicates, quartz, feldspar, mica, etc.). With all the water and exotic elements excluded from the main mass of igneous magma, this very-fluid, exotic-element rich “gem soup” forms gem pockets on the outer margin of the larger igneous rock bodies. These very fluid magmas create pegmatites. Figure 1 Igneous rock formation by intrusion into layered sedimentary rocks. Note pegmatite is to the outside of the intrusion.

B.

1 Classification of Igneous rocks

Some examples:

Igneous rocks are of two main types volcanic (extrusive) rocks formed on the surface and plutonic (intrusive) formed below the surface (see Figure 1 above).

Volcanic rocks can form from flowing lava or may be fragmental meaning that they are caused by explosive volcanism that can have horrific consequence. The city of Pompeii was destroyed in AD 79 by a volcanic eruption of Mt. Vesuvius.

Some gems such as diamonds and pyrope garnet are carried to the surface by volcanic eruptions referred to as diatremes (Literally, a hole filled with breccia [broken pieces of rock], formed by a subterranean gaseous explosion). The rock carrying diamonds to the surface is kimberlite. It is fragmental in nature (see pyroclastic below), but no one has directly witnessed a kimberlite eruption and the fragmental texture may in some cases even be intrusive having never reached the surface. Other gems that are carried up with lava outpouring include peridot (olivine) which may be carried upward from where it crystallized at a much slower rate than diamonds in kimberlite. Diamonds and peridot carried up as solids from a deeper rock are called, Xenoliths (which means foreign rocks). The peridot crystals have to be broken out of the lava or glassy obsidian that oozed out with them in tow. Obsidian itself, a volcanic glass caused by rapid cooling, is occasionally used as an ornamental gem. Snow flake obsidian is very attractive when tumbled or polished. The snowflakes in the obsidian are crystals of feldspar that grow in the magma or lava as it rises and are not xenoliths. They grow in the same melt and are larger than the rest of the surrounding rock. Sometimes these are called phenocrysts. Phenocrysts differ from xenoliths in that phenocrysts form from the same magma as the surrounding rock creating a texture called porphyritic texture, while xenoliths did not crystalize in the magma they are now found in.

Some gems form after beds of ash and lava settle because the ash is soluble in the hot fluids that may rise from hydrothermal (hot water) vents emanating from below (see geyser in Figure 1). Hot gases also rise and can carry mineral forming elements. Because the hot water is filled with dissolved minerals, on cooling a precipitation of crystals into spaces in the rock may occur as the hot gases and water rise and cool. Some opals and nodules such as thunder eggs form in this way. Though often called geodes, many quartz and other mineral lined pockets are really formed is cavities between pillow lavas. Much of the Brazilian amethyst and citrine “geodes” may have this origin. Pillow lavas form when lava flows into water (see Figure 2, 3 & 8) below.

Gold and other minerals also may precipitate from hydrothermal solutions that emanate from igneous activity and much prospecting for metals takes place in igneous regions as well. However, some people consider these to be metamorphic rather than igneous processes.

For geologist to have conversations about rocks special vocabulary and descriptive terminology has been developed.

2 Figure 3. The pillows have hollows between them that can be mineralized. Similar to Figure 2. Pillows formed as lava erupts into geodes. water.

Igneous rocks are classified and named by their texture and color. These properties help us to interpret their cooling history and the source of the magma. Knowing the history of the rock allows us to consider its economic potential as well.

Volcanic rock cool rapidly

The terminology can be complex. We will say that fine-grained rocks are made of microscopic crystals. Geologist call this texture aphanitic. Aphanitic rocks and glass (no crystals/minerals) are mostly of volcanic origin as are fragmental rocks (sometimes called pyroclastics). Pyroclasts are literally “fire fragments. ” Pyroclastics rocks are explosively emplaced. Usually this means a volcanic eruption. Sometimes the rock material comes out of fissure (crack) or just explodes without a prior volcano.

Fine-grained rocks may not have formed on the surface, but slightly below it (close to the surface), they are considered volcanic because of their microscopic interlocking crystal texture.

The reason why fine-grained rocks (and volcanic glass) are considered volcanic is that they must have cooled relatively quickly, not giving enough time for larger crystals to form.

Sometimes however, a finer-grained rock will include coarser crystals that rose with the lava or glass. The larger crystals called phenocrysts encased in the finer groundmass, give the rock a special composite texture called porphyritic. The constituents of porphyritic rocks indicate two separat e rates of cooling in differe nt environments (below the surface for the Figure 4. Porphyritic texture pheno crysts, and nearer the is due to two rates of cooling. surfac e for the groundmass). Larger crystals are called phenocrysts that are surrounded by ground mass.

3 Phenocrysts and xenoliths (foreign rocks) carried up can be of value, consider peridot and diamonds. Both may exist in large quantities in the mantle but are rare on the surface and in the earth’s crust. As well, corundum, ruby and sapphire and moonstones can be carried up with volcanic eruptions (such as alkali basalt) that flood the earth’s surface with large volumes of volcanic rock making gem-rich regions as in Thailand’s Chantbaburi-Trat area that includes the Hill of Gems (rubies) area, the Pailin ruby and sapphire gem field in Cambodia. Peridot Mesa near San Carlos, Arizona, and Australia’s sapphire and zircon-rich Ankie and New England districts (Yellow and blue sapphires) have similar origins. The liquid magma acts like an elevator bringing these precious crystals to the surface.

Plutonic Rocks Cool Slowly

Plutonic rocks on the other hand must have cooled relatively slowly because they have large, visible crystals (This texture is called phaneritic texture = coarse-grained crystals). The growth of larger crystals takes time and indicates slower cooling below the surface. The surrounding rocks act as a sort of thermos only allowing the heat to escape slowly. Since a hot fluid allows for more fluidity, the elements that make up crystals can travel more freely to hook up with each other and the crystals have more time to grow large. For this reason course-grained rocks, such a pegmatite (which is a textural term meaning having crystals larger that 1 inch [2.5 cm]) are more likely to have large mineral grains/crystals that potentially could yield gems of sufficient size for cutting.

Some pegmatites apparently form from very fluid remnants of cooling magma with incompatible elements that are driven off of the main cooling plutonic rock body. These mineralogically complex pegmatites are especially sought after targets for gem hunters and may contain large crystals of quartz, tourmaline, beryl, and spodumeme, and many other rare minerals that fascinate the mineral collector. They are also economically important for certain chemicals, such as lithium and beryllium that are mined from them.

Exercise Begins Here

After a discussion of the rocks, you will be given a box of rocks and asked to identify their origin and name them based on a brief description. You should learn to recognize these rocks for an exam. Your instructor will emphasize certain rocks and will help you to correctly identify the

4 samples.

List of rock numbers for you to learn.

Rock numbers Igneous

Look at the texture and Composition (color).

Texture types of igneous rocks

You should locate, 1) glassy texture, 2) fine-grained (aphanitic) texture, and 3) coarse texture (phaneritic), 4) pegmatitic texture which has crystals larger than about an inch in diameter, 5) the bubbly or vesicular texture, and 6) lastly pyroclastic texture made of broken pieces of rock, such as pumice, and ash mixed together. Use the characteristics described below.

Glassy texture–overall sheen is glassy, often transparent on the thin edges. Aphanitic texture–overall a flat or dull appearance, typically a uniform color. Phaneritic texture–may have several colors (different visible minerals), sparkles when moved in the light. Pegmatite texture–too coarse to be in boxes, there is a side table example. Looks like granite. Vesicular texture–has openings like Swiss cheese, or cotton candy. The gray rock, pumice (AKA: the bathroom stone) may float on water. The darker rock scoria will not float. Pyroclastic texture–broken pieces of rocks (pumice, etc.) fused together by heat. The pieces may be different colors. Sometimes layered (usually lighter pumice and darker layers of glass). Often this rock is difficult to recognize in small specimens.

List the numbers of the rocks in the table below.

Glassy Aphanitic Phaneritic Pegmatite Vesicular Pyroclastic (fine) (coarse) (very coarse) (bubbly) (fragments)

Use the “Igneous Rocks Classification Table” below to name the rocks you have.

Texture and composition

Texture

5 Step 1. First recognize that texture is along each row, Use column 1 (down) to put in all textures in this table to figure out what you have in your tray. Put the sample numbers in the first column, such as coarse, fine, glassy, etc. (note pegamatite is in with granite).

Once you have the textures from the box, check with the instructor because some samples must be on the side table because we do not have enough samples of them.

Composition

Step 2. Composition is a result of the minerals present and often is reflected by the color of the rock as listed below. Four color groups are used, felsic (light colors), intermediate (file cabinet gray or salt and pepper), mafic (dark colors such as red, dark brown, or black), and ultramafic (having green colors).

As you will see in the chart of igneous classification on the next page’s chart, you need texture and composition to identify an igneous rock. However, composition is basically the same as color (composition = color)!

The amount of dark minerals is all you need to classify color (= composition)!

Remember from earlier that dark colored minerals are: Red, Brown, Dark Gray, Black, and Green, look for the percentages of these colors in the rock. These dark silicate minerals are called the ferromagnesian silicates. So the ferromagnesian silicates can be used to classify the composition of the ordinary igneous rocks! There are a few oddball igneous rocks that have no ferromagnesians. Remember that significant green is ultramafic!

We then classify the igneous rocks base on composition (color) as:

Felsic–less than 25% dark minerals; quartz is often present (usually pink and white colors). Intermediate–more than 25% and less than 45% dark minerals (usually an intermediate gray). Mafic–more than 45% and less than 85% dark mineral (usually dark gray or black, some brown). Ultramafic–more than 85% dark mineral, olivine often present (usually green with some red or brown.)

6 1) put the rock numbers in the texture box on the left (check to see if there are side table rocks) 2) move the numbers over to the correct compositional box. Go over these with your instructor 3) later record the number and its description on the next two pages of charts

Igneous Rocks Classification Table

Compositions Felsic Intermediate Mafic Ultramafic Light colored Gray colored Rocks Dark colored Very dark colored ------> Igneous rocks mainly gray, from igneous rocks rocks containing (across) mainly pink, white, 25-45% dark mainly dark green, almost all iron- and and gray less than minerals, quartz black, or brown magnesium-rich 25% dark minerals uncommon >45% dark minerals minerals, greater (see chart) but less than 85% than 85%

Textures (down) Plutonic rocks with coarse texture

Granite if grains Diorite Gabbro Dunite Coarse grained are less than 2.5 cm. (visible minerals) Usually a dark gray Black, but crushed Green, usually Pegmatite if grains with black specks grains on broken grains are sugary are larger than 2.5 (salt & pepper look) surfaces may appear looking. cm much lighter

Volcanic rocks with fine-grained texture or glassy texture

Fine grained Rhyolite– usually Andesite–usually a Basalt–usually (microscopic pink or whitish medium gray often black or dark redish no rocks in this minerals) gray. May have with splintery dark brown with even category small dark minerals minerals (shiny texture bits)

Glassy texture Obsidian–a natural glass used in arrow uncommon in this no rocks in this heads category category

Non-glassy, bubbly no rocks in this no rocks in this Scoria–dark brown, no rocks in this texture category category red, or black may category have olivine grains

Glassy, bubbly Pumice–the bathroom stone, floats on no rocks in this no rocks in this texture water category category

Pyroclastics texture Tuff–a volcanic glass and layered rock. Kimberlite–this is the rock that contains Layers are subtle look at all sides carefully diamonds. Fragments of many rocks possible. May have some calcite, so may fizz with acid!

Use the table below for your notes (next page). Mark appropriate boxes to coincide with the rock names.

7 8 Some Final Considerations of the Composition of Igneous Rocks.

As we will discuss, felsic rocks are much more common on the continents. Because they have reduced amounts of ferromagnesian (iron and magnesium) or dark minerals, they are low density and float on the denser mafic and ultramafic rocks. Density differences are one reason for plate tectonics. A differentiation by density makes the continents behave differently than the ocean and underlying mantle rocks. The dark minerals (ferromagnesian minerals) have higher melting points and thus sink, as the rocks sink the non-ferromagnesian or light minerals melt and the created magma rises back up. So the ferromagnesians which are still solid and dense sink back into the mantle and the lighter elements (essentially minerals) in the magmas rise.

Plate tectonics will be discussed in lecture and is important for the overturn of materials between the crust and underlying mantle and explains the movement of continents (continental drift) over geologic time. The churning caused by plate tectonics cause less denser (felsic; lighter) minerals to rise up and form the continental crust. Thus plate tectonics does ultimately control where rocks and gem minerals will be present. It also is an encompassing theory that explains the origins or earthquakes, mountain belts, and the chemical evolution of the earth, but this is a topic

9 for physical geology and will not be discussed in detail here. However, geoscientists have the best grasp of the reason for gem, precious metal and all other economic deposits and this course might be a gateway to future studies of the earth!

Figure 5. The plates shown here make up the earth’s outer surface and move on a plastic layer below. Plate boundaries, like the edges of broke ice on a lake, see most of the action.

Figure 6A Figure 6B

Figures 6A & B show two important boundaries of plate tectonics. Figure 6A shows a ridge in the middle of the Atlantic Ocean where new igneous rock is rising to create two new plates moving the USA away from Europe. Figure 6B shows where plates go under continents after growing from a midocean ridges and sink and melt. The regions near to these sinking (or Subducting plates) experience igneous activity (rocks melt) including volcanic eruptions.

Living near a plate boundary has its negatives, such a volcanoes and earthquakes, but much mineral wealth is generated by the processes, including gold and gemstones.

Samples for You to Study as a Group are Provided on the Side Tables

10 Once you have completed your tray identifications, each table will be assigned a side table sample. You will identify it and look up some of its characteristics and then use these characteristics and terminology to present your findings to the class using guide questions. You do not have to use all the guide questions and you can make your own questions and answer them instead if you so choose!

Your side Kimberlite Pillow lava Pegmatite Volcanic Vein quartz table samples rock with gold

1) Kimberlite Several samples are given. They appear coarse grained, but careful inspection shows that the samples are composed of fragments of both older rocks and broken minerals. Kimberlite is the rock in which most primary diamonds are found. The rock is created by an ultramafic magma that is extremely explosive due to carbon dioxide and other volatile elements that cause it to explosively rise from depths around 100 miles (160 km) below the crust. That is the magma has its source in the mantle. The magma drags diamonds up with it from this depth. The diamonds are already solid when they are taken up by the kimberlite magma, but as the magma rises it takes many fragments of other solid rocks up with it till it reaches the earth’s surface. Do not expect to see a diamond in your sample, as the concentration of diamonds in kimberlite is typically 0.1 grams per ton.

Use the Internet to find some illustrations and answers to the following questions.

A) Kimberlite–where is it named from?

B) What is the texture of kimberlite? What is the color?

C) Is kimberlite volcanic?

D) What does the word, “Xenolith” have to do with diamonds in kimberlite?

E) Where did the diamonds form? Figure 7. Kimberlite formation. F) Are any diamonds really formed from “Coal?” Coal is plant material that has fossilized to

11 make a fossil hydrocarbon fuel. How could the pressure get strong enough to do this?

2) Pillow lava and related “geodes” (There are sedimentary geodes; these are not!) is a rock created when lavas, typically basaltic lavas, flow into a lake or the ocean. Small pillows or blocks of lava pile up (Figures 2 & 3) , but there are typically spaces between the pillows that allow later mineralization to occur in between Figure 8. the pillows. These spaces can be filled with many minerals, but a typical infilling is quartz, such as amethyst and zeolite minerals such as natrolite and stilbite. The figure (Figure 8) above is from Paterson, New Jersey, less than 25 miles from New York City (Sinkankas, 1974).

Amethyst is the purple form of quartz, usually forming good crystals. These grow into a hollow between the pillows. Pillow lavas have such “openings” as do “geodes.” The term pillow has to do with the shape of the lava, not the holes or openings between them, but other lavas flow like sheets and don’t have openings of this size for minerals to grow in.

Consider the following in making your report.

A) Pillow lava–why this name?

B) What rock type is the lava?

C) Can you find a “Youtube” video on pillow lava.

D) Name or describe some locations and say a little bit about them (hint: Hawaii, New Jersey, Brazil). In Brazil, amethyst cathedrals really are the result of pillow lava flows, let’s find some pictures and discuss the evidence.

E) Some of the minerals associated with pillow lavas include, quartz, zeolite minerals, and calcite. Let’s see if we can find something out about these minerals and their local availability, such as in New Jersey.

12 3) Quartz Veins Be Careful! Warning, the valuable mineral on the quartz veins is very soft and could be scraped off with your fingernail! So what is it?

Well before you answer this question, there are things to consider. People have gone prospecting for this valuable mineral for thousand of years, and though it mainly is found in streams/rivers, there are primary deposits of this metal found in some igneous provinces. This precious mineral has an affinity for the watery solutions found circulating above an igneous intrusion. Some people would say it is not truly igneous since it is not from a direct magma, but precious metal still has an igneous association.

A) Your samples was part of a larger vein of quartz. Describe a vein of quartz and find a picture on the Internet that we can project. Figure 9. Quartz vein. B) Where in the USA might this valuable mineral be found? Mention a place or two (not a stream deposit, but a vein area).

C) What are some properties of the materials involved? Valuable versus what is called the gangue mineral? Notice that in the illustration above dark areas represent open space. What might minerals that grow into that space look like compared to if all the space was filled?

D) What is a nugget compared to crystallized material from primary deposits?

E) Borrow a microscope and set it up so that people can look at your specimens. Could this be a fake? Give some opinions after you have looked at it with the microscope.

4) Pegmatite First look up or find a description of pegmatite. It is a lot like granite. Your samples have blue/blue-green minerals in it that are sometime valuable. These are not as clear as some gem material. Yours may have fractures.

A) Can you recognize some minerals in it? Look up pictures and try and figure out what the

13 blue mineral is. What the pink mineral is. What the gray mineral with no cleavage is. What the platy mineral with one cleavage is. The blue/blue- green mineral was not in your mineral box, but the others were.

B) Ask your instructor for last weeks tray of minerals and identify the gray mineral, flaky mineral, and pink mineral in your tray samples. These same mineral are in the pegmatite.

C) Discuss with the group your definition of pegmatite and what kind of gems you might find in a pegmatite.

D) Can we find pictures or film of pegmatite on “Youtube,” etc? (May not be easy!).

E) What is a gem pocket?

Figure 10. Pegmatite pocket (gem pocket).

5) Peridot from Arizona and Hawaii Peridot is a green gem quality olivine (as a mineral it is called olivine) that does not form on the earth’s surface or crust, but comes from the mantle, sometimes starting as deep as 75 Km below the earth’s surface. Olivine and high pressure structural variants constitute over 50% of the Earth's upper mantle, and thus olivine is one of the Earth's most common minerals. The rise of these deep (mantle formed) minerals is unusual and so peridot is a rare gem. Both diamond and peridot come from the mantle.

A) Explain, in simple terms, what the mantle is? Give or find an illustration of the earth’s

14 concentric layers and the mantle should be a focus of your picture. We may project this.

B) Find something on the internet about mantle sources for minerals like peridot. Here is one link http://www.gemsociety.org/info/igem17.htm Look for “Gems formed in the mantle on this page.

C) Your igneous rock sample is one that we studied today. What is it called? Why is it volcanic or plutonic (is there evidence in the rock that supports your statement)? What is its classification in terms of texture? What is its classification in terms composition (but remember the peridot is not a phenocryst, but a xenotlith)? Being that the peridot is a xenolith, is this rock truly ultramafic? Just consider the matrix or ground mass and not the xenoliths.

D) What is a flood basalt? Find some gems that come from these? Name a few locations.

References

Sinkankas, J., 1974, Prospecting for Gemstones and Minerals, 2nd edition, New York, Van Nostrand Reinhold, 350 P.

15 Evaluation Questions for Igneous Rocks

) Would a denser rock (one that has denser minerals) generally lie toward one side of the classification of rocks (Igneous Rocks Classification Table)? Think about composition.

) Could the phenocrysts in a rock be potential gemstones? Explain.

) Would finding a particular igneous rock type get you excited about potentially becoming rich? Explain.

) Lavas tend to have what type of grain size?______. Is the grain size a negative for finding gems in A) some instances, B) all instances, C) let me research that--What gems are found in lavas?

) What is the general origin of obsidian? Can you find two present/past uses of obsidian that make it valuable?

16 ) What attribute sets pegmatite aside from other rocks? What gems might one hope to find in it?

) Locally pegmatites have produced gems. Can you find a local example of a gem producing pegmatite in New York, Connecticut, or Pennsylvania? (Hint try: “Gemstone Occurrences in Connecticut”; see if that includes pegmatites). What was produced?

) Why might it be easier to get a gemstone crystal (specimen that looks nice) out of a pegmatite than out of a granite or basalt?

) A very important mineral in igneous rock classification is a green mineral. As we learned last time this mineral is considered a dark-colored, or ferromagnesian silicate. What is this mineral? Towards which side of the chart is it found (left or right)?

17 ) Quartz is almost as important as the green mineral mentioned above. What side of the classification chart of igneous rocks does it tend to be on? (Felsic or Ultramafic)

) Pillow lavas form when lava______and tend to have______that minerals can grow in. See if you can fill this in.

) Plate tectonics has moved things around. Continents have drifted. Continents have been torn asunder (apart by rifting). Think about it. Could gems prospecting and an understanding of plate tectonics have a potential to help/explain each other.

) What about volcanoes and plate tectonics? Where is “The Ring of Fire?” Could volcanoes help create gemstones?

18 Look at Figure 6.

19