origins of minerals

ORIGIN AND OCCURRENCE

Gems are found worldwide. When there is a sufficient quantity to be worked, it is termed a deposit. A deposit, as well as the location of discovery of a single find, is termed the gem occurrence.

Deposits can be primary or secondary in nature. A primary deposit indicates the gem occurrence is still at the original location or at the place of formation. Primary deposits ..."have their original relationship with their host rock"[i]. Crystals can be well preserved, but much of the "deaf (nongem-bearing) rock" must be removed to recover the gems[ii]. A secondary deposit indicates the gem occurrence has been transported to another or "second" location and deposited. During this process, harder gems become rounded, whereas softer gem material is destroyed or reduced in size. The transportation and deposition can be by rivers (fluvial), the sea (marine), along the coast (litoral), or by wind (aeolian). As the water or wind currents diminish, the heaviest gems are dropped first and become concentrated (e.g., diamond, zircon, garnet, sapphire, chrysoberyl, topaz, peridot, tourmaline).

Most gemstones are minerals and originate from melts, solution, or vapors. The atoms arrange themselves from a disordered state into an orderly crystalline state, with growth dependent upon the availability of free space. Magmatic minerals crystallize from magma, lava, or gases and are found in igneous rock. Minerals that crystallize from hydrous solutions, at or near the surface, are found in sedimentary rock. New minerals formed from a recrystallisation of existing minerals beneath the surface are found in metamorphic rock.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1       Earth’s Structure

Evidence from seismology tells us that the Earth has a layered structure. Seismic waves generated by earthquakes travel through the Earth with velocities that depend on the type of wave and the physical properties of the material through which the waves travel. 

 

Types of Seismic Waves

 

Body Waves -  travel in all directions through the body of the Earth. There are two types of body waves:

 

P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel.

 

 

Vp = Ö [(K + 4/3m )/r ]

 

Where, Vp is the velocity of the P-wave, K is the incompressibility of the material, m is the rigidity of the material, and r is the density of the material.

 

P-waves are the same thing as sound waves. They move through the material by compressing it, but after it has been compressed it expands, so that the wave moves by compressing and expanding the material as it travels. Thus the velocity of the P-wave depends on how easily the material can be compressed (the incompressibility), how rigid the material is (the rigidity), and the density of the material. P-waves have the highest velocity of all seismic waves and thus will reach all seismographs first.

S-Waves - Secondary waves, also called shear waves,  travel with a velocity that depends only on the rigidity and density of the material through which they travel:

 

 

Vs = Ö [( m )/r ]

 

S-waves travel through material by shearing it or changing its shape in the direction perpendicular to the direction of travel. The resistance to shearing of a material is the property called the rigidity. It is notable that liquids have no rigidity, so that the velocity of an S-wave is zero in a liquid. (This point will become important later). Note that S-waves travel slower than P-waves, so they will reach a seismograph after the P-wave.

 

 

Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth. Thus they can give us information about the properties of rocks near the surface, but not about the properties of the Earth deep in the interior.

 

 

Because seismic waves reflect from and refract through boundaries where there is sudden change in the physical properties of the rock, by tracing the waves we can see different layers in the Earth.  This allows us to look at the structure of the Earth based on layers of differing physical properties.  Note that we know that density must increase with depth in the Earth because the density of crustal rocks are about 2,700 kg/m3 and the average density of the Earth is about 5,200 kg/m3.  Also note from the velocity equations that if density increases, wave velocity decreases.  Thus, the other properties, incompressibility and rigidity must increase with depth in the Earth at a greater rate than density increases.

           

Once we know the seismic wave velocities throughout the Earth, then we can perform experiments on different possible materials and make estimates of what the chemical composition.  Thus, we can also divide the Earth into layers of differing chemical composition.    

Layers of Differing Chemical Composition

Crust - variable thickness and composition

 

 

Continental 10 - 70 km thick, underlies all continental areas, has an average composition that is andesitic.

 

 

Oceanic 8 - 10 km thick, underlies all ocean basins, has an average composition that is basaltic.

 

Mantle - 3488 km thick, made up of a rock called peridotite (Olivine + Opx + Cpx). Evidence comes from Seismic wave velocities, experiments, and peridotite xenoliths (foreign rocks) brought to the surface by magmas. Experimental evidence suggests that the mineralogy of peridotite changes with depth (ant thus pressure) in the Earth. At low pressure, the mineral assemblage is Olivine + Cpx + Opx + Plagioclase (plagioclase peridotite).  At higher pressure the assemblage changes to Olivine + Cpx + Opx + Spinel [(Mg,Fe+2) (Cr, Al, Fe+3)2O4] (spinel peridotite). At pressures above about 30 kilobars, the assemblage changes to Olivine + Cpx + Opx + garnet (garnet peridotite).  This occurs because Al changes its coordination with increasing pressure, and thus new minerals must form to accommodate the Al.

 

At greater depths, such as the 400 km discontinuity and the 670 km discontinuity, olivine and pyroxene likely change to high pressure polymorphs.  Despite these changes in mineral assemblage, the chemical composition of the mantle does not appear to change much in terms of its major element composition.

 

 

Core  - 2883 km radius, made up of Iron (Fe) and small amount of Nickel (Ni). Evidence comes from seismic wave velocities, experiments, and the composition of  iron meteorites, thought to be remnants of other differentiated planets that were broken apart due to collisions.

 

 

 

 

 

Layers of Differing Physical Properties

Lithosphere - about 100 km thick (up to 200 km thick beneath continents, thinner beneath oceanic ridges and rift valleys), very brittle, easily fractures at low temperature.  Note that the lithosphere is comprised of both crust and part of the upper mantle. The plates that we talk about in plate tectonics are made up of the lithosphere, and appear to float on the underlying asthenosphere.

 

 

Asthenosphere  - about 250 km thick - solid rock, but  soft and flows easily (ductile).  The top of the asthenosphere is called the Low Velocity Zone (LVZ) because the velocities of both P- and S-waves are lower than the in the lithosphere above.  But, not that neither P- nor S-wave velocities go to zero, so the LVZ is not completely liquid.

 

 

Mesosphere - about 2500 km thick, solid rock, but still capable of flowing.

 

 

Outer Core  - 2250 km thick -  liquid.  We know this because S-wave velocities are zero in the outer core.  If  Vs = 0, this implies  m = 0, and this implies that the material is in a liquid state.

 

 

Inner core  - 1230 km radius,  solid

2       Mineral

Definition of a Mineral

 

A mineral is a naturally occurring homogeneous solid with a definite (but not generally fixed) chemical composition and a highly ordered atomic arrangement, usually formed by an inorganic process.

One of the consequences of this ordered internal arrangement of atoms is that all crystals of the same mineral look similar.  This was discovered by Nicolas Steno in 1669 and is expressed as Steno's Law of constancy of interfacial angles - angles between corresponding crystal faces of the same mineral have the same angle. This is true even if the crystals are distorted as illustrated by the cross-sections through 3 quartz crystals shown below.

Another consequence is that since the ordered arrangement of atoms shows symmetry, perfectly formed crystals also show a symmetrical arrangement of crystal faces, since the location of the faces is controlled by the arrangement of atoms in the crystal structure.

 

 

 

 

 

 

 

 

 


3       Classification of Rocks

 

All rocks fall into one of three classifications, igneous, sedimentary, or metamorphic. Igneous form from the cooling and crystallization of molten magma.  Sedimentary rocks are composed of materials that result from mechanical and chemical weathering of preexisting rocks. Sediment is deposited in layers become rock when cemented or compacted together. Metamorphic rocks were once igneous or sedimentary and underwent a change in texture and/or mineral composition. The change is in a solid state, caused by high heat and pressure resulting from burial deep within the crust of from intrusive igneous rock. Gems can also form from solution as vein or secondary minerals.

3.1     Igneous

 

The Earth is at least 4.5 billion years old, and probably had beginnings as a hot, fiery sphere of molten material. As the molten mass cooled, crystals formed and the Earth's surface was covered by a layer of igneous rock. Therefore, in the beginning, all rock was igneous (from Latin, meaning "fire"). Most igneous rock is made up of minerals, such as quartz, feldspar, nepheline, mica, amphibole, pyroxene, and olivine. The texture of the grains and the relative amounts of these minerals help to classify the rock. In addition to the minerals used for classification, igneous rock contains other minerals referred to as accessory minerals. These minerals can form large, perfect crystal gems, such as zircon, sapphire, and pyrope garnets. After the rock weathers, the minerals are recovered from unconsolidated sands and gravels. Igneous rocks can be classified into intrusive, extrusive, hypabssal, or pegmatite environments.

 

3.2     Metamorphic

Metamorphic rocks form from igneous, sedimentary, or other metamorphic rocks, changing in a solid state as a reaction to high pressure and temperature and/or hot circulating fluids. The hot chemical fluids, pressure and temperature, add new minerals and textures to the existing rocks with two common methods of metamorphism, regional and contact.

 

3.3     Sedimentary

In the beginning, the Earth was composed of igneous rock, but the processes of mechanical and chemical weathering broke down the rock into sediment. Mechanical weathering creates sediment that is transported via wind, water, and ice. After transportation, sediment is deposited, compacted, and cemented into solid rock. Mud turns to shale, sand into sandstone, and coarser pebbles cement together into conglomerates. Chemical weathering changes or liberates minerals in rocks. For example, igneous and metamorphic rocks have abundant feldspar, which chemically weathers into clay minerals, and quartz, which is inert to the chemical attack and freed from the parent rock. Chemical sedimentary rocks are precipitated by chemical reactions, evaporation, or organisms, resulting in gypsum, rock salt, and limestone to name a few. Rock weathers into sedimentary deposits that include eluvial, alluvial, and placer.


Formation

No matter what process is involved, a particular mineral cannot form unless the chemical ingredients necessary to make the mineral are present. Thus, the most common minerals are minerals that have a chemical composition made of the common elements found in their environment.

 

In the following sections we will examine how gems form. Minerals can form by any of the following processes:

3.4     Mineral Crystallisation

 

3.4.1    Magma

3.4.2    Gas

3.4.3    Hydrothermal

 

3.5     Molten rocks and associated Fluids

 

Pegmatites

Pegmatites are unusual magma bodies.

 

As the main magma body cools, water originally present in low concentrations becomes concentrated in the molten rock because it does not get incorporated into most minerals that crystallize. Consequently, the last, uncrystallized fraction is water rich. It is also rich in other weird elements that also do not like to go into ordinary minerals.

 

When this water-rich magma (also rich in silica and unusual elements) is expelled in the final stages of crystallization of the magma, it solidifies to form a pegmatite.

 

The high water content of the magma makes it possible for the crystals to grow quickly, so pegmatite crystals are often large. Of course, this is important for gem specimens!

 

When the pegmatite magma is rich in beryllium, crystals of beryl form.

 

If magmas are rich in boron, tourmaline will crystallize.

 

You should note that beryllium and boron are extremely rare elements in most rocks and it is only because the above process efficiently concentrates these unusual elements that crystallization of boron and beryllium-rich minerals can occur.

 

·  PEGMATITE is a common plutonic rock, of variable texture and coarseness, that is composed of interlocking crystals of widely different sizes.

The most spectacular pegmatites contain abnormally large crystals mixed with medium sized and smaller crystals. Crystals up to many meters long have been reported.

Some pegmatite bodies are small irregular patches less than 1 cm. (0.4 in) across in larger masses of plutonic or metamorphic rocks. Others may be thousands of meters in length and hundreds of meters thick. Some appear as dikes, veins, or sills. Many have irregular outlines.

Most pegmatites show symmetrical internal zonation.

·  MINERALOGY AND COMPOSITION:

Pegmatites may be composed of a variety of minerals. Terms such as granite pegmatite, gabbro pegmatite, syenite pegmatite, or names with any other plutonic rock type as prefix are used.

Compositions in the range from granodiorite to granite are common. Large crystals of quartz, potassium feldspar, sodium rich plagioclase, and micas (e.g., muscovite and lepidolite) may be abundant.

Simple pegmatites contain few, if any, exotic minerals. The center zones of complex pegmatites, however, may contain a wide variety of minerals such as tourmaline, topaz, garnet, spodumene, scapolite, beryl, apatite, fluorite, zircon, and various rare minerals some limited to only a few localities in the world. GEM quality stones are sought in such rocks.

·  COMMERCIAL ELEMENT SOURCES

Elements such as tungsten, boron, tantalum, columbium, bismuth, tin, uranium, radium, sheet mica, and sulfide minerals of various metallic elements are among substances obtained from pegmatite deposits.

·  GEOLOGICAL AGES:

Pegmatites have provided radioactive minerals for use in radiometric age dating of many rock complexes.

·  THE ORIGIN OF PEGMATITES:

As a result of the bewildering variety of shapes, sizes, appearances, and field relationships, many origins have been proposed for pegmatites. Some dikelike bodies showing clear INTRUSIVE relationships must be of igneous origin. These frequently cut across all other associated rocks and therefore represent material from late stages of crystallization of plutonic complexes. They were probably rich in volatile materials such as water, fluorine, chlorine, phosphorus, and sulfur. This highly fluid, aqueous melt provided an environment for concentration of chemical elements with ionic sizes too great to fit into crystalline structures of major rock forming minerals; these elements were thus concentrated in pegmatite deposits.

The occurrence of pegmatite corresponding to most plutonic rock compositions gabbros, diorites, syenites, anorthosites further recommends this possibility.

Other pegmatites grade into the rocks that surround them and show no intrusive relationships. Such bodies may represent material produced by melting (anatexis) during metamorphism at high temperatures and pressures. Some elements and fluids may be literally "sweated out" of a rock complex during metamorphism. well known because it contains crystals of many different minerals. This rock is pushed up as large veins of magma that was rich in volatile elements resulting in large crystals, usually surrounded by grantic rocks.

3.6     Environmental Changes

Metamorphic rocks are rocks changed by heat, pressure, and interaction with solutions. There are a number of types of metamorphic environments:

3.7     Surface Water

 

Water near the Earth's surface interacts with minerals and dissolves them. The ability of these solutions to maintain elements in solution varies with physical conditions. If the solution conditions change (for example if the solution cools or evaporates), minerals will precipitate. A similar, familiar processes is formation of salt crystals by evaporation of sea water.

 

 The mineral that forms is determined by what the dissolved elements are. If the water has interacted with silica-rich rocks (e.g., sandstone), silica-rich minerals will form:

 

 

If the water has interacted with copper-rich rocks, copper minerals will form:

 

 

3.8     Gems formed in the Earth’s Mantle


4       Deposits

4.1     Major Types of Gem Deposits

 

 

4.1.1    Primary

 

 

 

 

 

 

4.1.1.1   Pegmatite

 

 

 

 

 

 

 

4.1.1.2   Diamond Pipe

 

 

 

 

 

 

 

 

4.1.1.3   Hydrothermal Vein


4.1.2    Secondary

 

 

 

 

4.1.2.1   Weathering

 

 

 

 

 

 

4.1.2.2   Alluvial/Placer Deposits

After rock is brought to the surface, gems may be released from the rock by weathering (some minerals dissolve, others are transformed to clay minerals, and some others survive unchanged).  The minerals that survive unchanged may be washed into streams, etc., where they are concentrated by river / ocean processes.

 



[1] Must be naturally occurring.

[2]Check out what is diagenesis

[3]Sulphur, though strictly speaking, it is not a gem material



[i] Schumann, 1997, p. 53

[ii] Schumann, 1997, p. 53