Crystallography of Gemstones: The Crystal System Make Massive income from Google at www.online-home-jobs.com Crystalline (Crystal Lattice) Structures Structural Properties of a Crystal Crystallography is the science of determining the arrangement of atoms within any solid structure, including, but not limited to gemstones. All gemstones are crystalline structures made from a mixture of different elemental compounds, and the shape of a crystal is based on the atomic structure of these elemental building blocks. Atoms within a mineral are arranged in an ordered geometric pattern called a "motif" which determines its "crystal structure." A gem's crystal structure will determine a its symmetry, optical properties, cleavage planes, and overall geometric shape. The recipe, or mixture of these compounds becomes the blueprint for how the crystal will grow. A crystal's growth pattern is referred to as its "Crystal Habit." Individual Crystal Systems and the Axial System The crystal system is a grouping of crystal structures that are categorized according to the axial system used to describe their atomic "lattice" structure. A crystal's lattice is a three dimensional network of atoms that are arranged in a symmetrical pattern. Each crystal system consists of a set of three crystallographic axes (a, b, and c) in a particular geometrical arrangement. The seven unique crystal systems, listed in order of decreasing symmetry, are: 1. Isometric System, 2. Hexagonal System, 3. Tetragonal System, 4. Rhombohedric (Trigonal) System, 5. Orthorhombic System, 6. Monoclinic System, 7. Triclinic System. Bravais Lattices Bravais lattices describe the geometric arrangement of the individual lattice points within each of the seven crystal systems. There are fourteen Bravais lattices which are distinct from one another in the translational symmetry they contain, and all crystalline minerals fit in one of these unique fourteen arrangements. The Unit Cell The "unit cell" is the smallest divisible unit of a given mineral with symmetrical characteristics that are unique to its crystalline structure. A structure's unit-cell is a spatial arrangement of atoms (motifs) which are "tiled" in a three-dimensional space to form the crystal. The unit-cell's form is determined by its unique "lattice" parameters, the length of the cell edges, and the angles between them. The positions of the atoms inside the unit-cell are described by the set of atomic positions (xi,yi,zi) measured from a given lattice point. The Seven Crystal Systems 1. Cubic The cubic crystal system is also known as the "isometric" system. The cubic (Isometric) crystal system is characterized by its total symmetry. The Cubic system has three crystallographic axes that are all perpendicular to each other, and equal in length. The cubic system has one lattice point on each of the cube's four corners. 2. Hexagonal The hexagonal crystal system has four crystallographic axes consisting of three equal horizontal or equatorial (a, b, and d) axes at 120¼, and one vertical (c) axis that is perpendicular to the other three. The (c) axis can be shorter, or longer than the horizontal axes. 3. Tetragonal A tetragonal crystal is a simple cubic shape that is stretched along its (c) axis to form a rectangular prism. The tetragonal crystal will have a square base and top, but a height which is taller. By continuing to stretch the "body-centered" cubic, one more Bravais lattice of the tetragonal system is constructed. 4. Rhombohedral A rhombohedron (aka trigonal system) has a three-dimensional shape that is similar to a cube, but it has been skewed or inclined to one side making it oblique. Its form is considered "prismatic" because all six crystal faces are parallel to each other. Any faces that are not squared at right angels are called "rhombi." A rhombohedral crystal has six faces, 12 edges, and 8 vertices. If all of the non-obtuse internal angles of the faces are equal (flat sample, below), it can be called a trigonal-trapezohedron. 5. Orthorhombic Minerals that form in the orthorhombic (aka rhombic) crystal system have three mutually perpendicular axes, all with different, or unequal lengths. 6. Monoclinic Crystals that form in the monoclinic system have three unequal axes. The (a) and (c) crystallographic axes are inclined toward each other at an oblique angle, and the (b) axis is perpendicular to a and c. The (b) crystallographic axis is called the "ortho" axis. 7. Triclinic Crystals that form in the triclinic system have three unequal crystallographic axes, all of which intersect at oblique angles. Triclinic crystals have a 1-fold symmetry axis with virtually no discernible symmetry, and no mirrored or prismatic planes. Crystal Forms Closed & Open Forms Any grouping of crystal faces or facets that are arranged in the same symmetry is called a "form." There are approximately 48 crystal forms broken down into "open" or "closed" categories. There are 30 "closed" and 18 "open" crystal forms. "Closed Forms" are those groupings of facets that are related by symmetry and completely enclose a volume of space. Vectorial Properties of Crystals Although a crystal structure is an orderly arrangement of atoms on a lattice-like structure, the order may be different along different directions or axes in the crystal.

The rock cycle.

The Rock Cycle

Planet earth is made up of a molten rock interior, with cooler outer regions terminating in a thin, solidified skin called the "crust," or lithosphere. Earth's outer crust is thinner underneath the ocean - called the "oceanic crust" - with an average thickness of 10 kilometers, while the portions of the crust that contain land masses - called the "continental crust" - have an average thickness of 20 to 80 kilometers.

The outer crust of the earth is fractured into 14 individual sheets of rock called "plates," that are floating on a sea of molten rock. As these floating islands of rock move along a molten sea they separate and collide with each other in a process called "plate tectonics."

Plate Tectonics

Through gravitational and inertial forces, the individual plates are in a constant state of motion, causing the cyclical opening and closing of ocean basins. Over 130 million years ago, all of the continents on earth today started out as one giant land mass called "Pangaea." As the plates separate or collide with each other they create three distinct types of "boundaries" which are called:

1. Convergent Boundaries Plates colliding with each other, causing subduction (Himalayas)
2. Divergent Boundaries One plate that is separating or tearing into two plates (Rift Valley)
3. Transform Boundaries Plates slide against each-other along transform fault (San Andreas)

When two plates collide the subordinate plate will be consumed underneath the dominant plate in a process called "subduction." The subduction process within the "collision zone" of a convergent boundary creates enormous upward pressure, causing mountain-building activity through a process called "uplifting" or "collision orogeny," and may also cause volcanic activity.

Mountains that are created by tectonic orogeny can occur as a single mountain range (ie. the Urals) or as a "thrust belt" consisting of several mountain ranges (ie. North American Cordillera - Rocky Mountains, Sierra Nevada, and Cascades).

Subduction also drags surface material and sediments deep underground along with the subordinate crust in a process called "deep burial," where the surface material is re-congealed into molten rock.

Basic Rock Types

The earth and its geology is referred to as a "closed system," meaning that all rock and minerals are constantly being recycled in a process of evolution called the "rock cycle." Starting with its initial formation as cooled and solidified lava, rock is worn away by ice, rain, and wind (weathering), then transported by erosion, until the material reaches its final resting place (deposition). During this evolutionary process there are three basic states of being (forms) that a rock can have:

1. Igneous Parent Rock (felsic, intermediate, mafic or ultramafic)
2. Metamorphic Formed by Heat and Pressure
3. Sedimentary Formed by Deposition

When the earth was formed several billion years ago, there was only molten material (Igneous rock), which was the "parent" to all other rock varieties. Each of the three types of rock can evolve into the other in a bi-directional process (see chart at top of page).

Gem Occurrences

All gemstones are found in roughly four types of situations or occurrences, which describe their physical location at the time of their discovery. When prospecting for precious metals, ore bodies, or gemstones, each of these unique situations calls for a different method of exploration, identification and extraction.

1. In Situ Gems found "in place" in their parent, or host rock
2. Eluvial Gems exposed by weathering or erosion
3. Colluvial Gems found in loose deposit of debris at base of slope
4. Alluvial Gems that are transported and redeposited by erosion, or running water

Gems that are found in situ, which is Latin for "in place," can occur in both igneous pegmatites and kimberlites, or in metamorphic host rock, which are considered to be a "primary" deposit. Gems that have remained roughly in place, but the surrounding soil and/or rock has eroded/weathered away are considered to be "eluvial," or "eluvium." When a gem is part of a debris field that was deposited by a landslide or "spill" it is considered to be "colluvial." Lastly, when the gem has been completely transported to a new, secondary location by running water, and redeposited within sediments, sands, or river gravels it is considered to be "alluvial," or "alluvium."

Gemstones are typically hard, highly resistant to weathering, and have a specific gravity that is higher than that of common minerals, rock or sedimentary soil, therefore, they tend to concentrate in alluvial deposits in much the same way as gold placers develop.