Geology and Mineralogy of Gemstones. David Turner R.

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СКАЧАТЬ in a mineral that correlate to weaknesses in the internal bonding and atomic structure of that mineral. Cleavage is often described as perfect (as in micas), good (as in epidote), imperfect (as in beryl), or indistinct (as in tourmaline). Cleavage planes will be straight and repeated in different orientations of a mineral as dictated by that mineral’s overall symmetry, and therefore can be described using crystallographic orientations and patterns (e.g., octahedral cleavage, as in fluorite).

Parting is when a mineral will preferentially disaggregate in a somewhat consistent manner but in a way that is not controlled by the atomic arrangement of atoms and therefore will not be repeated based on a mineral’s underlying symmetry (Figure 1.5). Fracture is described as the irregular breakage (commonly curved) of a crystal and can sometimes be diagnostic, as in quartz, which exhibits conchoidal fracturing, or kyanite, that exhibits splintery fractures.

Tenacity is the resistance of a mineral to break or bend. Easily breakable minerals are termed brittle (as in kyanite, Figure 1.6) while those that can bend and return to their shape are termed elastic (as in mica‐group minerals). Bendable minerals that do not return to the shape but that do not break apart are termed flexible. Minerals with metallic bonding can be malleable (hammered into thin sheets, as in gold), ductile (can be drawn out into wires), or sectile (can be cut into slices).

      Hardness is a measure of a mineral’s resistance to scratching against another mineral and is related to its bonding characteristics. The Mohs hardness scale is a relative ranking of common minerals and their hardness. Gemstones are generally high up on the ranking, as it is important for them to not be easily scratched. In order from soft to hard, the Mohs scale (developed in the early 1800s) is defined by the following index minerals: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase (6), quartz (7), topaz (8), corundum (9), and diamond (10). Half increments are often used, as in the case of beryl that has a hardness of ~7.5–8. Because hardness is a function of bonding within a mineral, it is also technically a property that may vary depending on the direction of scratching. For example, kyanite shows a hardness of 5 parallel to its length and 7 across the length, while garnet exhibits a hardness of 7.5 in all directions. Hardness can also be measured by other methods and scales, such as Vicker’s Hardness or the use of a sclerometer, an instrument that measures the width of a scratch made by a diamond on the sample under controlled conditions.

Figure 1.5 This crystal of corundum shows rhombohedral parting patterns and underlying irregular fractures.

      Photo by D. Turner.

Specific Gravity (SG) is a measure of how heavy a material is for a given volume, defined by the weight of the material compared to the weight of water for an equal volume. Specific Gravity is unit‐less, which differs from density that is measured in g/cm³ or kg/m³. The SG of water is 1, while that of diamond is 3.52. Most rock‐forming minerals (like quartz, SG = 2.65) have SG values between 2 and 3.5 while metal sulfides (like pyrite, SG = 5.0) and native metals (like gold, SG = 19.3) have higher SG values. This is sometimes referred to as heft.

Figure 1.6 This cluster of bladed kyanite crystals shows brittle tenacity and splintery parting, yellowish‐grey to blue coloration, and would exhibit lower hardness along the length of the crystals than across.

      Photo by D. Turner.

      Fluorescence is a consistent property of some minerals while in others it only occurs when certain impurities are present. Fluorescence is a phenomenon where light with greater energy (and shorter wavelength) excites electrons within a material and upon deexcitation (or relaxation) of the electron to ground state, a photon of lesser energy (and longer wavelength) is emitted. It is a type of luminescence. This is normally tested using ultraviolet light and observed in the visible range with the human eye; however, the process can be observed across a range of activating and fluorescent wavelengths. Fluorite is a common fluorescent mineral and some diamonds can be strongly fluorescent, yet neither of these minerals will always display fluorescence. Other types of luminescence include phosphorescence, thermoluminescence, triboluminescence, and cathodoluminescence.

References

      1 Ball, S. H. (1935). A historical study of precious stone valuations and prices. Economic Geology, 30(5), 630–642.

      2 Haxel, G. B., Hedrick, J. B., Orris, G. J., Stauffer, P. H., & Hendley II, J. W. (2002). Rare earth elements: Critical resources for high technology. Fact sheet No. 087‐02. United States Geological Survey.

      3 Hazen, R. M., Grew, E. S., Downs, R. T., Golden, J., & Hystad, G. (2015). Mineral ecology: Chance and necessity in the mineral diversity of terrestrial planets. The Canadian Mineralogist, 53(2), 295–324.

      4 Mason, B. & Moore, C. (1982). Principles of Geochemistry. New York: John Wiley & Sons.

2 Basics of Rocks and Geology

      2.1 Earth System Science

Earth System Science views the Earth as a working system, each part having an impact and an effect on the other through geological time. To understand how the Earth creates beautiful and inspiring gems, all aspects of the Earth system must be appreciated, including the atmosphere, oceans, surface tectonic processes, processes deep in the Earth, and life (Figure 2.1).

      The significance of these components varies for the creation and preservation of different precious materials but all aspects tend to be tied together in one way or another. Diamonds, for example, predominantly form deep within the Earth in a region called the Upper Mantle, where very high pressures and temperatures exist. However, other processes, such as volcanism, are required to bring these diamonds through the mantle and crust to the surface. Natural processes on the Earth’s surface, such as glaciation, can move the diamonds away from their original source and leave a trail of ground kimberlite rock leading back to where the original deposit resides. Alternatively, if enough diamonds were moved by natural processes (e.g., river transport) from their primary geological location to a new secondary location, a diamond deposit could be formed far away from the original source rock. Even in this very limited example, the complexity and interconnectedness of the Earth system is obvious.

2.2 The Earth’s Structure and Plate Tectonics

Our solid planet is not homogeneous but is made up of a number of very distinct layers (Figure 2.2). These layers, from exterior to interior, are:

       Crust. The Earth’s crust is the uppermost layer. It represents ~1% of the total volume and generally consists of continental and oceanic crust. This uppermost layer is separated into a number of rigid sections, known as tectonic plates. Continental crust and oceanic crust have different overall compositions; continental crust has a higher silicon (Si) content but is more heterogeneous while oceanic crust has СКАЧАТЬ