Читать книгу Geology and Mineralogy of Gemstones - Karl Wallace, David Turner R. - Страница 24

Each mineral has a distinct chemical composition and internal arrangement of atoms and bonding. Accordingly, every mineral will exhibit distinct physical properties.

Color is the most familiar of the physical properties and often what draws people to minerals and gemstones. In a simple sense, color is described as the outward appearance of mineral as observed by our eyes. It is a function of the nature of the incident light and its interaction with the mineral, including effects from transmission, reflection, refraction, scattering, and absorption of visible light. Minerals that display little to no absorption of visible light will appear white if light is scattered off the surface (as in kaolinite) or transparent if light is transmitted through the crystal (as in pure quartz). Despite being an easy to observe property, color is actually not a very good diagnostic property on its own. This is because many minerals can exhibit a range of colors depending on the impurities within them. This concept is developed in greater detail later, as it is critical to the world of gemstones.

Luster refers to how visible light interacts with the surface of a mineral. Minerals with metallic luster show strong reflection of light off their surfaces, as in the case of polished gold or the mineral pyrite (iron sulfide). Minerals with nonmetallic luster generally absorb at least some of the incident light in addition to reflection. Types of nonmetallic luster include vitreous, resinous, dull, earthy, pearly, greasy, silky, and adamantine.

Streak refers to the color of a mineral after it has been ground along the surface of a ceramic or porcelain streak plate. The process of grinding the mineral into finer particles results in a more even display of a mineral’s color under incident light. Streak is often more diagnostic for minerals than color.

Habit describes the common ways that a mineral crystallizes into macroscopic forms. Habit can be described through the examination of the external form of a mineral specimen, which can be either an individual crystal or an aggregate of crystals that grew together. Terms used to describe individual crystals include platy, pyramidal, bladed, lamellar, acicular, tabular, or prismatic. Terms used to describe aggregates of crystals include fibrous, reniform, botryoidal, dendritic, radiating, concentric, massive, or stalactitic.

Cleavage, parting, and fracture describe the ways in which minerals break under force. Cleavage is the occurrence of discrete planes of weakness 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.