Chapter 2: Physical Properties of Minerals

specific gravity (G)

relative density; a number that expresses the ratio of the weight of a substance and the weight of an equal volume of water at 4 degrees Celsius (the temperature of the maximum density of water; ratio of densities so this number has no units

polymorphs

compounds that have the same composition but different structures

conchoidal fracture

fractures that produce the smooth, curved ridges resembling the interior surface of a shall; commonly observed in materials such as glass and quartz

hackly fracture

fractures that produces a jagged and sharp edge

descriptions of tenacity

-brittle -malleable -sectile -ductile -flexible -elastic

fibrous or splintery fracture

fractures that result in pieces that are fibrous or splinters

prismatic

geometric form name: for a crystal with one dimension much longer than the other two

cubic

geometric form name: with the external form of a cube

rhombohedral

geometric form name: with the external form of a rhomnbohedron

octahedral

geometric form name: with the external form of an octahedron

asterism

inclusions that are arranged in three directions at 120 degrees to each other; a stone cut from such a crystal shows a six-pointed star (star rubies, sapphires and garnets); caused by the scattering of light from small mineral inclusions properly oriented in three directions

bladed

individual crystals (or grains) that are flattened, elongated crystals (kyanite, stibnite)

stressed

applied external force to a mineral (force applied)

3 directions of cleavage

cubic; rhombohedral

opaque

described a mineral that is impervious to visible light, even on thin edges of the mineral; most metallic minerals

6 directions of cleavage

dodecahedral

determination of the hardness of a mineral

using a fresh mineral surface, observe the following: 1. sometimes, when one mineral is softer than another, portions of the first will leave a mark on the second that may be mistaken for a scratch; such a mark can be rubbed off, whereas a true scratch will be permanent 2. the surfaces of some minerals are frequently altered to a material that is much softer than the original mineral; a fresh surface of the specimen to be tested must be used 3. the physical nature of a mineral may prevent a correct determination of its hardness; for instance, if a mineral is granular, or splintery, it may break but appear to be scratched by a mineral much softer than itself; it is always advisable when making the hardness test to confirm it by reversing the order of procedure; that is, try to scratch mineral A by mineral B, but also try to scratch B by A

fibrous

very thin, flexible thread-like grains or fibers (chrysotile)

cibcebtruc

when bands or layers are arranged concentrically about one or more centers (agates)

color

when the chemical element causing the color is essential to the mineral, this can be used as a diagnostic tool (constant and consistent); often times will vary

types of nonmetallic lusters

-adamantine -vitreous -resinous -pearly -greasy -silky -earthy

types of mechanical properties

-cleavage -fracture -parting -hardness

quality of development of the external planes or faces

-euhedral -subhedral -anhedral

types of luster

-metallic -nonmetallic -submetallic

types of cleavage

-perfect -good -fair -poor -absent

play of colors

interference of light either at the surface or in the interior of a mineral may produce a series of colors as the angle of the incident light changes; this creates a brilliant flashes of varied color, against a white or black background (opal and diamond); caused by diffraction of light from closely spaces features such as packed spheres (opal) or fine lamellae (plagioclase)

silky

appears as a skein of silk or a piece of satin (fibrous gypsum); this is a characteristic of some minerals in fibrous aggregates

greasy

appears as if covered with a thin layer of oil (some milky quartz and nepheline); this luster results from light scattered by a microscopically rough surface (halite)

earthy

appears dull (soil); this characteristic of aggregates of very fine-grained materials (goethite and limonite)

adamantine

appears to have the luster of a diamond; from the Greek word adamas, meaning diamond; this is the brilliant luster shown by some minerals that also have a high refractive index, and as such refract light strongly, like a diamond; other examples are some garnets and the lead carbonate, cerussite

perfect cleavage

breaks easily and the resulting surface is flat and reflects light well (mica)

good cleavage

cleavage surfaces are less continuous and may have some irregularies

rupture

cleavage, parting, and fracture

stalactitic

colloform; made up of small stalactites, which are conical or cylindrical in form as is common on the ceilings of caves; from the Greek word stalaktos, meaning dripping

fluorescent

luminescence in the visible light spectrum during exposure to ultraviolet light, X-rays, or cathode rays

phosphorescent

luminescence in the visible light spectrum during exposure to ultraviolet light, x-rays, or cathode rays that continues after the exciting rays are cut off

streak plate

unglazed white porcelain plate used to determine the streak of a mineral

transparent

describes a mineral that transmits light and through which an object may be seen; quartz and calcite are commonly transparent; most gemstones are transparent and many are priced on the quality of their transparency

subhedral

describing a crystal or mineral grain that is partly bounded by crystal faces and partly by irregular surfaces; from the Latin root sub, meaning less than

banded

a mineral aggregate in which a single mineral may show thin and roughly parallel banding (banded malachite), or in which two or more minerals form a finely banded intergrowth (chert and hematite bands in banded iron-formation)

dendritic

a mineral exhibiting a branching pattern like a fern; from the Greek root dendron, meaning tree (manganese oxides)

radiating

a mineral in which acicular crystals radiate from a central point (wavellite and goethite)

massive

a mineral specimen totally lacking crystal faces, often found in very fine-grained minerals (goethite)

sectile

a mineral that an be cut into thin shavings (chalcocite)

flexible

a mineral that bends but does not return to its original shape when pressure is released (sheets of chlorite, talc)

brittle

a mineral that breaks and powders easily (halite)

ductile

a mineral that can be drawn into a wire (gold)

malleable

a mineral that can be hammered out into a thin sheet (copper)

micaceous

a mineral that separates into thin plates easily (mica)

chatoyancy

a mineral that shows a band of light at right angles to the length of the fibers, or to the direction of the inclusions, displaying a silky appearance when viewed in reflected light

elastic

a mineral that, after being bent, will resume its original position upon the release of the pressure (micas)

acicular

a mineral with a needlelike shape; from the Latin root accicula, meaning needle (sillimanite)

density

a mineral's mass (m) per unit volume (v); expresed in units of grams per cubic centimeters (g/cm3) or in kg/m3

absolute hardness scale

a quantitative measurement of a mineral's hardness; exponential graph

geode

a rock cavity lined with mineral matter but not completely filled; they may be handed, as in agate, containing a central portion filled with euhedral crystals projecting into an open space

heft

a sense of the relative weight of the mineral

Mohs relative hardness scale

a series of 10 common minerals was chosen for use in comparing a mineral's relative hardness

lamellar

a specimen made up of layers, like the leaves in a book (graphite, molybdenite)

compact

a specimen so fine-grained that individual grains are not obvious to the eye (kaolinite)

oolitic

a specimen that is made up of oolites, which are small, round or ovate (meaning egg-shaped) accretionary bodies and resemble the roe of fish; from the Greek word oon, meaning egg; this appearance is common in some iron-rich specimens, made of hematite, known as oolitic iron ore

globular

a surface made of little spheres or globules (smithsonite)

pearly

appears as a mother-of-pearl with a iridescent sheen; this is characteristic of mineral surfaces that are parallel to well-developed cleavage planes (talc and apophyllite)

virteous

appears as a piece of polished glass (quartz and emerald)

resinous

appears as a piece of resin (sphalerite)

euhedral

describing a mineral that is completely bounded by crystal faces (well-formed); from the Greek roots eu, meaning good, and hedron, meaning plane; no interference of other minerals while the mineral is growing

anhedral

describing a mineral that lacks crystal faces and that may show rounded or irregular surfaces; from the Greek root an, meaning without

poor cleavage

difficult to see and the cleavage surfaces are not well developed (beryl or apatite)

uneven or irregular fractures

fractures that produce rough and irregular surfaces

translucent

describes a mineral that is capable of transmitting light diffusely but is not transparent; allows light to be transmitted, it will not show a sharp outline of an object seen through it; some varieties of gypsum are translucent

pisolitic

made of pea-sized grains; from the Greek word pisos, meaning pea; having a texture similar to that of an oolitic aggregate but somewhat coarser in grain size (bauxite, the major source of aluminum ore)

botryoidal

mammillary; when a mineral appears as a bunch of grapes; from the Greek root botrys, meaning bunch or cluster of grapes (smithsonite, chalcedony, and prehnite)

absent cleavage

mineral does not demonstrate the properties of cleavage (quartz)

granular

mineral grains that are of approximately equal size; the term is mainly applied to minerals whose grains range in size from about 2 to 10 mm; if the individual grains are larger, the aggregate is described as coarse-granular; if smaller, it is fine-granular

diaphaneity

minerals are commonly described in terms of the amount of light they can transmit; the light-transmitting qualities of a mineral; originates from the Greek word diaphanes, meaning transparent)

aggregates

minerals that are intergrown and tend to be made of many smaller grains with crystals ranging from euhedral to anhedral

submetallic

minerals with an intermediate luster

basal parting

observed in many minerals; occurs parallel to the "base" of a mineral

break

occurs when the strain exceeds the overall strength of a mineral

4 directions of cleavage

octahedral

1 direction of cleavage

pinacoidal; basal

2 directions of cleavage

prismatic

quality of cleavage

refers to the ease with which the mineral breaks and the perfection of the broken surface

fair cleavage

reflects properties between poor and good

mechanical properties

reflects the strength of the internal forces (atomic structure) that hold the individual atoms together (bond strength)

Mohs scale of hardness

relative hardness from soft to hard: 1. Talc: very easily scratched by the fingernail; has a greasy feel 2. Gypsum: 2.2 fingernail: can be scratched by the fingernail 3. Calcite: 3.2 copper penny: very easily scratched with a knife and just scratched by a copper coin 4. Fluorite: easily scratched with a knife but not as easily as calcite 5. Apatite: 5.1 pocket knife, 5.5 glass plate: scratched with a knife, with difficulty 6. Orthoclase: 6.5 steel file: cannot be scratched with a knife, but scratches glass 7. Quartz: 7.0 streak plate: scratches glass easily 8. Topaz: scratches glass very easily 9. Corundum: cuts glass 10. Diamond: used as a glass cutter

nonmetallic luster

shown by many minerals that transmit light, at least through their edges; varies widely in appearance ranging from a highly polished glass surface to a dull earthy-like appearance

morphology

terms that relate to their overall appearance

streak

the color of the finely powdered mineral on a streak plate

luster

the general appearance of a mineral surface in reflected light

crystal habit

the general shape of a mineral, which also includes irregularities due to growth

strain

the internal structure of the crystalline substance is deformed due to the stress (resultant deformation)

labradorescence

the iridescence that is the result of light scattered by extremely thin exsolution lamellae; this play of colors is similar to that produced by soap bubbles or thin films of oil on water

metallic luster

the luster of a polished metal surace, such as the appearance of chrome, steel, copper, and gold; reflect light like metals and are opaque to transmitted light

crystal form

the outward appearance of a mineral takes on a regular geometric shape; the external form is the outward expression of the internal ordered atomic arrangement

luminescence

the phenomenon that results from a material absorbing a form of energy (thermal, mechanical, electromagnetic) and re-emitting that energy as visible light; most can only be seen in the dark

tenacity

the resistance of a mineral to breaking or deforming, or its cohesiveness

hardness

the resistance of a smooth surface of a mineral to scratching; determined by observing if a mineral is scratched by another material of known hardness; "scratchability"; expression of a mineral's weakest bonding; H=#

parting

the response of a crystalline material to an external force; mineral breaks along specific planes of structural weakness; usually has discontinuities and are not smooth

cleavage

the response of a crystalline material to an external force; the tendency of minerals to break along parallel planes; the mineral breaks preferentially along the specific directions that the mineral has weaker bonds holding the atoms together

fracture

the response of a crystalline material to an external force; with some crystals, the strength of the bonds is approximately the same in all directions and breaking of these minerals generally will not follow a particular direction; the breakage of minerals when they do not yield along cleavage or parting surfaces