PROPERTIES OF MINERALS
Characteristics are used in the identification & study of minerals. The most common characteristics used when describing minerals are: COLOUR, STREAK, LUSTER, SPECIFIC GRAVITY, CRYSTAL FORM, CLEAVAGE, FRACTURE, TENACITY, HARDNESS, TRANSPARENCY
Few minerals have special properties like MAGNETISM, CHATOYANCY, FLUORESCENCE, ODOR, CONDUCTIVITY and BURN PROPERTIES
Colour varies depending on the chemicals present and is the least informative in identifying a mineral variety
This is the colour of the line drawn when a mineral is rubbed across a piece of unglazed tile. Often the line is the same or similar colour of the specimen.This test is useful because in a few cases, similar looking minerals have different streaks - so this test is a definitive identifier.
Luster is what the surface looks like when the light reflects off of the mineral. Some words to describe luster are:
- DULL / EARTHY - very dull, mainly minerals that are porous eg kaolinite, orthoclase
- WAXY - like the surface of a candle, like wax paper eg opal, chalcedony
- GREASY / OILY - a little bit shiny eg nephaline
- PEARLY - like a pearl, play of colours on a white background eg talc, muscovite mica
- SILKY - has a shiny surface like a piece of silk cloth eg some varieties of gypsum, kernite, ulexite, some fibrous minerals
- GLASSY / VITREOUS - looks like glass, shiny & reflective eg quartz, many rock-forming minerals, obsidian ("nature's glass")
- RESINOUS - looks like freshly-broken shellac, usually yellow-brown eg sphalerite
- ADAMANTINE - high luster, almost brilliant, "diamond-like" eg sphalerite
- SUB-METALLIC -silvery or metallic luster but mineral is transparent or translucent when in small slivers eg hematite
- METALLIC - very shiny, like processed metals, highly reflective, opaque minerals eg pyrite, gold, silver
- other words - shiny, sparkly, shimmering, opalescent, frosted, milky
4) SPECIFIC GRAVITY
Specific Gravity (SG) indicates how many times more the mineral weighs compared to an equal amount of water (SG 1). If you have a bucket of silver, it would weigh 10 times as much as a bucket of water. That is why we think of metals as being "heavy". They are heavy compared to other things that we are used to picking up.
The average rock you would pick up has an SG of about 2.75 because most of the earth’s crust is made up of quartz, calcite & feldspar. If you have a bucket of calcite, it would only weigh about 2 1/2 times as much as a bucket of water. When something feels heavy, it feels heavier than expected for something of that size. This is also known as the "heft" of an object. How hefty a specimen feels has to do with how dense it is, its mass compared to its volume. The following defines what we mean by this:
- very light: less than 2 SG eg borax
- light:2 - 2.5 SG eg gypsum, halite, selenite, ulexite
- average: 2 – 3 SG eg calcite, dolomite, feldspar, muscovite mica, quartz, talc, turquoise,
- above average /slightly heavy: 3 - 4 SG eg biotite mica
- heavy: 4 – 5 SG eg almandine garnet, apatite, barite, celestite, chalcopyrite, fluorite
- very heavy: 5 – 10 SG eg galena, hematite, magnetite, nickel-iron, pyrite
- extremely heavy even for a metallic mineral: greater than 10 SG eg gold, silver
- super heavy: 20 plus SG must be platinum!
5) CRYSTAL FORM
Shape of crystal, shape the mineral would take if it had room to grow in a cavity, not massive – some minerals have a number of different crystal shapes.
When minerals have the time & space to grow into their crystal forms, they grow to beautiful regular shapes that are easy to recognize once you have seen a few examples. Some words used to describe crystal forms or shapes are:
- acicular / radiating needles ~ crystals that grow in fine needles
- blebs ~ rounded blobs
- botryoidal ~ looks like top of bunch of grapes
- concretion ~ spherical, round shape that is solid, the same all the way through or filled with layers or agate
- cubic ~ 6 equal, square faces
- dendritic ~ branching, tree-like, looks like the veins in a leaf or like a painted “tree shape”
- dodecahedron ~ 12 sided, like a 12 sided die
- dog-tooth ~ shaped like the canine tooth, like a dog's tooth
- fibrous ~ looks like fibers, threads, parallel lines
- geode ~ spherical, round shape that is hollow inside, often lined with crystals
- hexagonal prism with pyramid termination ~ hexagonal cross-section, with pointy ends (terminations)
- hexagonal prism with rounded ends ~ 6 sided cross-section, with rounded ends
- hexagonal pyramid ~ sharp 6 sided pyramid, often seen in clusters
- mamilliary ~ rounded like botryoidal but a bit bigger than a bunch of grapes
- massive ~ a chunk of mineral with no crystal shape evident
- octahedral ~ 8 sided
- prismatic ~ like a prism with flat ends, longer than it is wide
- pyritohedral ~ 12 sided with 5 sided pentagon faces
- rose shaped ~ looks like a flattened flower or rose with petals
- tabular ~ divide easily into thin plates or sheets, a stack is know as a “book”
- termination ~ the end of a complete crystal
This is the pattern when mineral is broken – in planes or conchoidal.
This describes what the edges look like when the mineral breaks.
This describes toughness, how cohesive the mineral is, if it falls apart.
The hardness of a mineral is tested by seeing what it can scratch and what scratches it.
This is the ability to transmit light. Depending on a number of things, rocks & minerals can also transmit light. Many rocks that are opaque when in a chunk, are translucent when cut into very thin slices.
Few minerals are magnetic. Only lodestone, the magnetic variety of magnetite generates a magnetic field.
This occurs when minerals display luminous bands that seem to move when the mineral is rotated. Tiger's Eye is a well known example of this property.
Fluorescent minerals emit visible light, often in vivid different colours, when exposed to ultraviolet light.
Some minerals have distinctive odours - for example sulfur which is said to smell like rotten eggs.
Some minerals can allow electric currents to travel through them.
16) BURN PROPERTIES
When powdered minerals are burned, they sometimes have a distinctive flame colour which helps identify the mineral.