Minerals play an essential part of our everyday lives, from pencil lead made of granite to gold jewelry worn on fingers and iron found in cars. Each mineral possesses unique properties which help us classify it and distinguish one mineral from the next.
Crystalline structures feature crystal form and chemical composition; other important properties include density and streak test results.
Physical Properties
Minerals possess various physical characteristics. This includes their luster (how light reflects off of them), streak (how the mineral appears when spread out on paper), color and refractive index. Some minerals also bend the way light passes through them, known as refraction; this property can be measured with its refractive index. Some magnetic minerals can also help identify them due to their internal structure; this magnetic signature can also help identify them. Other physical attributes may include their tenacity (how resistant to stress the mineral is), specific gravity as well as smell or taste characteristics – for instance Halite has its characteristic salty flavor while sulfur emits strong smells of rotten eggs.
Most minerals are solids with inherent weaknesses within their atomic structures that allow for breakage when hit with a hammer, known as cleavage. Cleavage quality may range from perfect (biotite) to poor (pyrite).
Chemical Composition
Chemical composition of minerals is one of the primary criteria used to classify them into individual species. To assess its chemical makeup, an empirical formula must be divided by its molecular weight (or sum of its atomic weights) to produce a percentage breakdown for each element present within it and listed accordingly in a table.
Minerals stand out from other materials in part by their unique atomic structures. Atoms often join together into molecules and compounds; positively charged cations attract negatively charged electrons (anions) in other atoms to form ionic bonds which provide cohesion within minerals.
Some minerals exhibit alternative bonding mechanisms as well, including hydrogen and van der Waals bonds that do not involve valence electrons and can be found in clay minerals for example. Minerals tend to have isotropic physical properties – meaning their physical properties remain constant regardless of where you look at it from.
Color
Color of minerals can be an extremely significant indicator of identification and value assessment of specimens. A mineral’s color is determined by its chemical makeup; specifically the presence of certain elements known as chromophores such as copper and iron atoms.
White light enters crystals, some wavelengths may be absorbed while others reflected. This produces the colors we observe today.
Absorpting wavelengths is caused by electrons in the outermost shells of mineral atoms occupying vacant energy orbitals to fill them up, thus absorbing wavelengths in some form or another; some can be visible, others not so.
Many other colors in minerals are caused by impurities or structural defects within their crystal structures, creating pseudochromatic minerals such as iridescence seen in iris quartz and pearls; chatoyancy seen in malachite and chrysoberyl; asterisim seen by some garnets; aventurescence from aventurine quartz; play of colors (feldspar microcline variety called amazonite); often not entirely understood, precise spectroscopy analysis down to ppm levels as well as magnetic resonance, radiation or laboratory synthesis may provide definitive explanations.
Cleavage
Minerals that exhibit cleavage break along planes of weakness within their crystal structures, known as cleavage surfaces. Cleavage differs from fracture, in which pieces break off irregularly that cannot be described using Miller indices, and parting which occurs due to impurities or inclusions within the mineral itself. Quality and quantity of cleavage surfaces help distinguish one mineral from another – for instance micas often exhibit basal cleavage that causes them to split open into flat sheets.
Cleavage can also provide valuable insights into a mineral’s atomic arrangement. Micas contain layers of silicon and oxygen atoms stacked up closely together, creating zones of weakness called cleavage surfaces which later break away to reveal book-like mica pages. Quartz, on the other hand, does not exhibit this behavior when hit by a rock hammer and instead breaks randomly with each impact.
