Acicular- Needle-like crystal habit.

 

Acid - Chemical compound that yields cations of hydrogen when dissolved in water.  Common strong acids include hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4).  A common weak acid is acetic acid (CH3COOH); vinegar is a 3% solution of acetic acid.  
 Acids will attack many minerals and can be used to help identify them.  For example, acids attack carbonate minerals, such as calcite, causing carbon dioxide bubbles to be given off in an affect known as effervescence.  Many acids are dangerous and should be handled with care.

Adamantine - Hard brilliant luster; like that of a diamond.

Adularescence, Adularescent - Strong pearly-to-blue floating sheen seen in the moonstone varieties of the feldspars Orthoclase, Albite, and Oligoclase.

Aggregate - A combination of minerals joined in irregular fashion to form a material similar to concrete.  

 

Albite Law Twin - Type of lamellar twin on the {010} plane typical of albite and exhibited by other members of the plagioclase series.

Aliphate - Hydrocarbon in which carbon atoms are linked into chains.

Alloy - Homogeneous mixture of two or more metals in variable proportions.

Alteration - Change in the chemical composition and/or atomic structure of a mineral brought about by chemical attack or changing physical conditions.

Alumino- Compound containing aluminum usually in combination with an anionic group (i.e. aluminosilicate).

Amalgam - Alloy of mercury with another metal.

Amide - Organic compound containing the amino group (NH2 or NH).    

                                

Amorphous, Metamict - Amorphous materials are non-crystalline, that is they lack long-range regularity in their atomic structure.  By definition they cannot be minerals.  Nevertheless, some natural amorphous substances have been accepted as minerals.
 Some minerals containing radioactive atoms have become amorphous as a result of radiation damage to their atomic structures; they are called metamict.

Analogue - A mineral which is identical to another, usually with respect to both chemical composition and atomic structure with the exception of a single attribute, typically one the elements essential to its composition.  For example, Calcite, Magnesite, and Strontite are analogues, containing respectively calcium, magnesium, and strontium.                                        

 

Anastomosing - Network of branching and rejoining veins.                                           

 

Anhedral - Crystal with no well-formed external faces.

Anhydrous - Mineral with no water (H2O) in its formula.

Anion – See atom.

Anionic group – See atomic group.
 
Anisotropic - Crystal which affects light differently when light passes through the crystal in different directions.  See index of refraction.  

 

Arborescent - Intergrowth of crystals in the shape of slender divergent branches like a plant.   

Archeotype - General type of atomic structure.  SnS-archeotype refers to the atomic structure of the mineral herzenbergite.  PbS-archeotype refers to the atomic structure of the mineral galena.

Arcata - Curved or bent.  

Arsenate - Compound containing the arsenate group (AsO4).

Arsenite - Compound containing the arsenite group (AsO3).

Asbestiform - Crystal intergrowth, similar to that commonly exhibited by asbestos, consisting of fibers generally aligned in a single direction, often at right (90º) angles to the walls of a vein.  See cross-fiber.

Atom, Element, Electron, Proton, Neutron, Nucleus, Ion, Anion, Cation - An atom is the smallest particle of matter that can combine with other atoms to form chemical compounds.  

 At the center of an atom is the nucleus containing positively-charged protons and uncharged neutrons.  The nucleus is surrounded by negatively-charged electrons.  The number of protons in the nucleus determines the chemical behavior of the atom.  With two exceptions atoms with every number of protons between 1 (hydrogen) and 92 (uranium) have been found in nature; these are known as the 90 naturally occurring elements.

 An atom that has the same number of electrons as protons is uncharged (electrically neutral).  Most atoms, when mixed with atoms of a different element, have a tendency to lose or gain certain small number of electrons to form ions.  Negatively charged ions (those with extra electrons) are called anions; positively charged ions (those with too few electrons) are called cations.  

 

Atomic Bonding - It is useful to classify the bonds that hold atoms to one another in chemical compounds and crystals into three types - ionic, covalent and metallic.  

 As the name implies, ionic bonding is due to the attractions between positively and negatively charged ions.  Some atoms tend to lose electrons to become cations, ions with a positive charge; others tend to gain electrons to become anions, ions with a negative charge.  Oppositely charged ions attract one another forming ionic bonds.  In ionic bonding the oppositely charged ions can be thought of as hard balls are packed closely together, almost in contact, with several ions of the opposite charge surrounding each ion.

 When the different atoms are almost equal competitors for electrons, atoms bond together by sharing electrons.  This kind of bonding is called covalent bonding.  It holds two partners together in a very precise geometrical arrangement.  The two bonded atoms can be thought of as an ellipsoid with nuclei at the two foci.  Discreet covalently bonded atom groups are found in many minerals.  These usually have a net negative charge and are called anionic groups.  An example is the phosphate group in which four oxygen atoms surround a central phosphorous atom; the whole unit behaves like an anion with a charge of -3.  Many of these groups form the basis for the common mineral classification schemes (Strunz, Dana).

 Atoms in metals bind together by another sort of electron sharing, but in these compounds the electrons are shared between large numbers of atoms and are actually free to move from atom to atom throughout the material.  This type of bonding is weaker.  Shear forces can cause the atoms to slip with respect to one another and then re-bond in the new position.  This explains the malleability of many metals.  The free flow of electrons explains the electrical and heat conductivity of metals. 

 

 Atomic Group, Anionic Group, Polyatomic Ions - Certain atoms form especially tightly bonded atomic groups.  In minerals, these usually consist of a small cation surrounded by three or four oxygens.  These groups usually have a net negative charge, in which case they are called anionic groups.  An example is the phosphate group, (PO4)-3, in which four oxygen atoms surround a central phosphorous atom and the whole unit behaves like an anion with a charge of -3.

Atomic Structure - Atomic structure refers to the orderly, repetitive, 3-dimensional arrangement of atoms in a crystalline substance such as a mineral.  Nowhere is the orderliness and perfection of nature more apparent than in
 atomic structure.  In an atomic structure, atoms come together into stable configurations by forming networks of bonds between one another.  Knowledge of the atomic structure of a mineral helps us to understand all of the mineral’s properties.

Barrel-shaped - Habit of a stout prismatic crystal in which the prism faces are bowed so that the crystal has a greater diameter at its center than at either end.  The crystal usually also has flat terminations.

Basal Pinacoid - Crystal form consisting of the two parallel faces, (001) and (00-1), which transect the c- axis.

Base- Compound that when dissolved in water yields free hydroxyl (OH-) anions.

Baveno Law Twin - Type of simple contact twin on the {021} plane exhibited by orthoclase, microcline and other members of the feldspar group.

Beryllo- Compound containing beryllium usually in combination with an anionic group (i.e. beryllosilicate).                                       

 

Birefringence - See index of refraction.  

Birefringent - Having different indicies of refraction along different crystal directions.

Bladed - Habit of a crystal that is flat and long; like a knife blade.  Also an intergrowth of blades stacked together.

Bleb - A rounded crystal grain of one mineral imbedded in another.

Blocky -   Habit in which a crystal roughly block-shaped and nearly equidimensional.

Borate - Compound containing the borate group (BO3) or linked borate groups.

Boro - Compound containing boron usually in combination with an anionic group (i.e. borosilicate).  

                   

Botryoidal, Reniform - Crystalline intergrowth with smooth, rounded surfaces.

Brazil Law Twin - Type of penetration twin in quartz in which right- and left-handed crystal segments are twinned across the {11-20} plane.  It is usually only possible to detect Brazil twinning under special polarized light.                                            

 

Brittle - Breaks or powders easily.  A type of tenacity.

Butterfly twin - Simple contact twin consisting of two wedge-shaped individuals, exhibiting no prominent reentrant angle, and having an overall shape resembling a butterfly.

Capillary -Very thin, hair-like crystal habit.

Carbide - Compound of a metal with carbon.

Carbocycle - Hydrocarbon in which carbon atoms are linked into rings.

Carbonate - Compound containing the carbonate group (CO3).

Carlsbad Law Twin - Type of penetration twin about the [001] axis exhibited by orthoclase, sanidine and other members of the feldspar group.   Cathodoluminescence - Emission of visible light by a mineral when it is bombarded by electrons.

Chatoyancy - An optical phenomenon, possessed by certain minerals in reflected light, in which a movable wavy or silky sheen is concentrated in a narrow band of light that changes its position as a mineral is turned. It results from the reflection of light from minute, parallel fibers, cavities or tubes, or needlelike inclusions within the mineral. The effect may be seen on a cabochon-cut gemstone, either distinct and well defined (such as the narrow, light-colored streak in a fine chrysoberyl cat's-eye) or less distinct (such as in the usual tourmaline or beryl cat'seye).
 
Chatoyant - a mineral or gemstone possessing chatoyancy or having a changeable luster or color marked by a narrow band of light.

Cation – See atom.

Chalky - Having the color, luster, fracture, or general appearance of chalk.

Chemical Formula, Chemical Composition - The chemical formula the standard way of stating the chemical composition of a mineral, that is, the relative numbers of atoms of each element contained in that mineral.  The formula is given as a series of element symbols, often followed by subscripts.  The subscript indicates the number of atoms of that element in the formula.  The presence of the element symbol indicates the presence of at least one atom, so subscripts of 1 are considered redundant and are omitted.
  With the exception of the native metals and carbon, minerals are usually composed of positively and negatively charged units called ions.  Many ions have only one typical charge state, e.g. oxygen is always -2 (O-2) in minerals, and sodium is always +1 (Na+1).  If an atom is capable of forming ions with more than one charge state, e.g. iron which can form both a +2 or +3 ion (Fe+2 or Fe+3), a superscript may be used to indicate the charge.  (Standard chemical notation is to use a Roman numeral in parentheses for this purpose.  Notice that only some of the ions in a formula will have their charges indicated.  However, the total of all the ionic charges in the formula must equal 0; positive and negative charges must exactly balance.  
  Parentheses in chemical formulas are used either to indicate atomic groups, e.g. the carbonate group (CO3), or to indicate a solid solution relationship between two or more atoms, e.g. (Fe, Mg).

Chromate - Compound containing the chromate group (CrO4).

 Cleavable- Able to be easily split into smaller fragments, especially along cleavages.  The term is usually used in reference to massive intergrowths of a mineral.

Cleavage, Parting - Cleavage and parting are ways in which certain minerals to break along flat surfaces.  The tendency to cleave is a characteristic feature of all crystals of a particular mineral species.  Cleavage occurs parallel to planes in the atomic structure that correspond to relatively weak bonds between atoms.  For example, a mineral is likely to cleave along  a plane which leaves equal numbers of + and - charges on the two new  surfaces, but it unlikely to cleave along a plane which leaves all +  charges on one and all - charges on the other.  If the bonds in the   structure are of nearly equal strength in all directions, the mineral  will fracture, but not cleave.
  Parting differs from cleavage in that parting is only developed in certain samples of a mineral in response to twinning or applied pressure.  The planes along which cleavage and parting occur are designated by Miller indices.  The perfection with which cleavage and parting are seen in a mineral is usually described with the following terms:    perfect, imperfect, distinct, indistinct, excellent, good, fair, and poor                                         

 

Coarse-grained - Consisting of an aggregate of relatively large crystals or grains.

Cockscomb- Intergrowth of long, slightly offset crystals in semi-circular fans.

Cogwheel Twin - Penetration twin with a circular pattern of protruding crystal edges resembling the teeth of a gear or cogwheel.
 
 
Colloform - Crystalline intergrowth with smooth, rounded surfaces.

 Colloidal- Aggregates of extremely small grains thought to have formed from a gel.

 Color, Streak -Color is one of the most obvious of mineral properties, but as a guide to recognition it must be used with caution.  Some minerals always exhibit the same color, while others can be found in many different colors.  Trace color impurities can produce dark color when viewed through a large thickness of a mineral, thereby masking the true color of the mineral.  
 By scraping the mineral against a piece of white unglazed porcelain, the streak of the finely powdered mineral is obtained.  The color of the streak can be much more distinctive of the mineral
Light absorption related to electrons within minerals causes most of the colors we see in minerals.  Such electrons may be associated with transition elements, charge transfer interactions, or color centers.  Transition elements, such as iron, copper and manganese, contain electrons that can interact with visible light, absorbing certain wavelengths.  A mineral that always contains a certain transition element as an essential constituent will always have color imparted by that element, although the exact color is also dependent on the neighbors of the transition element (e.g. all iron -containing minerals will be colored, but not all will be the same color).  A mineral that in pure form is colorless may exhibit any of a variety of colors if it contains small amounts of various transition elements.  Some very intense colors in minerals are the result of light absorption by electrons that jump between atoms in the mineral.  An example is the blue color of the sapphire variety of corundum which results from an electron jumping between Fe+2 and Ti+4 in a process called charge transfer.  Irradiation by natural or artificial radioactive sources can impart color to minerals by knocking atoms or electrons out of position.  The remaining vacancies, called color centers, result in the absorption of light.

Columnar - Subparallel intergrowth of prismatic crystals.

Compact - Dense, close-packed crystalline texture in which individual crystals cannot be distinguished without magnification.   Complex - Group of tightly bonded atoms behaving as unit.

Conchoidal - Manner in which certain minerals fracture along smoothly curved surfaces. 

 

Concretion -A hard, rounded mineral mass that usually forms in sedimentary rocks surrounding a fragment of organic material.  In a general sense, any spherical crystal aggregate.

Contact Twin - Twin in which the crystals meet along a well-defined plane (composition plane).

Cross-fiber - Consisting of parallel fibers oriented at right (90º) angles to the walls of a vein.  See asbestiform.

Cruciform Twin - Penetration twin consisting of two prismatic crystals assuming the configuration of a cross.

Crust, Incrustation - A coating of minerals formed on a surface.

Cryptocrystalline - Consisting of crystals too small to see under an ordinary microscope.

Crystal, Crystalline - When people talk about crystals, they usually are referring to solid pieces of matter that are bounded by regularly arranged flat faces.  Crystal faces result as the solid grows by adding atoms in a completely orderly, repetitive, 3-dimensional array called its atomic structure.  Scientists call a substance crystalline if it possesses an atomic structure, whether it exhibits external faces or not.  A crystal formed as the result of geological processes is a mineral.

Crystal class – See symmetry.  

Crystal Face, Crystal Form -The flat exterior surfaces of a crystal are called crystal faces.  Crystal faces are oriented parallel to planes of atoms in the crystal’s atomic structure.  The symmetry that relates the atoms within the crystal also relates the faces on the crystal.  A crystal form is a group of crystal faces that are equivalent to one another because they are related by the crystal’s symmetry.  Crystal faces and forms can be designated by numerical notations known as Miller indices.  By studying the geometrical relationships between crystal faces, one can determine the symmetry of the crystal and its crystal class.

Crystal Growth - A mineral crystal begins to form when the proper atoms arrange themselves in the pattern that defines the atomic structure of that mineral.  Crystal growth continues as the same kinds of atoms bond themselves to the original cluster following the same pattern.  Crystal growth may take place from molten rock, from a water solution, or from a gas.  The forces that attract the atoms to the surface of a growing crystal usually favor the formation of flat faces that parallel certain planes of atoms in the atomic structure.  Depending  upon natural conditions during growth, atoms will be attracted to one  plane more than another, causing growth to occur more rapidly in that  direction.  The faces that we observe on crystals actually represent the atomic planes to which atoms have been added most slowly.
 
Crystal Habit- Crystal habit refers to the general shape of a crystal.  The crystal habit is determined both by the atomic structure of the crystal and by the environment in which the crystal grows.  Because of variability in the growth environment, natural crystals rarely grow in ideal geometric shapes.  Nevertheless the angular relationships between crystal faces will always provide evidence of the symmetrical relationships between crystal faces.  Some of the terms used to describe crystal habits are:
        Acicular: needle-like 

        Fibrous, capillary, filiform: very thin; hair-like        

        Prismatic: longer than wide, surrounded by parallel faces; like a column 

        Bladed, lath-shaped: flat and long; like a knife blade

        Platy, lamellar: in very thin sheets

        Tabular: like a tablet of paper 

        Equant: nearly equal in all three dimensions                         

 

Crystal system –  See symmetry.

 
Crystallite- Minute incompletely crystalline particle.

Cube – A three-dimensional shape bounded by 6 equivalent, square faces at 90o to each other.

Cubic – The three crystallographic axes are all equal in length and intersect at right angles (90 degrees) to each other.  

Cuboidal - Similar to a cube in shape.

Cubo-octahedron - A crystal shape which combines the shape of a cube and the shape of an octahedron.

Cuneiform -Wedge-shaped.

Cyclic Twin -Repeated twin in which the crystals are related by two or more symmetry equivalent planes or axes ideally resulting in a complete circular array, such as a disk, ring, or star-like group.   Cyclo- Applied to minerals containing anion groups linked into rings (i.e. cyclosilicate)

Cyclosilicate - Mineral characterized by rings of silicate groups: 3-membered rings (Si3O9), 4-membered rings (Si4O12), 6-membered rings (Si6O18).

 Dauphiné Law Twin - Type of penetration twin in quartz in which two right-handed (or two left-handed) crystals are rotated by 60º to one another about [0001] axis.  The twin regions are separated by irregular internal boundaries, which can be observed on the crystal surface as interruptions in the horizontal striations on the prism faces.

Deliquescent - Dissolves and become liquid by absorbing moisture from the air.                                         

 

Deltoid - Used to refer to a four-sided polygon with two adjacent and equal long sides and two adjacent and equal short sides, which consequently is somewhat similar in appearance to a triangle (the Greek letter "delta").

 Dendritic - Intergrowth of crystals in the shape of slender divergent branches like a plant.                         

 

Density (D), Specific Gravity (S.G.) - The same volume of two different minerals will generally not weigh the same.  This is because the minerals contain different atoms and/or because their atoms are packed together more or less closely.  Density is a measure of the weight of a given volume of a material, usually expressed in grams per cubic centimeter (gm/cm3).
 Commonly we report the ratios of mineral densities to the density of water which is 1 gm/cm3 at 4°C.  Such relative measurements give the specific gravity, which is essentially the same numerically as the density, but is a dimensionless number.
  One way to measure a mineral’s density is to weigh it suspended by a thread in air and then to weigh it immersed in water.  It will weigh less in water by an amount equal to the weight of the water that its volume displaces.  The volume of the mineral is equal to the difference between the two weights because each cubic centimeter of displaced water weighs 1 gram.  Finally divide the weight of the mineral by its volume to obtain the density.

 Devitrified - Changed from glass to crystalline.

 Dichroism, Pleochroism, Trichroism - Pleochroism is an optical property observed in the crystals of certain minerals in which light is absorbed differently as it passes through the crystals in different directions.  Differences in the atomic structure of a crystal in different directions account for the differential light absorption.  Three distinct colors (trichroism) or two distinct colors (dichroism) may be seen as a crystal is held in front of a light and turned.  Most pleochroic mineral crystals exhibit only small differences in color intensity as they are turned.

 Dihexagonal - Literally double-hexagonal, this indicates a form in which the faces are related by a 6-fold rotational axis combined with parallel mirror planes and/or perpendicular 2-fold axes.

 Dimorphism, Dimorphous - Two minerals that have the same chemical compositions but different atomic structures.  See polymorphism.

 Dioctahedral - Type of layered atomic structure in which only two of three possible octahedrally coordinated sites are occupied by cations.  An octahedrally-coordinated site is a position in the structure in which a cation can form bonds to six anions.  The anions can be thought of as positioned at the corners of an octahedron.  See also   trioctahedral.

 Diploid - A crystal form consisting of 24 nonparallel faces related by the combined symmetry of the 2/m B3 (diploidal) crystal class.

 Dipyramid, dipyramidal - crystal form consisting of two identical pyramids joined base to base.   

                                      

Discredited - Term used for once accepted mineral specie that has been determined not to meet the requirements of a mineral species.  A mineral species is often discredited by proving that it actually corresponds to another known mineral species or that it is a mixture of two or more known mineral species.                                       

 

Dispersion - The color of light depends upon its wavelength.  Normal white light contains a mix of all visible wavelengths and includes red, orange, yellow, green, blue, and violet (from longer to shorter wavelength).  When light enters a transparent solid, it changes angle or is refracted.  Longer wavelengths of light are refracted more than shorter wavelengths, so the material is able to separate white light into its component colors.  This phenomenon is called dispersion.  Minerals differ in their ability to create dispersion.  Diamond produces strong dispersion, which is the reason that one is able to see distinct  flashes of color in an otherwise colorless diamond gem.                                            

 

Disphenoid- A crystal form consisting of four nonparallel faces related either by three 2-fold rotational axes or by one 4-fold rotational-inversion axis.                                          

 

Disseminated - Scattered as small particles throughout a rock.                                           

 

Ditetragonal - Literally double-tetragonal, this indicates a form in which the faces are related by a 4-fold rotational axis combined with parallel mirror planes and/or perpendicular 2-fold axes.                                        

 

Ditrigonal - Literally double-trigonal, this indicates a form in which the faces are related by a 3-fold rotational axis combined with parallel mirror planes and/or perpendicular 2-fold axes.

 Divalent - Cation having a charge (valence) of 2.

Divergent -Intergrowth in which crystals radiate from a common center.                                           

 

Dodecahedron – A three-dimensional shape bounded by 10 equivalent faces.

 Dome - A crystal form consisting of two nonparallel faces related by mirror symmetry.

 Double Refraction - When light enters or leaves a transparent crystal, it is refracted (bent).  If the crystal is of low enough symmetry, light traveling along different axes of the unit cell is refracted to different angles.  An image viewed through such a crystal appears to come from two sources, it is doubled.                                       

Doublet Twin - Two crystals intergrown in a twin relationship

Druse, Drusy - Intergrowth of small projecting crystals that line the walls of a cavity in rock.  Usually only the terminations of the crystals are visible.

 Ductile - Able to be drawn into a wire without breaking.  See tenacity.  

  Dull - Lowest mineral luster typified by no reflectance; light disperses in all directions from rough granular surface.