Compiled from various sources by Richard Graeme


Earthy - Used to describe a dull luster or a slightly rough fracture like that of hard clay.                                   


Effervescence, Effervescent - Giving off small bubbles.  Carbonate minerals usually effervesce by giving off carbon dioxide bubbles when they are expos

ed to acid.                                  


Efflorescence, Efflorescent - Formation of a fluffy or powdery crystalline coating on the surface of a rock or mineral that occurs as the result of loss of water from the mineral.


Eightling Twin - Group of eight crystals intergrown in a twin relationship.                                              


Elastic -   Able to return to original shape when released after bending.  See tenacity.
Element Symbols - The one and two letter abbreviations for the elements used in chemical formulas.  The first letter is always upper case; the second is always lower case, if one is part of the abbreviation.  For example:
                         Ag = silver                         Al = aluminum                         As = arsenic   The symbol is an abbreviation of the element name, but the element  name used in defining the symbol may be in English, Latin, German,  or French, so that information is not always helpful.

Epitactic, Epitaxy - The growth of a crystal of one mineral on the surface of a crystal of a different mineral in a definite orientation determined by the atomic structures of the two minerals.  Compare twinning.

Equant, Equidimensional -   Habit in which a crystal is about the same size in all directions.                                        


Equigranular - Consisting of grains of roughly equal size.                                            


Euhedral - Crystal completely surrounded by well-formed faces.                                          


Feldspathoid - Group of minerals chemically related to the feldspars, but containing relatively less silicon.                         


Felted, Felt-like - Intergrowth consisting of interwoven fibers.                                            


Fibroid - Intergrowth resembling fibrous tissue.                                         


Fibrolamellar -Intergrowth consisting of layers of flattened fibers.                           


Fibrous, Filiform - Very thin, hair-like crystal habit.  Also an aggregate of fibers.

Fine-grained- Consisting of an aggregate of relatively small crystals or grains.  

Fishtail Twin, Swallow-tail Twin - Contact twin that results in a v-shaped termination similar in  appearance to the tail of a swallow or fish.

Fiveling Twin - Group of five crystals intergrown in a twin relationship.  


Flexible- Bends without breaking but does not return to its original shape.  See tenacity.

Fluorescence, Phosphorescence - When some minerals are subjected to invisible ultraviolet (UV) radiation, they emit visible light, seeming to "glow-in-the-dark."  This effect, called fluorescence, was named after the mineral fluorite, which often spectacularly displays the effect.  Relatively few mineral species exhibit fluorescence, and those that do are usually not invariably fluorescent; they often require a certain impurity ion which serves as an activator.  Some minerals may fluoresce under short- wave (SW) ultraviolet radiation, but not under long-wave (LW), or vice-versa.  Many different fluorescent colors have been observed in minerals, and the same mineral species may exhibit different fluorescent colors depending on its chemical impurities.
 If the emission continues after the ultraviolet radiation has been turned off, the mineral is said to be phosphorescent.

Foliated, Lamellae, Lamellar, Micaceous - Intergrowth consisting of thin, leaf-like layers.

Fourling - Group of four crystals intergrown in a twin relationship.                                             


Fracture - The manner in which a mineral breaks along relatively irregular or non-flat surfaces.  The following terms are used to describe fracture:

Conchoidal: smoothly curved surfaces     

Subconchoidal: more irregular curved surfaces    

Hackly: jagged surface like that of broken metal     

Uneven, irregular: more or less flat surfaces with some roughness     

Even, regular: relatively flat surface, but not cleavage     

Splintery, fibrous: surface composed of slender fragments or fibers     

Earthy: slightly rough, irregular surface like that of hard clay                                             


Friable - Crumbly; easily broken or pulverized.   


Front Pinacoid - A crystal form consisting of the two parallel faces, (100) and (-100), which transect the axis.                                            


Fusible -Capable of being melted by heating.                                      


Geniculated Twin - Repeated contact twin consisting of prismatic crystals in a back-and- forth configuration yielding knee-shaped forms.                                              


Glass - An amorphous substance that is actually a highly viscous (slow- flowing) liquid.  Globular, Nodular - Crystalline intergrowth with smooth, rounded surfaces.                                          


Granular - Consisting of grains of roughly equal size.

Greasy - Poorly reflective mineral luster; similar in appearance to the surface of grease.                                        


Gyroid - A crystal form consisting of 24 nonparallel faces related by the combined symmetry of the 432 (gyroidal) crystal class.    


Hackly - Type of mineral fracture that produces a jagged surface.                                            


Halide - Compound of a metal with a halogen (F, Cl, Br, I).                                           


Hardness (H) - Hardness is the resistance of a mineral to scratching.  This property is indicative of the strength of crystals because it requires the breaking of bonds between atoms.  Hardness varies greatly from species to species and it can, therefore, be a very useful property for identification.  The most common way of determining a mineral’s hardness is to try to make a scratch on it using another material whose hardness is known.  If you are able to make the scratch, then the mineral has a hardness less than that of the material used to scratch it.  The test may, of course, be reversed by trying to scratch the material of known hardness with the mineral.  
 The German mineralogist Friedrich Mohs (1773-1839) developed a scale of hardness based upon 10 common minerals:
      (1) talc

      (2) gypsum

      (3) calcite 

     (4) fluorite 

     (5) apatite
     (6) orthoclase

      (7) quartz

      (8) topaz 

     (9) corundum

     (10) diamond


By successively testing the unknown mineral with each of the minerals in the list, you can determine its Mohs hardness.  For example, a mineral of hardness 6 ½ can be scratched by quartz, topaz, corundum and diamond, but not by orthoclase or any of the minerals of lesser hardness.  Common non-mineral materials are often useful for hardness testing: fingernail (2½), copper penny (3), pocket knife (5-5½), window glass (5½), and steel file (6½).  A few cautions are in order.  The best results are obtained by using sharp corners (points) to scratch smooth even surfaces.  The difference in hardness between successive minerals in the Mohs scale is not equal.  In some minerals hardness varies with direction.  Intergrowths of small crystals may appear harder or softer than the surfaces on individual crystals of the same mineral.

 Heart-shaped Twin - Simple contact twin consisting of two wedge-shaped individuals, exhibiting a prominent reentrant angle, and having an overall shape resembling a heart.                                            


Hemihedral - Exhibiting only half of the faces corresponding to the maximum symmetry possible for that crystal system.

Hemimorphic - Lacking a center of symmetry such that identical faces are not found on directly opposite sides of crystals.   


Hemispherical - In the shape of half a sphere; usually in reference to a radial intergrowth of crystals.

Hexagonal - Crystal system characterized by one 3-fold or 6-fold symmetry axis.

Hexahedron - A three-dimensional shape bounded by 6 equivalent faces.                          


Hexoctahedron - A three-dimensional shape bounded by 48 triangular faces.  The trapezohedron is a crystal form in the isometric crystal system.

Holohedral - Exhibiting all of the faces corresponding to the maximum symmetry possible for that crystal system. 


Hopper - Crystal habit in which the growth of crystal faces has been quicker along their edges than at their centers, so that the centers of the faces are depressed.           


Hydrate - Compound containing water (H2O).                                            


Hydrocarbon - Organic compound composed entirely of carbon and hydrogen.                                             


Hydrous - Mineral with water (H2O) in its formula.                                             


Hydroxide - Compound containing the hydroxyl group (OH).                                         


Hygroscopic - Readily absorbing moisture, as from the atmosphere.                                          


Idiomorphic - As individual euhedral crystals.                                           


Igneous - Pertaining to rocks formed by solidification from molten material.                                            


Imide - Organic compound containing nitrogen with two attached carbonyl groups (CO).                                            


Inclusion - A foreign body within a crystal.  Inclusions usually represent material trapped during crystal growth.  They commonly are earlier-formed crystals of other minerals or remnants of the fluid from which the crystal grew.
 Incrustation, Crust - A coating of minerals formed on a surface.   


Index of Refraction, Double Refraction, Birefringence- When a beam of light strikes the surface of a transparent material at an angle, part will be reflected away and part will penetrate the material.  The part of the beam that enters the material will be bent or refracted by an amount related both to the angle at which the beam strikes the material (the angle of incidence), to the density of the material, and to the light absorbing properties of the material.
 In general, the denser a material, the more the light entering it will be bent, but because additional factors affect the bending, this determination is not the same as a measurement of the density.  By measuring the angles of incidence and refraction, a quantity called the index of refraction can be determined.  This index can be used as an identifying characteristic for the material.   The atomic structures of many mineral crystals are quite different in different directions.  A light beam entering such a crystal will be split into two beams - each bent at a different angle.  This is called double refraction or birefringence.  An image viewed through a birefringent crystal will appear to be two images.                                                


Ino- Applied to minerals containing anion groups linked into chains  (i.e. inosilicate)                                           


Inosilicate - Mineral characterized by chains of silicate groups: single chains (Si2O6), double chains (Si4O11).                                   


Intergrowth, Aggregate - Most natural crystals form as intergrowths or aggregates.  Where crystals come in contact with one another as they are growing, they form irregular contacts rather than regular crystal faces.  The term massive refers to intergrowths in which the mineral crystals have grown in contact with other crystals on all sides leaving no empty space in between.  Most rocks are massive intergrowths of one or more different minerals.  Many minerals grow in distinctively shaped aggregates.  Crystal faces are sometimes present at the periphery of such an aggregate.  Some of the terms used to describe crystal aggregates are:
     Drusy: surface covered with a layer of intergrown crystals with terminal faces

     Fibrous, felted, asbestiform: aggregate of slender fibers

     Botryoidal, colloform, mammillary, globular, reniform: large rounded masses

     Radiate, divergent: crystals radiating from a common center

     Lamellar, foliated: thin plates stacked together

     Bladed: blades stacked together

     Dendritic: arborescent: in slender divergent branches like a plant 

    Cockscomb: long, slightly offset crystals in semi-circular fans 

    Rosette: numerous platy crystals overlapping like flower petals

Intermetallic - Refers to a compound composed of two or more metals in definite proportions.  (An alloy is a mixture of two or more metals in variable proportions.)                                          


Interstitial - Between grains or in pores in rock.                                            


Iodate - Compound containing the iodate group (IO3).

Ion – See atom.                            


Iridescent - Exhibiting rainbow-like colors in the interior or on the surface of a mineral.                          


Iron Cross Twin - Twin consisting of two penetrating pyritohedra related by rotation about the [110] axis; typical of pyrite.                                           


Isomorphic, Isomorphism - Two different minerals that possess the same atomic structure are called isomorphs.  The minerals may be related chemically through the replacement of the atoms of one element for those of another.  An example is the olivine series consisting of the minerals fayalite, Fe2SiO4, and  forsterite, Mg2SiO4.  Isomorphic minerals may have nothing in common chemically.  For example halite, NaCl, and galena, PbS, have identical atomic structures. 


Isostructural -Having the same atomic structure (but different chemical composition.

 Japanese Law Twin - Type of contact twin on the {11-22} plane of quartz.  The two crystals are at a nearly right angle (84º33') forming a "V.”  Both crystals are usually flattened parallel to their front-facing prism faces.         


Lamellae, Lamellar, Foliated, Micaceous - Intergrowth consisting of thin, leaf-like layers.

Lamellar Twin - Polysynthetic twin in which the individual crystals are thin plates.                               


Lath-like - Habit of a crystal that is flat and long; like a knife blade.

Left handed, Right-handed - Crystals in those crystal classes with no center of symmetry or mirror planes can occur in right-handed or left-handed variations.  These variations are identical except that they are mirror images of one another in the same way that your right and left hands are related.  In the same way, certain crystal forms in these crystal classes can be termed right (-handed) or left (-handed).

Lenticular - Habit of a crystal that resembles a double-convex lens in  cross-section.                                                                                                            


Lozenge - Rhombus or diamond (as in one of the four suits in a deck of cards).

Luminescence -   Emission of light, often colored as a response to UV light, X-ray or other source.                                      


Luster, Sheen - Luster refers to the appearance of a mineral surface as a result of light reflecting from it.  To a trained eye luster can be one of the most important clues for the sight-identification of minerals.  Minerals with higher indicies of refraction reflect more of the light that strikes them and, therefore, have a higher or brighter luster.  The following terms are often used to describe luster:
      Dull, earthy: no reflectance; light disperses in all directions from rough granular surface

      Waxy: slightly reflective; typical of minutely granular surfaces

      Greasy, oily: poorly reflective; similar in appearance to the  surface of grease or oil

      Vitreous: brightly reflective; similar to the luster of freshly  broken glass

      Resinous: very reflective; like the luster of broken resin or shellac

      Adamantine: hard brilliant luster; like that of a diamond 

     Submetallic: silvery or nearly metallic luster

      Metallic: brilliant, highly reflective luster of metals; material is always opaque
 Characteristics just below a mineral's surface can affect the way it reflects light and give the mineral an unusual sheen.  Examples include:
      Pearly: caused by tiny partly-developed cleavages parallel to the surface

      Silky: caused by reflections from a fibrous growth structure or from parallel hair-like inclusions                                          


Magnetism - Very few minerals are noticeably attracted to a magnet.  Certain elements, most notably iron, have electrons that tend to align themselves in the same direction in a magnetic field.  When the atomic structure of an iron-rich mineral allows most of its iron atoms to align their electron fields in the same direction, it will be attracted by a magnet.  Under certain conditions, the iron nuclei can be locked in place, thereby producing a magnet.  Magnetite, an iron oxide, is attracted to magnets and can itself be naturally magnetized.  Many meteorites are attracted by magnets because they contain native iron.                                             


Malleable - Able to be hammered into thin sheets without breaking.  See tenacity.

Mammillary – Crystalline intergrowth with smooth, rounded surfaces.                                


Manebach Law Twin - Type of simple contact twin on the {001} plane exhibited by orthoclase, microcline and other members of the feldspar group.                                        


Massive - Uniform intergrowth of crystals.                                             


Mealy - Resembling meal in texture or consistency; granular.                                         


Metal - Element whose atoms form metallic bonds with one another.  Metals tend to lose electrons to form positively charged ions (cations).

Metallic - Brilliant, highly reflective luster of metals.                                         


Metalloid - Non-metallic element that exhibits some metallic characteristics.                                              

 Metamict - Amorphous as the result of radiation damage.

Metamorphic - Pertaining to rocks that have been significantly modified by heat and pressure, for the most part without melting.   


Micaceous, Lamellae, Lamellar, Foliated - Intergrowth consisting of thin, leaf-like layers.                     


Microcrystalline - Made up of crystals so small that they can only be seen with a microscope.                                   


Micron, Micrometer, µm - One-millionth of a meter (0.000001 m); one-thousandth of a millimeter (0.001 mm).                                 


Miller Indices - Three integers (sometimes four in the hexagonal crystal system) used to indicate the orientation of a plane or direction in a crystal such as those corresponding to a crystal face or cleavage.  The three numbers are related to the three (or four) axes that define the unit cell.  The three numbers are enclosed in parentheses, as (111), to indicate a single face or plane.  They are enclosed in braces, as {111}, to indicate a crystal form (set of planes related by symmetry).  They are enclosed in brackets, as [111], to indicate a direction.                                          


Mimetic - Appearing to have a higher degree of symmetry as the result of twinning.

Mineral, Mineral Species - A mineral is a chemical element or combination of chemical elements that is normally crystalline and which has formed by natural geological processes.  Being crystalline means having an atomic structure.  For the mineral to be a distinct mineral species, it must differ from every other mineral species either in its combination of chemical elements or in its atomic structure.                              


Mineral Class, Mineral Group - Minerals are usually classified according to aspects of their chemical composition and atomic structure.  Most schemes are based principally upon the major anions and anionic groups in minerals.  This leads to the following major classes:
      native elements

      sulfides and sulfosalt

      oxides and hydroxides




      sulfates and chromates

      phosphates, vanadates and arsenates

      tungstates and molybdates 

 The silicates are further divided into the following subgroups depending on different types of silicate linkages:





 These classes are then split into groups of more closely related minerals.  The numerical scheme employed in this presentation is based upon that developed by the German mineralogist Hugo Strunz.                                          


Mineral Name - Each distinct mineral species is given a distinct name.  Names are chosen in many ways.  Minerals have been named after persons, places, properties, and similarities with other minerals.                                             


Modified - Term used to describe a crystal shape or form that varies from another.  For example, a cubooctahedron is an octahedron modified by a cube.                                            


Molybdate - Compound containing the molybdate group (MoO4).

Monoclinic - Crystal system characterized by one two-fold symmetry axis and/or one mirror plane.                                             


Monovalent - Cation having a charge (valence) of 1.                                        


Morphology - Crystal shape.

Multiple Twin - Twin consisting of three or more crystals.    Myrmekitic, Myrmekite-like – Intergrowth consisting of blebs, drops, or vermicular grains of one mineral in another.  

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© 2013 by Doug Graeme