GLOSSARY OF COMMON MINERAL TERMS

N-Z

Compiled from various sources by Richard Graeme

 

Nanometer, nm - One billionth of a meter (0.000000001 m); one-millionth of a millimeter (0.000001 mm).


Negative Form, Positive Form - Terms used to distinguish symmetrically similar, but distinct forms in certain crystal classes.   

                                          
Neso- Applied to minerals containing individual isolated anion groups (i.e. nesosilicate) 

                                           

Nesosilicate - Mineral characterized by single isolated silicate groups of general formula SiO4.


Neutron – Uncharged particles which, along with protons for the nucleus of atoms. 

                              

Nitrate - Compound containing the nitrate group (NO3).  

                                            

Nitride - Compound of a metal with nitrogen.   

                                        

Nodular - In the form of or consisting of nodules (small rounded lumps).


Non-metal - Element whose atoms form covalent bonds with one another.  Non-metals (except the noble gases)

tend to gain electrons to form negatively charged ions (anions).


Nucleus – At the center of an atom is the nucleus containing positively-charged protons and uncharged neutrons.  The nucleus is surrounded by negatively-charged electrons. 

                                   

Ocherous - Resembling ochre, an earthy, pulverulent, red, yellow, or brown iron oxide (hematite).


Ocher-like –  Earthy, red, yellow or brown material.

                         

Octahedron - A three-dimensional shape bounded by eight triangular faces and having six corners.  The octahedron is a common crystal form in the isometric crystal system.   

                              

Oily - Poorly reflective mineral luster; similar in appearance to the surface of oil.                                             

 

Oolitic - Consisting of many spherical bodies.


Opalescent, Opaline - Having a milky or rather pearly sheen such as that shown by some kinds of opal.     

                                         

Opaque - Impervious to visible light even in thin fragments.  

 

                                      

Organic - Compound containing carbon.  Most naturally occurring organic compounds were formed by a living organism.  Carbonates are not classified as organic compounds.


Orthorhombic - Crystal system characterized by three 2-fold symmetry axes.  

                                        

Ovoid - Egg-shaped   

 

Oxalate - Compound containing the oxalate group (C2O4).  

                                          

Oxide - Compound of an element with oxygen.                                                                     


Oxidize - To chemically change through contact with oxygen or oxidizing conditions typical on or near the earth's surface.  This type of alteration generally results in the formation of minerals having compositions with greater amounts of oxygen or with one or more cations in a higher charge state than in the original mineral.

 

                                           

Paramorph - Pseudomorph with the same composition as the mineral it replaces.                                         

 

Parting, Cleavage - 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


Pearly - Pearl-like sheen caused by tiny partly-developed cleavages parallel to the surface.

 

Pedion - A crystal form that consists of only one face.                          

 

Penetration Twin - Twin in which two or more crystals appear to interpenetrate each other with the surface between them being irregular or poorly defined. 

                                  

Pericline Law Twin - Type of lamellar twin about the [010] axis exhibited by the members of the plagioclase series. 

                                      

Phosphate - Compound containing the phosphate group (PO4).                                            

 

 Phosphide - Compound of a metal with phosphorous.
 Phosphorescence - See fluorescence.  

                                            

Phyllo - Applied to minerals containing anion groups linked into sheets  (i.e. phyllosilicate.) 

                                                       

Phyllosilicate - Mineral characterized by sheets of silicate groups of general formula Si4O10.   

                            

Piezoelectricity, Pyroelectricity - The atomic structures of certain minerals that lack a center of symmetry become electrically polarized when heat or pressure is applied.  When heat is involved it is called pyroelectricity.  When pressure is involved it is called piezoelectricity.  The effects are generally reversible.  For example, by applying an alternating electrical current to opposite ends of a piece of quartz, you can make it vibrate.  The piezoelectricity of quartz has proven useful for accurately tuning frequencies in radios and clocks.  The pyroelectricity of tourmaline has made it useful in devices that measure high pressures.  Tourmaline was used to measure the pressure of the first atomic bomb test.                                        

 

Pinacoid - A crystal form that consists of two parallel faces.                                            

 

Pisolitic - Composed of pea-like grains.                                            

 

Pitchy - Rather dull luster similar to that of pitchblende, a massive variety of uraninite. 

                                            

Platelet - A very small platy crystal.  

 

Platy - Very thin sheet-like crystal habit.                        

 

Pleochroism, Dichroism, 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.

Plumose – An aggregate of many small scales spreading into a formation that looks like a feathery plume.                                         

 

Poikiloblast - A large crystal of metamorphic origin that contains within it many small grains of other minerals.


Polished Section - A slice of rock that has been highly polished for viewing under the microscope.  Polished sections are usually made for studying opaque minerals using light reflected from the polished surface of the mineral grains embedded in the rock.                                          

 

Polymorphism - Two or more minerals that have the same chemical compositions but different atomic structures are called polymorphs.  Such a polymorphic relationship exists between the minerals andalusite, kyanite, and sillimanite which all have the chemical formula, Al2SiO5.  Typically, polymorphs form under differing conditions of temperature and/or pressure.                                  

 

Polysynthetic Twinning – Repeated twin in which the crystals meet along parallel planes.

 
Polytype - Mineral that differs from another only in the stacking of similar structural units in its atomic structure.


Porcelaneous - Dull white luster resembling unglazed porcelain.


Porphyroblast - Euhedral crystal formed in a metamorphic rock.


Positive Form, Negative Form - Terms used to distinguish symmetrically similar, but distinct forms in certain crystal classes.


Prism, Prismatic - Crystal habit characterized by greater length than width, with prism faces parallel to and completely surrounding the long direction of the crystal.  Also, a crystal form consisting of three or more symmetryrelated faces parallel to a common axis.


Proton – Positively charged particles, which, along with neutrons form the nucleus of atoms. 

 

Pseudo - This prefix is used in combination with many roots:
       pseudocubic

       pseudocubo-octahedron

       pseudododecahedral

       pseudohexagonal 

       pseudo-isometric

       pseudo-octahedral 

       pseudo-orthorhombic 

       pseudorhombohedral

       pseudotetragonal

       pseudotetrahedral

       pseudotrigonal


 The prefix indicates that the visible shape of the sample falsely implies that the sample has more symmetry than is indicated by the internal atomic structure of the mineral.  
 For example, the distinction between an orthorhombic crystal and a lower symmetry monoclinic crystal is the value of one angle in the unit cell.  If the angle is 90°, then the mineral is orthorhombic; any other value makes the mineral monoclinic.  However, the angle in a monoclinic mineral may be so close to 90° that the mineral appears to be orthorhombic.  Such a mineral would be termed pseudo-orthorhombic.                                               

 

Pseudomorph - Any given mineral will form only within a specific range of conditions of heat, pressure and chemical composition.  When the mineral’s environment changes, the mineral will often change or alter to a different mineral.  Even though the new mineral would normally form in its own distinctive crystal shape, occasionally the new mineral not only takes the place of the original, but also retains its external crystal shape.  The new mineral is then said to be a pseudomorph (false form) of the original.                                   

Pseudo-Spinel Law Twin - Twin relationship closely resembling the Spinel law, but relating crystals not in the 4/m B3 2/m crystal class.                                            

 

Pulverulent - Easily powdered.                       

 

Pyramid, Pyramidal – A crystal form consisting of 3, 4, 6, 8, or 12 on parallel faces meeting at a point. 

 

Pyritohedron - A dodecahedron (three-dimensional shape bounded by 12 faces) with five-sided faces.  The pyritohedron is so named because crystals of pyrite sometimes take this shape.  It is a crystal form in the isometric crystal system.  

 

Pyroelectricity, Piezoelectricity - The atomic structures of certain minerals that lack a center of symmetry become electrically polarized when heat or pressure is applied.  When heat is involved it is called pyroelectricity.  When pressure is involved it is called piezoelectricity.  The effects are generally reversible.  For example, by applying an alternating electrical current to opposite ends of a piece of quartz, you can make it vibrate.  The piezoelectricity of quartz has proven useful for accurately tuning frequencies in radios and clocks.  The pyroelectricity of tourmaline has made it useful in devices that measure high pressures.  Tourmaline was used to measure the pressure of the first atomic bomb test.                                       

 

Quatravalent - Cation having a charge (valence) of 4.                        

 

Radial-fibrous - Intergrowth in which fibrous crystals radiate from a common center.


 Radiate, Radial - Intergrowth in which crystals radiate from a common center.


 Radioactivity - Some kinds of atoms have nuclei that are unstable and break apart to form atoms of other elements.  In the process of breaking apart these atoms release radiation in the form of alpha and beta particles and gamma rays.  The most common radioactive elements in minerals are uranium and thorium.  Minerals rich in these elements will be very radioactive, causing a Geiger counter to react strongly.  Minerals containing small amounts of these elements are only slightly radioactive.
 While the number of protons in an atomic nucleus determines what element it is, the number of neutrons in the nucleus is somewhat variable for a given element.  Uranium nuclei always have 92 protons (atomic number = 92) and they most commonly have 146 neutrons, giving them an atomic weight of 92 + 146 = 238.  More rarely uranium nuclei in minerals have only 143 neutrons yielding an atomic weight of 235.  These are the different isotopes U238 and U235.  They breakdown at different rates, known as half lives, and therefore give off radiation at different rates.  Some other elements whose most common isotopes are not radioactive also exist as in the form of rarer radioactive isotopes.


 Re-entrant Angle - V-shaped depression marking the junction between two twinned crystals.

 

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


 Repeated Twin, Multiple Twin - Twin consisting of three or more crystals.


Resinous - Very reflective luster; like the luster of broken resin or shellac. 

 

Resorbed - Dissolved.


Reticulated – Intergrowth pattern consisting of criss-crossed slender crystals; often indicative of a twin relationship.


 Rhomb - A three-dimensional shape that can be thought of as a cube stretched or compressed in the direction of two opposite corners 

                                
Rhombic - Having the shape of a rhombus (like the diamond in a deck of playing cards). 

 

 

Rhombohedron -.  The rhombohedron is a common crystal form in the hexagonal crystal system.  

 

Right-handed, Left-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).  

 

Rock - An aggregate of minerals.  Most rocks fall into three general categories depending on processes by which they formed:
     Igneous: rocks formed by solidification from molten material.    

     Sedimentary: rocks formed by the deposition of sediments.    

     Metamorphic: rocks that have been significantly modified by heat and pressure, for the most part

     without  melting.


Rosette - Intergrowth consisting of numerous platy crystals overlapping like flower petals.


Saddle-shaped – Crystal habit with curved faces and edges so as to resemble a saddle.


Salt - Compound that results when an acid reacts with a base.  Note that "table salt" is sodium chloride (NaCl), the mineral halite.  NaCl results from the reaction between hydrochloric acid (HCl) and the base, sodium hydroxide (Noah); HCl + NaOH -> NaCl + H2O 

                                    
Scalenohedron
- A crystal form consisting of symmetry-related faces that are scalene triangles (three unequal sides).  The tetragonal scalenohedron has 8 faces related by the combined symmetry of the B42m (tetragonal scalenohedral) crystal class.  The trigonal scalenohedron has 12 faces related by the combined symmetry of the B32m (trigonal scalenohedral) crystal class.


Scepter - Unusual prismatic crystal habit in which the diameter of the prism is greater near the termination of the crystal than near its base.  

                                  

Schiller - Optical effect in which flashes of bright colors appear as the mineral is turned.  Schiller is caused by the diffraction of light from closely spaced layers.                                           

 

Schistose - Layered texture of an aggregate of fibrous or platy mineral grains.      

                                       

Sectile - Can be cut into thin shavings with a knife.  See tenacity.                                            

 

Sedimentary - Pertaining to rocks formed by the deposition of sediments.                                      

 

Selenite - Compound containing the selenite group (SeO3).                                             

 

Semi-metal - Element whose atoms form bonds with one another that are intermediate between metallic bonds and covalent bonds.


Semimetallic, Submetallic - Luster somewhat less than metallic.


 Series – See solid solution


 S. G. (Specific Gravity) – See density


 Sheaf - Bundled intergrowth of fibers.

 


Sheen – See luster.                                      

 

Side Pinacoid - A crystal form consisting of the two parallel faces, (010) and (0-10), which transect the b axis.                                              

 

Silicate - Compound containing the silicate group (SiO4) or linked silicate groups.                                            

 

Silicide - Compound of a metal with silicon.


Silky - Silk-like sheen caused by reflections from a fibrous growth structure or from parallel hair-like inclusions.                                  

 

Simple Twin – Twin composed of only two crystals.
 

Sixling - Group of six crystals intergrown in a twin relationship.                                             

 

Slag - Residual material from the refinement of a metallic ore.


Slickensided - Having a polished and smoothly striated surface usually the result of movement along the surface.                                                   

 

Solid Solution, Series, Substitution - The chemical formula given for each mineral provides the atomic components in the pure mineral.  In fact, natural minerals almost always contain atoms of some other elements in partial substitution for those provided in the formula.  Certain ions, such as Fe+2 and Mg+2, are similar enough that they can take each other’s place in any proportion in an atomic structure.  The minerals fayalite, Fe2SiO4, and forsterite, Mg2SiO4, have the same atomic structure and any composition intermediate between them is possible.  This is called a solid solution series (or simply a mineral series), and such a substitutional relationship can be indicated by providing the formula as, (Fe,Mg)2SiO4.  The name applied to the mineral compound depends upon which element is present in greatest amount.  A separate name may be applied to the series.  For example, fayalite and forsterite belong to the olivine series.                                            

 

Soro- Applied to minerals containing finite linkages of anion groups (i.e. sorosilicate)   

 

 Sorosilicate - Mineral characterized by finite linkages of silicate groups: double groups (Si2O7), triple groups (Si3O8), etc.     Specific Gravity (S. G.) – See density                                  

 

Sphenoid - A crystal form consisting of two nonparallel faces related by 2-fold rotational symmetry.


 Spherulitic - As spherical intergrowths of crystals.


 Spinel Law Twin - Type of twinning occurring in octahedral crystals of spinel and many other minerals of the isometric system.  The octahedron can be thought of as cut through its center along a plane parallel to a set of octahedral faces and then one of the halves of the octahedron rotated by 60° relative to the other.                                  
 Splendent – A luster of the highest intensity.


 Stalactitic - As concentrically layered intergrowths with conical or cylindrical shapes; similar to stalactites.


 Stellate - Star-like intergrowth of crystals, often resulting from cyclic twinning.


 Streak - 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 than the specimen color.  This only works for minerals softer than the ceramic tile used ~ 7.  See also color.


 Striated - Marked by a series of parallel straight lines (striations).  On crystals striations generally represent the oscillation between two crystal forms.   

                     

Strunz Classification, Strunz Number - Scientists have developed various schemes for classifying minerals in order to better visualize and understand the relationships between them.  One of the most popular modern schemes, developed by Professor Hugo Strunz, is based upon a combination of chemical composition and atomic structure.
 The Strunz number is composed of four parts.  The first three parts correspond to the class, group, and subgroup.  In the complete Strunz system, the 4th part of the number is a three-digit integer specific to an individual mineral species.
  For example, in the full Strunz classification, almandine is assigned the number 9.AC.110, which can be decoded as follows:                   9.        Silicate class

                                A       Nesosilicate group

                                C.     Garnet subgroup                           

                                110  Almandine


 Subadamantine - Luster somewhat less than adamantine.

 

Subconchoidal - Fracture along surfaces that are not quite as smoothly curving as for conchoidal fracture. 

 

Subhedral - Crystal partially bounded by well-formed faces.


 Submetallic – Luster somewhat less than metallic.


Subparallel - Not quite parallel, usually used in reference to intergrowths of prismatic crystals.

 
Subresinous - Luster somewhat less than resinous.

 
Substitution –  See solid solution.

 
Subvitreous - Luster somewhat less than vitreous.


Sulfarsenates - Compounds based on AsS4 groups.


Sulfarsenites - Compounds based on AsS3 groups.  Sulfantimonites (SbS3) and sulfbismuthinites (BiS3) are closely related compounds.     Sulfate - Compound containing the sulfate group (SO4).


Sulfide - Compound of a metal or semi-metal with sulfur.

            Sulfite - Compound containing the sulfite group (SO3).


Sulfosalt -Compound of both a metal and semi-metal with sulfur.


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

 

Symmetry, Crystal Class, Crystal System - Hold up your hands side-by-side in front of your face with your fingers spread.  At least in an approximate way, your hands are identical, except that they are flipped as though one were the reflection of the other in a mirror.  This is a type of symmetry called mirror symmetry.  Objects can exhibit rotational symmetry so that rotation around a symmetry axis repeats the object two or more times in each full rotation.  If the object assumes the same appearance four times (every 90°) as it is rotated a full 360°, it is said to have four-fold symmetry.  A center of symmetry is present if every point on an object has a corresponding identical point on the opposite side of the object the same distance from its center.
 Symmetry is an important property of mineral crystals.  The symmetry we see in the external shape of a crystal results from the symmetry of the mineral’s atomic structure.  The symmetry of the crystal may not be obvious because of irregular growth; however, the angles between the crystal’s faces will always be related by the true symmetry of the mineral.  Careful study, sometimes along with angular measurements, may be necessary to determine the symmetry of a crystal.  Recognizing the symmetry of a crystal will be a big help in identifying the mineral.
 There are ten kinds of symmetry (symmetry elements) that can be seen in the external form of crystals:
           1: no symmetry; identity

           2: two-fold rotation

           3: three-fold rotation

           4: four-fold rotation 

           6: six-fold rotation

           B1: center of symmetry

           m: mirror plane

           B3: three-fold rotation with inversion

           B4: four-fold rotation with inversion 

           B6: six-fold rotation with inversion


 There are only 32 possible ways in which the various symmetry elements can be combined.  These are called the classes, and they can in turn be organized into six crystal systems.  The symbols used to represent the symmetry
elements are combined to represent each of the crystal classes.  The rules for interpreting these combined symbols are:  
    (1) if a mirror plane symbol "m" immediately follows a rotational       

         axis symbol, the rotational axis lies in the mirror plane;  


    (2) if a slash "/" separates the rotational axis and the mirror       

         symbol, the mirror plane is perpendicular to the axis;

 

     (3) if two or three mirror plane symbols or two or three rotational

          axis symbols immediately follow one another, they are        perpendicular to one another.
 These conventions are not rigorously followed for the cubic system in which the symmetry relationships are more complex.  The 32 crystal classes are listed below, organized according to the six crystal systems.
        

         Triclinic: 1, B1

         Monoclinic: 2, m, 2/m

         Orthorhombic: 2/m 2/m 2/m, 222, mm2

         Tetragonal: 4, B4, 4/m, 4mm, B42m, 422, 4/m 2/m 2/m

         Hexagonal: 3, B3, 3m, B3 2/m, 32,6, B6, 6/m, 6mm, B6m2, 622, 6/m 2/m 2/m 

         Isometric or cubic: 23, 2/m B3, 4/m B3 2/m, B43m, 432                                            

 

Symplectic – Intimate intergrowth of two different minerals.
 

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Tabular - Crystal habit appearing like a tablet of paper.

 

                                             

Tarnish - Formation of a thin alteration film on the surface of certain minerals.  The thin film, itself called tarnish, affects the mineral's color and luster.                 

 

Tecto- Applied to minerals containing anion groups linked into three- dimensional frameworks (i.e. tectosilicate)                                                                    

 

Tectosilicate - Mineral characterized by a three-dimensional framework of silicate groups of general formula SiO2.  Aluminum commonly takes the place of some of the silicon.      

 

Tellurite - Compound containing the tellurite group (TeO3) or linked tellurite groups (Te2O5, Te3O8).          

 

Tenacity - Tenacity refers to the strength rather than the hardness of a mineral.  It actually represents its resistance to breaking, crushing, bending, or tearing.  The following terms are used to describe the tenacity of minerals:
     Brittle: breaks or powders easily

     Malleable: can be hammered into thin sheets without breaking 

    Sectile: can be cut into thin shavings with a knife    

    Ductile: can be drawn into a wire without breaking    

    Elastic: can be bent and then returns to its original shape     

    Flexible: can be bent but does not return to its original shape                                           

 

Termination - The tip of crystal; ideally made up of crystal faces.                                    

 

Tetartoid - A crystal form consisting of nonparallel faces related by the combined symmetry of the 23 (tetartoidal) crystal class.


 Tetragonal - Crystal system characterized by one 4-fold symmetry axis.


Tetrahedron - A three-dimensional shape bounded by four triangular faces and having four corners.  The tetrahedron is a common crystal form in the isometric crystal system.


Tetrahexahedron - A three-dimensional shape bounded by 24 faces that are isosceles triangles (two sides equal).  The tetrahexahedron is a crystal form in the isometric crystal system.

                                           
Thermoluminescence - Emission of visible light by a material when it is heated.                                             

 

Thiosulfate - Compound containing the thiosulfate group (SO).               

 

Translucent - Capable of allowing visible light to pass through, but not clearly enough that an object can be seen looking through the material.  

 

Transparent - Capable of allowing visible light to pass through clearly so that an object can be seen looking through the material.           

 

Trapezohedron - A three-dimensional shape bounded by 24 faces.  The trapezohedron is a crystal form in the isometric crystal system.                                  

 

Trapezohedron - A crystal form consisting of symmetry-related faces that are trapezoids (four unequal sides).  There are trigonal (6 faces), hexagonal (12 faces), tetragonal (8 faces), and cubic (24 faces) trapezohedra.
 

Triboluminescent - Emission of light due to mechanical stress on the crystal lattice.
 

Trichroism, Pleochroism, Dichroism - 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.
 

Triclinic – A crystal form where he three axes are all unequal in length and intersect at three different angles (any angle but 90 degrees).

 
Trigonal - A subgroup of the hexagonal crystal system characterized by one three-fold symmetry axis.                                  

 

Trilling - Group of three crystals intergrown in a twin relationship.                                           

 

Trimorphous - Three minerals that have the same chemical compositions but different atomic structures.  See polymorphism.                                         

 

Trioctahedral - Type of layered atomic structure in which all three of the 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 dioctahedral.                       

 

Trisoctahedron - A three-dimensional shape bounded by 24 faces that are isosceles triangles (two sides equal).  The trisoctahedron is a crystal form in the isometric crystal system.                        

 

Tristetrahedron - A three-dimensional shape bounded by 12 three-or-four-sided faces.  The tristetrahedron is a crystal form in the isometric crystal system.  The trigonal tristetrahedron can be constructed by splitting each of the four faces of a tetrahedron into three triangular faces.                                               

 

Trivalent - Cation having a charge (valence) of 3.


Tuft, Tufted - A crystal aggregate in the form of clumps of fibrous crystals.                                              

 

Tungstate - Compound containing the tungstate group (WO4).                                             

 

Turbid - Appearing cloudy or translucent because of suspended material.


Twinning - The intergrowth of two or more crystals of the same mineral in a definite orientation determined by the atomic structure of the mineral.  The twin relationship is indicated by specifying the Miller indices of a plane on which the structures meet (e.g. on {100}) and/or an axis about which one structure is rotated relative to the other (e.g. about [310] axis).  If a plane is specified without indicating an axis, the twinning is understood to be by reflection across that plane.                             

 

Uneven - Fracture characterized by more or less flat surfaces with some
roughness.                                            

 

Unit Cell - The unit cell is the smallest group of atoms in the atomic structure that if repeated in each of the three dimensions would completely generate the atomic structure.  The atoms in the unit cell and their arrangement are unique to a given mineral and define its atomic structure, chemical composition, external symmetry, and many of its properties.
 The shape and dimensions of the unit cell are given by three lengths (axes), a, b, and c, and the angles between them: alpha (angle between b and c), beta (angle between a and c), and gamma (angle between a and b).
 In the higher symmetry crystal systems certain lengths are equal and certain angles are precisely defined so that they need not be stated:
    Triclinic: a not equal to b not equal to c, alpha not equal to beta not equal to gamma (All dimensions need  to be given.)
    Monoclinic: a not equal to b not equal to c, alpha = gamma = 90° not equal to beta  (Only a, b, c, and beta  need to be given.)
    Orthorhombic: a not equal to b not equal to c, alpha = beta = gamma = 90° (Only a, b, and c need to be  given.)
    Tetragonal: a = b not equal to c, alpha = beta = gamma = 90° (Only a and c need to be given.)
    Hexagonal: a = b not equal to c, alpha = beta = 90°, gamma = 120° (Only a and c need to be given.)
    Isometric or cubic: a = b = c, alpha = beta = gamma = 90° (Only a needs to be given.)  

 

 Uranyl- Compound containing the uranyl group (UO2).
Vanadate - Compound containing the vanadate group (VO4).   Vanadin- Compound containing clusters of vanadium atoms surrounded by  oxygen atoms (exclusive of the vanadate (VO4) group).                                              Variety - Variety names have traditionally been given to minerals that have some distinctive physical characteristic, such as color, that sets them apart from other minerals of the same mineral species.  Variety names are most commonly used for minerals that have use as gems.  Amethyst, for example, is the purple variety of the mineral species quartz, which has many other varieties as well.                                               

 

Vein - A thin tabular or sheet-like body usually partially or totally filled with intergrown crystals.
 
Veinlet - A small vein.
Vermicular, Vermiform - Worm-like in shape.  

                                           

Vicinal - Crystal face that closely approximates a larger face in orientation.


 Vitreous - Brightly reflective; similar to the luster of freshly broken glass.


 Vug - Small cavity in rock, usually lined with crystals.      Waxy - Slightly reflective mineral luster; typical of minutely granular surfaces.


 Xenoblast - A mineral grain without faces that has grown in a metamorphic rock.   

                                       

Xenomorphic - Pertaining to crystals that are not bounded by faces; anhedral.

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