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This figure shows an example of Compound Twinning, where two orthoclase twins are twinned in parallel to their basal pinacoid. Compound Twins are produced by the presence of two or more twinning laws in the same group.

Compound Twinning

This figure shows an example of Compound Twinning, where two orthoclase twins are twinned in parallel…

Corundum.

Corundum

Corundum.

Corundum.

Corundum

Corundum.

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." — Ford, 1912

Corundum

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." —…

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." — Ford, 1912

Corundum

"Hexagonal-rhombohedral. Crystals usually prismatic in habit or tapering hexagonal pyramids." —…

"Monoclinic. Habit varied. Crystals usually nearly equidimensional in the three axial directions and often complex in development." — Ford, 1912

Datolite

"Monoclinic. Habit varied. Crystals usually nearly equidimensional in the three axial directions and…

Diamond.

Diamond

Diamond.

Diopside is a monoclinic pyroxene mineral with composition MgCaSi<small><sub>2</sub></small>O<small><sub>6</sub></small>. It forms complete solid solution series with hedenbergite (FeCaSi<small><sub>2</sub></small>O<small><sub>6</sub></small>) and augite, and partial solid solutions with orthopyroxene and pigeonite. It forms variably colored, but typically dull green crystals in the monoclinic prismatic class. It has two distinct prismatic cleavages at 87 and 93&deg; typical of the pyroxene series. It has a Mohs hardness of six and a Vickers hardness of 7.7.

Diopside

Diopside is a monoclinic pyroxene mineral with composition MgCaSi2O6. It forms complete solid solution…

Dioptase is an intense emerald-green to bluish-green copper cyclosilicate mineral. It is transparent to translucent. Its luster is vitreous to sub-adamantine. Its formula is CuSiO<small><sub>3</sub></small>&sdot;H<small><sub>2</sub></small>O (also reported as CuSiO<small><sub>2</sub></small>(OH)<small><sub>2</sub></small>). It has a hardness of 5, the same as tooth enamel. It specific gravity is 3.28&ndash;3.35, and it has two perfect and one very good cleavage directions. Additionally, dioptase is very fragile and specimens must be handled with great care. It is a trigonal mineral, forming 6-sided crystals that are terminated by rhombohedra.

Dioptase

Dioptase is an intense emerald-green to bluish-green copper cyclosilicate mineral. It is transparent…

"Hexagonal-rhombohedral. Crystals are usually the unit rhombohedron (cleavage rhombohedron). Faces often curved, and sometimes so acurately as to form "saddle-shaped" crystals. Other forms rare." &mdash; Ford, 1912

Dolomite

"Hexagonal-rhombohedral. Crystals are usually the unit rhombohedron (cleavage rhombohedron). Faces often…

In cases where the basal pinacoid is fixed by some physical property like cleavage, the elongation is in the direction of the clinodiagonal axis. This crystal of Feldspar shown is a common example of this elongation.

Elongation in the Direction of the Clinodiagonal Axis

In cases where the basal pinacoid is fixed by some physical property like cleavage, the elongation is…

"Monoclinic. Crystals are often much elongated parallel to the ortho-axis with a prominent development of the faces of the orthodome zone, giving them a prismatic aspect. Striated parallel to the ortho-axis. Terminated usually at one end of the ortho-axis and most commonly by the two faces of a pyramid." &mdash; Ford, 1912

Epidote

"Monoclinic. Crystals are often much elongated parallel to the ortho-axis with a prominent development…

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." &mdash; Ford, 1912

Garnet

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." — Ford, 1912

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." &mdash; Ford, 1912

Garnet

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination." — Ford, 1912

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination. Hexoctahedron observed at some times." &mdash; Ford, 1912

Garnet

"Isometric. Common forms dodecahedron amd trapezohedron, often in combination. Hexoctahedron observed…

Goethite is a mineral composed of an iron hydrate crystallizing in soil and other low temperature environments.

Goethite

Goethite is a mineral composed of an iron hydrate crystallizing in soil and other low temperature environments.

"An important part of the study of crystallography consists in the measuring and classifying of the interfacial angles on the crystals of all minerals. These measurements are accomplished by means of instruments known as goniometers." — Ford, 1912

Contact Goniometer

"An important part of the study of crystallography consists in the measuring and classifying of the…

Certain crystals of green fluorspar from Saxony, showing the form ∞ 03, {310}, have one half of their planes developed at the expense of the other half, so as to produce a hexagonal scalenohedron.

Green Fluorspar from Saxony

Certain crystals of green fluorspar from Saxony, showing the form ∞ 03, {310}, have one half of their…

Gypsum.

Gypsum

Gypsum.

Gypsum.

Gypsum

Gypsum.

"Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edges beveled by prism and pyramid faces." &mdash; Ford, 1912

Gypsum

"Monoclinic. Crystals usually tabular parallel to clinopinacoid; in diamond-shaped crystals with edges…

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing triangular markings." &mdash; Ford, 1912

Hematite

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing…

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing triangular markings." &mdash; Ford, 1912

Hematite

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing…

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing triangular markings. Edges of plates sometimes beveled with rhombohedral and pyramidal forms." &mdash; Ford, 1912

Hematite

"Hexagonal-rhombohedral. Crystals usually thick to thin tabular. Basal planes prominent, often showing…

Hemimorphite is a sorosilicate mineral consisting of hydrous zinc silicate.

Hemimorphite

Hemimorphite is a sorosilicate mineral consisting of hydrous zinc silicate.

Heulandite is the name of a series of tecto-silicate minerals of the zeolite group.

Heulandite

Heulandite is the name of a series of tecto-silicate minerals of the zeolite group.

Hornblende is a complex inosilicate series of minerals. Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole.

Hornblende

Hornblende is a complex inosilicate series of minerals. Hornblende is not a recognized mineral in its…

This figure exhibits a combination occurring on the ferro-magnesian meta-silicate, Hypersthene. Forms: (a), (b), (m), (n), (h), (o), (c), (i), and (u).

Hypersthene

This figure exhibits a combination occurring on the ferro-magnesian meta-silicate, Hypersthene. Forms:…

"Isometric. Trapezohedral habit." &mdash; Ford, 1912

Leucite

"Isometric. Trapezohedral habit." — Ford, 1912

"Isometric. Octahedral habit, sometimes twinned octahedrons. Dodecahedron at times, either alone or with octahedron." &mdash; Ford, 1912

Magnetite

"Isometric. Octahedral habit, sometimes twinned octahedrons. Dodecahedron at times, either alone or…

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example of malachite.

Malachite

The most common twinning plane is the orthopinacoid ∞P∞̄ , {100}, as may be seen in this example…

"Orthorhombic. Crystals commonly tabular parallel to basal plane, showing also short prisms and low brachydomes." &mdash; Ford, 1912

Marcasite

"Orthorhombic. Crystals commonly tabular parallel to basal plane, showing also short prisms and low…

"Orthorhombic. Crystals commonly tabular parallel to basal plane, showing also short prisms and low brachydomes. The brachydomes usually striated parallel to the brachy-axis. Often twinned, given coxcomb and spear-shaped groups." &mdash; Ford, 1912

Marcasite

"Orthorhombic. Crystals commonly tabular parallel to basal plane, showing also short prisms and low…

Orthorhombic Iron Disulphide (Marcasite) sometimes shows cyclic groups of five individuals, bounded by ∞ P, {110} (M); P∞&#774;, {011} (l); and 0P, {001}, and united into a pentagonal figure by (M) as twinning plane.

Marcasite

Orthorhombic Iron Disulphide (Marcasite) sometimes shows cyclic groups of five individuals, bounded…

"The symmetry of the Monoclinic System is as follows: The crystallographic axis b is an axis of binary symmetry and the plane of the a and c axes is a plane of symmetry." &mdash; Ford, 1912

Symmetry of monoclinic system

"The symmetry of the Monoclinic System is as follows: The crystallographic axis b is an axis of binary…

Orthoclase

Orthoclase

Orthoclase

Orthoclase.

Orthoclase

Orthoclase.

"Monoclinic. Crystals are usually prismatic in habit and have as prominent forms, clinopinacoid, base, prism, with often smaller orthodomes." &mdash; Ford, 1912

Orthoclase

"Monoclinic. Crystals are usually prismatic in habit and have as prominent forms, clinopinacoid, base,…

"Monoclinic. Crystals are usually prismatic in habit and have as prominent forms, clinopinacoid, base, prism, with often smaller orthodomes." &mdash; Ford, 1912

Orthoclase

"Monoclinic. Crystals are usually prismatic in habit and have as prominent forms, clinopinacoid, base,…

Pyroxene

Pyroxene

Pyroxene

Pyroxene

Pyroxene

Pyroxene

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally developed as to give the effect of a hexagonal pyramid." &mdash; Ford, 1912

Quartz

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally…

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally developed as to give the effect of a hexagonal pyramid. Sometimes one rhombohedron predominates or occurs alone." &mdash; Ford, 1912

Quartz

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally…

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally developed as to give the effect of a hexagonal pyramid. Sometimes one rhombohedron predominates or occurs alone. At times the prism faces are wanting, and the combination of the two rhombodendrons gives what appears to be a doubly terminated hexagonal pyramid, known as a quartzoid." &mdash; Ford, 1912

Quartz

"Terminated usually by a combination of a positive and negative rhombohedron, which often are so equally…

This illustration shows Quartz or Silica. Quarts is the most abundant mineral in the Earth's continental crust.

Quartz

This illustration shows Quartz or Silica. Quarts is the most abundant mineral in the Earth's continental…

"The quartz crystals are said to be right or left handed, depending upon whether these faces are to be observed trunctuating the edges between prism and rhombohedron." &mdash; Ford, 1912

Right-handed quartz

"The quartz crystals are said to be right or left handed, depending upon whether these faces are to…

Rhodonite.

Rhodonite

Rhodonite.

"Triclinic. Crystals commonly tabular parallel to base." &mdash; Ford, 1912

Rhodonite

"Triclinic. Crystals commonly tabular parallel to base." — Ford, 1912

An alternate combination of manganese meta-silicate (Rhodonite), showing forms: (c), (b), (a), (n), (h), (o), and (s).

Rhodonite

An alternate combination of manganese meta-silicate (Rhodonite), showing forms: (c), (b), (a), (n),…

Rutile.

Rutile

Rutile.

"This form consists of twelve scalene triangular faces. These faces correspond in their position to the alternate pairs of faces of a dihexagonal pyramid." &mdash; Ford, 1912

Scalenohedron

"This form consists of twelve scalene triangular faces. These faces correspond in their position to…

"A crystal of scapolite, upon which the faces of the third-order pyramid are shown." &mdash; Ford, 1912

Scapolite

"A crystal of scapolite, upon which the faces of the third-order pyramid are shown." — Ford, 1912

"Tetragonal; tri-pyramidal. Crystals usually simple pyramids of first order. Closely resemble isometric octahedrons in angles. Faces of the pyramid of third order are small and rare." &mdash; Ford, 1912

Scheelite

"Tetragonal; tri-pyramidal. Crystals usually simple pyramids of first order. Closely resemble isometric…

"Isometric; tetrahedral. Tetrahedron form." &mdash; Ford, 1912

Sphalerite

"Isometric; tetrahedral. Tetrahedron form." — Ford, 1912

Staurolite.

Staurolite

Staurolite.

"Orthohombic. Habit prismatic, showing usually a combination of prism with large angle, brachypinacoid, base and macrodome." &mdash; Ford, 1912

Staurolite

"Orthohombic. Habit prismatic, showing usually a combination of prism with large angle, brachypinacoid,…

"Monoclinic. Uniform in cruciform twins. Commonly tabular parallel to clinopinacoid. Crystals usually in sheaflike aggregates." — Ford, 1912

Stilbite

"Monoclinic. Uniform in cruciform twins. Commonly tabular parallel to clinopinacoid. Crystals usually…

Striation of crystal planes may be produced by repeated polysynthetic twinning. This is well illustrated in the case of the triclinic feldspar, Albite. The twinning plane here is the brachypinacoid, and the contact of a large number of fine lamellæ, alternately in twinning position, would evidently produce a striation on the basal plane parallel to the brachydiagonal axis

Striation of Crystal Planes

Striation of crystal planes may be produced by repeated polysynthetic twinning. This is well illustrated…

Sulphur.

Sulphur

Sulphur.

Sulphur.

Sulphur

Sulphur.