Glossary of Rock and Mineral Terms - H

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habit

The attributes of the appearance of a crystal or aggregate.

hackley fracture

Type of fracture resembling broken metal, exhibiting sharp, jagged surfaces. This fracture is sometimes known as "jagged" fracture

halides

Group of minerals containing one of the halogen elements (chlorine, fluorine, bromine, and iodine) as a building block. Most halides are soft and fragile, and some are soluble in water. Many crystallize in the isometric system.

half-life

The time necessary for half of the atoms of a parent isotope to decay into the daughter isotope.

halogen

Five chemically related elements belong in the halogen group. They are astatine, bromine, chlorine, fluorine, and iodine. Minerals that are composed of the halogen elements are known as halides.

hardness

The degree of resistance of a given mineral to scratching, indicating the strength of the bonds that hold the mineral's atoms together. The hardness of a mineral is measured by rubbing it with substances of known hardness.

hardness testing kit

Kit composed of minerals or rods with labeled harnesses used to scratch a mineral to test its hardness.

hard water

Water rich in calcium.

HCl

Abbreviation and chemical formula of hydrochloric acid.

headland

A cliff that projects out from a coast into deep water.

heat treated

describes a mineral or gem put under intense heat to enhance color or remove flaws.

hemimorphic

describes doubly terminated crystal with two differently shaped ends.

hexagon

Polyhedron with six sides and a top and bottom base.

hexagonal crystal system

Any mineral that falls under the following specifications belongs to the hexagonal crystal system: Four axes, three are equal in length and lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners.

hexagonal bipyramid (al)

Synonym of bipyramidal hexagon

Holocene Epoch

The second epoch of the Quaternary Period, beginning approximately 10,000 years ago and continuing to the pres-ent time. See also Pleistocene Epoch.

hook

A spit that curves sharply at its coastal end.

hopper

Crystal form exhibiting an indenting, terraced, structure penetrating towards the center.

horn

A high mountain peak that forms when the walls of three or more cirques intersect.

hornfels

A hard, dark-colored, dense metamorphic rock that forms from the intrusion of magma into shale or basalt.

horst

A block of rock that lies between two faults and has moved upward relative to the two adjacent fault blocks. See also graben.

host mineral

Mineral that is the chief constituent of a particular rock.

hot spot

An area in the upper mantle, ranging from 100 to 200 kilometers in width, from which magma rises in a plume to form volcanoes. A hot spot may endure for 10 million years or more.

hydrate

To absorb water and construct it as part of the crystal lattice.

hydration

The addition of water into a minerals' chemical structure.

hydraulic conductivity

The extent to which a given substance allows water to flow through it, determined by such factors as sorting and grain size and shape.

hydraulic gradient

The difference in potential between two points, divided by the lateral distance between the points.

hydraulic lifting

The erosion of a stream bed by water pressure.

hydrocarbon

A molecule that is entirely made up of hydrogen and carbon.

hydrochloric acid

(HCl) Corrosive acid used mainly for dissolving unwanted substances. It is a very destructive liquid and will destroy many minerals.

hydrofluoric acid

(HF) Corrosive acid used for the production of glass(, since it dissolves glass). It is a very destructive liquid and will some many minerals.

hydrogen bond

An intermolecular bond formed with hydrogen.

hydrologic cycle

The perpetual movement of water among the mantle, oceans, land, and atmosphere of the Earth.

hydrolysis

A form of chemical weathering in which ions from water replace equivalently charged ions from a mineral, especially a silicate.

hydrometallurgy

The separation of metals from ore or from alloys through a process in which a liquid is the primary factor, or the forming of alloys and purification of metals through a process in which a liquid is the primary factor.

hydrothermal

Hydro" means Water and "Thermal" means Heat. Hydrothermal solutions are solutions of hot water arising from underground sources. They may be as cool as cold water or hot as burning hot steam.

hydrothermal deposit

A mineral deposit formed by the precipitation of metallic ions from water ranging in temperature from 50º to 700ºC.

hydrous

Containing water, and in some cases, containing hydroxyl.

hydroxides

Compounds of metallic elements combined with water (H2O) or hydroxyl (OH). The hydroxides are a subgroup of the oxide group.

hydroxyl

Radical composed of hydrogen and oxygen,  formula = (OH).

hygroscopic

Describing a mineral that intakes and retains water from the atmosphere, and forms part of its structure. Hygroscopic minerals should be kept away from humid areas and water, and should preferably be kept in rice or silica gel which absorb moisture, since water can destroy such minerals.

hypothermal vein

Vein created at extreme depths and at a very high temperature.

hypothesis

A tentative explanation of a given set of data that is expected to remain valid after future observation and experimentation.