| Term | Definition | | | |
| 50/50 Rule | Some minerals are part of a "solid-solution series," with chemical composition and crystal structure that varies from one pure "end-member," through possible intermediate compositions, to another end-member. For example pure scheelite is calcium tungstate, CaWO4, and pure powellite is calcium molybdate, CaMoO4. There could be intermediate minerals with a mix of tungsten and molybdenum, but for simplicity, we say that if the mineral contains more tungsten ions than molybdenum ions – the mineral is termed scheelite. If instead there are more molybdenum than tungsten ions, it is called powellite, with no "50/50" minerals in the middle. | | | 1036 |
| Acicular | Crystals which appear as sharp, slender needles either individually or in a group. | | | 1041 |
| Adamantine | A mineral having a luster of the highest degree, similar to that of a diamond. | | | 1040 |
| Alteration | The chemical or physical changes in a mineral, through natural causes, resulting in the formation of a new mineral which replaces the old in whole or in part. An example would be mica replaced by grossular, locally called caswellite. | http://www.fomsnj.org/MineralImage.aspx?minIMID=1073&minName=Grossular | Mica replaced by grossular. | 1043 |
| Amorphous | A non-crystalline mineral having no clearly defined form nor shape. | | | 1042 |
| Amphibole | Is a group of closely related, generally dark colored rock forming silicate minerals. | | | 1019 |
| Association | The recurring presence of a mineral in the same combination with other minerals. It is an important aid in the visual identification of a mineral. e.g. Hardystonite is frequently associated with clinohedrite. Norbergite is never found in association with zincite. | | | 1044 |
| Augen | From German "eyes", are lenticular eye-shaped mineral grains or mineral aggregates visible in some foliated metamorphic rocks. In a cross section they have the shape of an eye. | | | 1038 |
| Basal Cleavage | The breakage of a mineral in only one direction, leaving a smooth flat surface where it was broken off. | | | 1045 |
| Botryoidal | A texture or mineral habit where the mineral has a globular external form resembling a bunch of grapes. | http://www.fomsnj.org/MineralImage.aspx?minIMID=1268&minName=Hemimorphite | Botryoidal mineral example | 1000 |
| Boxwork | Is a mineral-aggregate structure having plates or septa that are often coated with crystals and that intersect at various angles and enclose angular spaces. | http://www.fomsnj.org/MineralImage.aspx?minIMID=1349&minName=Calcite | An example of boxwork | 1032 |
| Brittle | A mineral that shatters easily. It is not to be confused with the hardness of the mineral. e.g. Quartz is brittle yet it has a Mohs hardness of 7. | | | 1046 |
| Calc-silicate | A term used to describe rock produced by metasomatic alteration of existing rocks, producing calcium silicate minerals such as diopside and wollastonite. Skarns or tactites are calcium-bearing calc–silicate rocks. | | | 1010 |
| Camptonite | A dark-coloured, medium-grained igneous rock, a common form of basalt, also called a lamprophyre, which characteristically forms dykes. Dikes of it cut through the limestone at Franklin. | | | 1047 |
| Cleavage | The natural tendency of some minerals to break in certain and definite directions and where the resulting pieces have smooth, flat surfaces. It is to be distinguished from fracture. | http://www.fomsnj.org/MineralImage.aspx?minIMID=1000&minName=Calcite | Example of cleavage. | 1048 |
| Conchoidal | Describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. In a mineral the result is a concave surface somewhat like the interior of a seashell. In a sub-conchoidal fracture the concavity is not so well defined. | | | 1049 |
| Concretionary | A concretion is a hard, compact mass of material formed by the precipitation of mineral cement within the spaces between various minerals, and is found in sedimentary rock or soil. | | | 1050 |
| Dike | Is a sheet of rock that formed in a fracture in a pre-existing rock body. They can be either magmatic or sedimentary in origin. Magmatic dikes form when magma intrudes into a crack then crystallizes as a sheet intrusion, either cutting across layers of rock or through an unlayered mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack. | | | 1026 |
| Druzy | Also drusy. A coating of fine crystals on a rock fracture surface, vein, or within a vug or geode. | | | 1052 |
| Exsolution | Is a process by which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials. | | | 1017 |
| Foliated | Refers to repetitive layering in metamorphic rocks that results from segregation of different minerals into roughly parallel layers. Each layer may be as thin as a sheet of paper, or over a meter in thickness. | | | 1015 |
| Fracture | Any irregular breakage in a rock or mineral. It is thus distinguished from cleavage where the breakage occurs along defined planes. | | | 1053 |
| Gangue | Is commercially worthless rock in which valuable minerals and metals occur in an ore deposit. At Franklin and Sterling Hill, a common example would be calcite. | | | 1055 |
| Granular | A rock composed of mineral grains of similar size. Much of the green willemite / franklinite ore from Franklin is granular. | | | 1056 |
| Hardness | The relative resistance of mineral to being scratched by another substance. Measured by the Mohs hardness scale. | | | 1057 |
| Hemimorphic | A doubly-terminated mineral crystal having different crystal forms at each end of an axis. Named after hemimorphite, a common mineral at Sterling Hill. | | | 1058 |
| Homogeneous | Refers to the uniformity of composition throughout a rock, texture. For sedimentary rocks, it is the size, shape, and arrangement of the grains. For igneous and metamorphic rocks, texture of a rock or crystals. | | | 1018 |
| Igneous | Solid rock formed from molten and fluid material called magma. | | | 1059 |
| IMA | The International Mineralogical Association (IMA) is the world’s largest organization promoting mineralogy. It was founded in 1958 and has 38 national mineralogical societies or groups as members. The Association supports the activities of Commissions and Working Groups involved on certain aspects of mineralogical practice and facilitates interaction among mineralogists by sponsoring and organizing meetings. The IMA Commission on New Minerals, Nomenclature and Classification approves and publishes a list of names and details of new minerals. | | | 1034 |
| Iridescent | A mineral displaying reflected light, the colors of the rainbow. An example is the "peacock ore" patina of franklinite. | | | 1060 |
| Joints | Fractures in rock along which little or no offset has taken place parallel to the plane of the fracture. | | | 1088 |
| Lamellae | Plural for lamella, is used to refer to collections of fine sheets of material held adjacent to one another. | | | 1016 |
| Luster | The appearance of the mineral, in reflected light, which is different from its inherent color. Examples include metallic, glassy, adamantine, pearly, or vitreous. | | | 1061 |
| Malleable | The capability of a metal of being beaten into thin sheets without breaking. Malleable copper masses were found at Franklin. | | | 1062 |
| Matrix | Is the finer-grained mass of material in which larger mineral grains, crystals, or clasts are embedded. Also called groundmass. | | | 1063 |
| Metamorphic | Any rock that has been changed from its original chemical composition or physical appearance by high temperature or great pressure. e.g. marble is a metamorphosed limestone (physical change). | | | 1064 |
| Metasomatic alteration | The chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. Some of the minerals which compose the rocks are dissolved, and new mineral formations are deposited in their place. | | | 1011 |
| Micaceous | A mineral of foliated or lamellar structure which can be split into thin sheets. e.g. phlogopite or biotite. | | | 1068 |
| MOHS Hardness Scale | The relative resistance of a mineral to being scratched by another substance will determine its hardness and thus aid in identification.
1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Quartz
8 Topaz
9 Corundum
10 Diamond | | | 1065 |
| Mylonitization | Is the deformation of a rock by extreme microbrecciation, due to mechanical forces applied in a definite direction, without noteworthy chemical changes to granulated minerals. Characteristically the mylonites produced in this manner have a banded, or streaked appearance, and augen and lenses of the parent rock in a granulated matrix. | | | 1039 |
| Pegmatite | Is a coarsely crystalline, high silica igneous rocks such as granite, seen in abundance at Franklin. Often composed of microcline or other feldspars, quartz, mica, and rarer minerals such as allanite, thorite and zircon. | | | 1066 |
| Pinacoidal | A mineral that breaks along only one cleavage plane, showing two parallel faces, e.g. mica. | | | 1067 |
| Poikiloblastic Texture | Is a term that refers to rounded grains of one or more minerals embedded in much larger grains of another mineral. | | | 1085 |
| Pseudomorph | Is a mineral that has been chemically altered in some way and is a new mineral which can have a new crystalline structure, but it still retains the shape of the original mineral. These changes occur when the mineral is reduced, oxidized, elements added, or when elements are completely replaced. | | | 1020 |
| Radiating | A crystal spray diverging from a common center. e.g. some white willemite. | http://www.fomsnj.org/MineralImage.aspx?minIMID=1189&minName=Willemite | Image of radiating willemite | 1069 |
| Resinous | A mineral whose surface luster closely resembles the congealed sap of a tree. e.g. some types of sphalerite. | | | 1070 |
| Sedimentary | The weathered or decomposed materials of an older rock mass deposited elsewhere by wind or water and then formed into new rock. e.g. sandstone, shale. Speleothemes found inside the Sterling Hill Mine are post-mining sedimentary deposits. | | | 1071 |
| Skarn | Are calcium-bearing calc–silicate rocks most often formed at the contact zone between intrusions of granitic magma bodies and carbonate sedimentary rocks such as limestone and dolostone. Hot fluids derived from the granitic magma are rich in silica, iron, aluminum, and magnesium. These fluids mix in the contact zone, dissolve calcium-rich carbonate rocks, and convert the host carbonate rock to skarn deposits in a metamorphic process called metasomatism. | | | 1022 |
| Slickensides | Refers to a naturally polished fault surface due to friction along that surface during faulting. The fault surface may or may not bear mechanical scratches from one fault block grinding against another, and it may or may not bear fibrous minerals that formed within growing voids as fault movement took place. Note that the term slickenside refers to the fault surface itself, not to any striations or mineral fibers on it, as is commonly assumed. | | | 1072 |
| Solid Solution Series | A solid solution series is the compositional range between end-member minerals that share the same basic chemical formula but experience substitution of elements in one or more atomic sites. An example would be meionite and marialite in the scapolite group. | | | 1081 |
| Streak | The color of the powder produced when a mineral is drawn across unglazed porcelain. It is an aid to identification. | | | 1073 |
| Striated | Extremely fine lines or grooves on the face of a crystal. | | | 1074 |
| Tabular | A general term applied to the crystalline habit where the mineral consists of flat plates like the pages of a book. e.g. phlogopite mica | | | 1075 |
| Terminated Crystal | A single terminated crystal ends in a point on one end of the crystal, whereas a double terminated crystal ends in a point on both sides of the crystal. | | | 1005 |
| Thermoluminescence | The emission of visible light when a mineral is heated. Only some minerals possess this property. e.g. some Franklin fluorite (var. chlorophane). | | | 1076 |
| Triboluminescence | The emission of light from some minerals, caused by either rubbing, scratching or beating. e.g. some sphalerite. | | | 1077 |
| Vug | A small cavity in a rock or mineral generally lined with crystals. | | | 1078 |
