| Tephroite | |
|---|---|
 Tephroite from Japan | |
| General | |
| Category | Silicate mineral |
| Formula (repeating unit) | Mn2SiO4 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Unit cell | a = 4.88(2) Å, b = 10.61(2) Å c = 6.24(2) Å; Z = 4 |
| Identification | |
| Color | Olive-green, bluish green, gray, °esh-red, reddish brown; pale green in thin section, may be colorless |
| Crystal habit | Crystals typically short, prismatic, to 4 cm, or anhedral, equidimensional. Commonly in disseminated grains, compact, or massive. |
| Twinning | Uncommon on {011} |
| Cleavage | {010}, distinct; {001}, imperfect |
| Fracture | Uneven to conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 6 |
| Luster | Vitreous to greasy |
| Streak | Pale gray |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.87 – 4.12 |
| Optical properties | Biaxial (-) |
| Refractive index | nα = 1.759 nβ = 1.797 nγ = 1.860 |
| Birefringence | δ = 0.101 |
| Pleochroism | Weak; X = brownish red; Y = reddish; Z = greenish blue. |
| 2V angle | Measured: 60° to 70°, Calculated: 78° |
| References | [1] [2] [3] |
Tephroite is the manganese endmember of the olivine group of nesosilicate minerals with the formula Mn 2 Si O 4. A solid solution series exists between tephroite and its analogues, the group endmembers fayalite and forsterite. Divalent iron or magnesium may readily replace manganese in the olivine crystal structure.
Manganese is a chemical element with the symbol Mn and atomic number 25. It is not found as a free element in nature; it is often found in minerals in combination with iron. Manganese is a transition metal with important industrial alloy uses, particularly in stainless steels.
An endmember in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity. Minerals often can be described as solid solutions with varying compositions of some chemical elements, rather than as substances with an exact chemical formula. There may be two or more endmembers in a group or series of minerals.
The mineral olivine is a magnesium iron silicate with the formula (Mg2+, Fe2+)2SiO4. Thus it is a type of nesosilicate or orthosilicate. The primary component of the earth's upper mantle, it is a common mineral in Earth's subsurface but weathers quickly on the surface.
It was first described for an occurrence at the Sterling Hill Mine and Franklin, New Jersey, United States. [2] It occurs in iron-manganese ore deposits and their related skarns. It also occurs in metamorphosed manganese-rich sediments. It occurs in association with: zincite, willemite, franklinite, rhodonite, jacobsite, diopside, gageite, bustamite, manganocalcite, glaucochroite, calcite, banalsite and alleghanyite. [1] It can also be found in England and Sweden.
Franklin is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 5,045 reflecting a decline of 115 (-2.2%) from the 5,160 counted in the 2000 Census, which had in turn increased by 183 (+3.7%) from the 4,977 counted in the 1990 Census.
Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or Shear zones that intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional, or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from either magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on the original composition of both the hydrothermal fluid and the original composition of the protolith.
Metamorphism is the change of minerals or geologic texture in pre-existing rocks (protoliths), without the protolith melting into liquid magma. The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis, and melting (~850°C).
Tephroite has a hardness of 6 and a specific gravity of approximately 4.1, which is heavy for non-metallic minerals. Its name comes from the Greek tephros, "ash gray", for its color. [3] It can also be found olive-green, greenish-blue, pink, or brown. Other names for tephroite include mangan olivine and mangan peridot.
Specific gravity is the ratio of the density of a substance to the density of a reference substance; equivalently, it is the ratio of the mass of a substance to the mass of a reference substance for the same given volume. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance for liquids is nearly always water at its densest ; for gases it is air at room temperature. Nonetheless, the temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa).
