Edenite or ''edenitic hornblende'' is a double chain silicate mineral of the

amphibole Amphibole ( ) is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is ...

group with the general chemical composition NaCa₂Mg₅(Si₇Al)O₂₂(OH)₂. Edenite is named for the locality of Edenville,

Orange County, New York Orange County is a List of counties in New York, county located in the U.S. state of New York (state), New York. As of the 2020 United States census, the population was 401,310. The county seat is Goshen (village), New York, Goshen. This count ...

, where it was first described.

Occurrence

Edenite has been found primarily in

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

rocks, occurring in pods of other magnesium-rich minerals within a

marble Marble is a metamorphic rock consisting of carbonate minerals (most commonly calcite (CaCO3) or Dolomite (mineral), dolomite (CaMg(CO3)2) that have recrystallized under the influence of heat and pressure. It has a crystalline texture, and is ty ...

formation or with

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

-rich

lherzolite Lherzolite is a type of ultramafic igneous rock. It is a coarse-grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser amounts of calcic chromium-rich clinopyroxene. Minor minerals include chromium and alumi ...

s from deep within the Earth's crust. Thus, finding edenite in the field can indicate high-temperature regional metamorphism of the surrounding rocks.

Uses and importance

While edenite is not important for commercial or industrial applications, it is often studied because of its unique chemical substitution properties. Results from research performed on amphiboles have shown that edenite is particularly suited for fitting chloride anions into its chemical framework. This makes edenite a good candidate for use in chlorine isotope fractionation in amphibole-bearing rocks. Many synthetic variations of edenite are also used in geochemical research to produce a

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

analogue of fluoroedenite.

Crystal habit

Edenite is a member of the

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

crystal system In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices (an infinite array of discrete points). Space groups (symmetry groups ...

and is in the crystal class 2/m (space group C2/m). This means crystalline edenite is symmetrical around a two-fold rotation axis that is then reflected across a mirror plane perpendicular to the long axis of the mineral.Klein, C., and Dutrow, B. (2007) The 23rd Edition of the Manual of Mineral Science, John Wiley and Sons, Inc. Hoboken, New Jersey, U.S.

Optical properties

Edenite is a biaxial positive mineral. When viewed in

thin section In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron ...

with a

petrographic microscope A petrographic microscope is a type of optical microscope used to identify Rock (geology), rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descr ...

, it is white-gray with pale green

pleochroism Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with Polarization (waves), polarized light. Etymology The roots of the word are from Greek (). It was first made compou ...

in plane-polarized light. Under crossed polars, its interference colors range from first order gray to first order blue.

References

{{Reflist Sodium minerals Calcium minerals Magnesium minerals Amphibole group Hydroxide minerals Monoclinic minerals Minerals in space group 12 Minerals described in 1858 Blendes