Chondrodite is a nesosilicate mineral with formula . Although it is a fairly rare mineral, it is the most frequently encountered member of the humite group of minerals. It is formed in

hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

deposits from locally metamorphosed dolomite. It is also found associated with

skarn Skarns or tactites are coarse-grained metamorphic rocks that form by replacement of carbonate-bearing rocks during regional or contact metamorphism and metasomatism. Skarns may form by metamorphic recrystallization of impure carbonate protoliths, ...

and

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

. It was discovered in 1817 at

Pargas Pargas () is a town and municipality of Finland, in the Archipelago Sea, the biggest archipelago in the world by the number of islands, 50,000. The big limestone mine in Pargas is the base of the main industry and except for the central parts, t ...

in Finland, and named from the Greek for "granule", which is a common habit for this mineral.

Formula

is the end member formula as given by the

International Mineralogical Association Founded in 1958, the International Mineralogical Association (IMA) is an international group of 40 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 5000 plus known mineral species. ...

, molar mass 351.6 g. There is usually some OH in the F sites, however, and Fe and Ti can substitute for Mg, so the formula for the naturally occurring mineral is better written .Gaines et al (1997) Dana's New Mineralogy Eighth Edition, Wiley

Structure

The chondrodite structure is based on a slightly distorted

hexagonal close packed In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occ ...

array of

anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

O, OH and F with metal ions in the octahedral sites resulting in zigzag chains of octahedra. Chains are staggered so that none of the independent tetrahedral sites occupied by Si has OH or F corners.Phillips, W R and Griffen, D T (1981) Optical Mineralogy, pages 142 to 144 Half of the octahedral sites are filled by divalent

cations An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

, principally Mg, and one tenth of the tetrahedral sites are filled by Si. There are three distinct octahedra in the array: Fe is ordered in the M1 sites but not in the larger M2 and smaller M3 sites.American Mineralogist (1970): 55: 1182-1194 Ti is ordered in the M3 positions, which are the smallest, but Ti concentration appears never to exceed 0.5 atoms Ti per formula unit in natural specimens.American Mineralogist (1979) 64:1027 In the humite series Mg²⁺ is replaced by Fe²⁺, Mn²⁺, Ca²⁺ and Zn²⁺ in that order of abundance, though Mg²⁺ always predominates.

Unit cell

Space group: P2₁/b

Unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector In mathematics, a unit vector i ...

parameters:
Synthetic F end member a = 7.80 Å, b = 4.75 Å, c = 10.27 Å, beta = 109.2^o. Synthetic OH end member a = 7.914 Å, b = 4.752 Å, c = 10.350 Å, beta = 108.71^o.
Natural chondrodite has a = 7.867 to 7.905 Å, b = 4.727 to 4.730 Å, c = 10.255 to 10.318 Å, beta = 109.0^o to 109.33^o. Z = 2.

Color

Chondrodite is yellow, orange, red or brown, or rarely colorless, but zoning of different color intensity is common, and intergrown plates of chondrodite, humite, clinohumite,

forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...

and monticellite have been reported.

Optical properties

Chondrodite is biaxial(+), with

refractive indices In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

variously reported as n_α = 1.592 – 1.643, n_β = 1.602 – 1.655, n_γ = 1.619 – 1.675, birefringence = 0.025 – 0.037, and 2V measured as 64° to 90°, calculated: 76° to 78°. Refractive indices tend to increase from norbergite to clinohumite in the humite group. They also increase with Fe²⁺ and Ti⁴⁺ and with (OH)⁻ substituting for F⁻. Dispersion: r > v.

Environment

Chondrodite is found largely 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 ...

contact zones between

carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and Dolomite (rock), dolomite rock (also kn ...

s and acidic or alkaline intrusions where

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

has been introduced by metasomatic processes. It is formed by the hydration of

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

, , and is stable over a range of temperatures and pressures that include those existing in a portion of the uppermost mantle.Physics and Chemistry of Minerals (1999) 26: 297-303 Titanian chondrodite has been found as inclusions in olivine in

West Greenland Kitaa, originally Vestgrønland ("West Greenland"), is a former administrative division of Greenland. It was by far the most populated of the divisions, being home to almost 90% of the total population. The divisions were de facto replaced by s ...

, where it is associated with clinohumite, olivine,

magnesite Magnesite is a mineral with the chemical formula ( magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Occurrence Magnesite occurs as veins in and an alteration product of ultramafic r ...

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

and Ni-Co-Pb sulfides in a matrix of antigorite.

References

Magnesium minerals Iron minerals Humite group Monoclinic minerals Minerals in space group 14 Luminescent minerals