Nambulite
Encyclopedia
The general formula for nambulite is (Li,Na)Mn4Si5O14(OH). It is named after the mineralogist, Professor Matsuo Nambu (1917- ) of Tohoko University, Japan, who is known for his research in manganese minerals. The mineral was first discovered in the Funakozawa Mine, northeastern Japan, a metasedimentary manganese ore.
Nambulite is formed from the reaction between a hydrothermal solution and rhodonite
, and commonly creates veins in the host rock. Other than a collector’s gem, however, it has little economic value.
It belongs to the crystal system Triclinic-Pinacoidal
(or Triclinic-Normal), meaning that is has three axes of unequal length (a, b, c), all intersecting at oblique angles with each other (none of the angles are equal to 90°). It belongs to the crystal class , meaning that any point on the crystal that is rotated 360° and then completely inverted will meet with an equal (but opposite) point on the crystal (see centrosymmetry
). It’s space group is P .
The three axes (a, b, c) have different indices of refraction, na=1.707, nb=1.710, nc=1.730. The index of refraction (RI) can be defined as n = cair/cmineral, where “n” is the index of refraction and “c” is the speed of light. The maximum birefringence
is .023, the difference between the highest (nc=1.730) and lowest (na=1.707) indices of refraction within the mineral.
In a medium with an index of refraction equaling 1.53, Nambulite has a calculated relief of 1.71-1.73, giving it a moderate to high relief. Relief is a measure of the difference between the index of refraction of the mineral and that of the medium (often Canada balsam or other epoxy with an RI of around 1.53-1.54).
Nambulite is an anisotropic crystal, where the velocity of light that passes through the crystal varies depending on the crystallographic direction. In contrast, an isotropic crystal includes all isometric
crystals, and the velocity of light is equal in all directions. The mineral exhibits slight pleochroism, likely shades of pink to clear due to the presence of lithium. Pleochroism
is an optical property observed when the mineral is viewed under the microscope in plane polarized light, and when it the stage of the microscope is rotated the observed colors change. The color change is due to different wavelengths being absorbed in different directions, and the color of the mineral depends on the crystallographic orientation.
Nambulite is formed from the reaction between a hydrothermal solution and rhodonite
Rhodonite
Rhodonite is a manganese inosilicate, SiO3 and member of the pyroxenoid group of minerals, crystallizing in the triclinic system...
, and commonly creates veins in the host rock. Other than a collector’s gem, however, it has little economic value.
It belongs to the crystal system Triclinic-Pinacoidal
Triclinic crystal system
thumb|180px|Triclinic In crystallography, the triclinic crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system...
(or Triclinic-Normal), meaning that is has three axes of unequal length (a, b, c), all intersecting at oblique angles with each other (none of the angles are equal to 90°). It belongs to the crystal class , meaning that any point on the crystal that is rotated 360° and then completely inverted will meet with an equal (but opposite) point on the crystal (see centrosymmetry
Centrosymmetry
The term centrosymmetric, as generally used in crystallography, refers to a space group which contains an inversion center as one of its symmetry elements. In such a space group, for every point in the unit cell there is an indistinguishable point...
). It’s space group is P .
The three axes (a, b, c) have different indices of refraction, na=1.707, nb=1.710, nc=1.730. The index of refraction (RI) can be defined as n = cair/cmineral, where “n” is the index of refraction and “c” is the speed of light. The maximum birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
is .023, the difference between the highest (nc=1.730) and lowest (na=1.707) indices of refraction within the mineral.
In a medium with an index of refraction equaling 1.53, Nambulite has a calculated relief of 1.71-1.73, giving it a moderate to high relief. Relief is a measure of the difference between the index of refraction of the mineral and that of the medium (often Canada balsam or other epoxy with an RI of around 1.53-1.54).
Nambulite is an anisotropic crystal, where the velocity of light that passes through the crystal varies depending on the crystallographic direction. In contrast, an isotropic crystal includes all isometric
Isometric
The term isometric comes from the Greek for "having equal measurement".isometric may mean:* Isometric projection , a method for the visual representation of three-dimensional objects in two dimensions; a form of orthographic projection, or more specifically, an axonometric projection.* Isometry and...
crystals, and the velocity of light is equal in all directions. The mineral exhibits slight pleochroism, likely shades of pink to clear due to the presence of lithium. Pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
is an optical property observed when the mineral is viewed under the microscope in plane polarized light, and when it the stage of the microscope is rotated the observed colors change. The color change is due to different wavelengths being absorbed in different directions, and the color of the mineral depends on the crystallographic orientation.