Nambulite

Nambulite
Gem quality nambulite from the Kombat Mine, Grootfontein District of Namibia (size: 0.7 x 0.6 x 0.4 cm)
General
Category	Inosilicate
Chemical formula	(Li,Na)Mn4Si5O14(OH)
Crystal symmetry	Triclinic pinacoidal, P 1
Unit cell	a = 7.621 Å, b = 11.761 Å, c = 6.731 Å; alpha = 92.77°, beta = 95.08°, gamma = 106.87°; Z = 2
Identification
Molar mass	612.13 g
Color	Reddish orange brown
Crystal habit	Prismatic
Crystal system	Triclinic
Twinning	infrequent
Cleavage	{001} Perfect, {100} Distinct, {010} Distinct
Tenacity	Brittle
Mohs scale hardness	6.5
Luster	Vitreous - Adamantine
Streak	Pale yellow
Diaphaneity	Transparent to Translucent
Specific gravity	3.51
Optical properties	Biaxial (+)
Birefringence	δ = 0.023
Pleochroism	Weak
2V angle	(Calculated) 44°, (measured) 30°
Dispersion	r > v weak
Other characteristics	the Li analogue of Natronambulite references = [1][2][3][4]

The general formula for nambulite is (Li,Na)Mn₄Si₅O₁₄(OH).^[1] It is named after the mineralogist, Professor Matsuo Nambu (1917- ) of Tohoko University, Japan, who is known for his research in manganese minerals.^[5] The mineral was first discovered in the Funakozawa Mine, northeastern Japan, a metasedimentary manganese ore. ^[1]

Nambulite is formed from the reaction between a hydrothermal solution and rhodonite, and commonly creates veins in the host rock.^[6] Other than a collector’s gem, however, it has little economic value.

It belongs to the crystal system Triclinic-Pinacoidal (or Triclinic-Normal)^[2], 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 1, 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).^[7] It’s space group is P 1.

The three axes (a, b, c) have different indices of refraction, n_a=1.707, n_b=1.710, n_c=1.730. ^{[2] [1]} 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 (n_c=1.730) and lowest (n_a=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). ^[8]

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. ^[8] The mineral exhibits slight pleochroism, likely shades of pink to clear due to the presence of lithium.^[4] 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.^[8]

References

1. ^ ^{a b c d} Nambulite. http://www.mindat.org/min-2835.html
2. ^ ^{a b c} Nambulite Mineral Data. http://webmineral.com/data/Nambulite.shtml
3. ^ Thomas, Arthur. (2008) Gemstones: Properties, Identification and Use. London. New Holland Publishers (UK) Ltd.
4. ^ ^{a b} Hanfbook of Mineralogy
5. ^ M. Yoshii, Y. Aoki, K. Maeda. 1972. Nambulite, a new lithium- and sodium-bearing manganese silicate from the Funakozawa mine, northeastern Japan. 2001 Mineral Data Publishing, version 1.2. [1]
6. ^ Nambulite, (Li,Na)Mn4Si5O14(OH), in the Sausar Group of rocks in Central India Journal of Mineralogical and Petrological Sciences, Volume 100, page 26