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Nambulite
Nambulite
	Gem quality nambulite from the Kombat Mine near Otavi, northern Namibia (size: 0.7 × 0.6 × 0.4 cm)
General
Category	Inosilicate
Formula; (repeating unit)	(Li,Na)Mn4Si5O14(OH)
IMA symbol	Nbl
Strunz classification	9.DK.05
Crystal system	Triclinic
Crystal class	Pinacoidal (1) ; (same H-M symbol)
Space group	P1
Unit cell	a = 7.621, b = 11.761; c = 6.731 Å; α = 92.77°; β = 95.08°, γ = 106.87°; Z = 2
Identification
Formula mass	612.13 g/mol
Color	Reddish orange brown
Crystal habit	Prismatic
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 (+)
Refractive index	nα = 1.707 nβ = 1.710 nγ = 1.730
Birefringence	δ = 0.023
Pleochroism	Weak
2V angle	(calculated) 44°, (measured) 30°
Dispersion	r > v weak
Other characteristics	the Li analogue of natronambulite
References

Nambulite is a lithium bearing manganese silicate mineral with the chemical formula (Li,Na)Mn₄Si₅O₁₄(OH).^[2] It is named after the mineralogist, Matsuo Nambu (born 1917) of Tohoko University, Japan, who is known for his research in manganese minerals.^[6] The mineral was first discovered in the Funakozawa Mine of northeastern Japan, a metasedimentary manganese ore.^[2]

Nambulite is formed from the reaction between a hydrothermal solution and rhodonite, and commonly creates veins in the host rock.^[7] Other than a collector's gem, however, it has little economic value.

It belongs to the triclinic-pinacoidal crystal system (or triclinic-normal),^[3] meaning that it 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).^[8] Its 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]^[3] The index of refraction (RI) can be defined as n = c_air/c_mineral, 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).^[9]

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.^[9] The mineral exhibits slight pleochroism.^[5] 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.^[9]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ ^a ^b ^c ^d Nambulite, Mindat.org
^ ^a ^b ^c Nambulite Mineral Data. http://webmineral.com/data/Nambulite.shtml
^ Thomas, Arthur. (2008) Gemstones: Properties, Identification and Use. London. New Holland Publishers (UK) Ltd.
^ ^a ^b Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (2005). "Nambulite" (PDF). Handbook of Mineralogy. Mineral Data Publishing. Retrieved 14 March 2022.
^ Yoshii, M.; Aoki, Y.; Maeda, K. (1972). "Nambulite, a new lithium- and sodium-bearing manganese silicate from the Funakozawa mine, northeastern Japan". Mineralogical Journal. 7 (1): 29–44. Bibcode:1972MinJ....7...29Y. doi:10.2465/minerj1953.7.29.
^ Mukhopadhyay, Subir; Das, Kaushik; Fukuoka, Masato (2005). "Nambulite, (Li,Na)Mn4Si5O14(OH), in the Sausar Group of rocks in Central India". Journal of Mineralogical and Petrological Sciences. 100 (1): 26–30. Bibcode:2005JMPeS.100...26M. doi:10.2465/jmps.100.26. S2CID 140684834.
^ Dyar, M. Darby; Gunter, Mickey E.; Tasa, Dennis (ill.) (2008). Mineralogy and optical mineralogy. Chantilly, VA: Mineralogical Society of America. pp. 77–78. ISBN 978-1-946850-02-7.
^ ^a ^b ^c Klein, Cornelis; Dutrow, Barbara (2007). The 23rd edition of the manual of mineral science : (after James D. Dana) (23rd ed.). Hoboken, N.J.: J. Wiley. pp. 289–296. ISBN 978-0471721574.