Wöhlerite

Wöhlerite
General
Category	Mineral
Formula (repeating unit)	NaCa2(Zr,Nb)(Si2O7)(O,OH,F)2
IMA symbol	Wöh
Strunz classification	9.BE.17
Dana classification	56.2.4.5
Crystal system	Monoclinic
Crystal class	H-M Symbol: 2 Sphenoidal
Space group	P21
Unit cell	764.21
Identification
Color	Honey yellow, wine yellow to sulfur yellow, light to dark yellow, brown, gray
Twinning	Twin planes common on {010}
Cleavage	Distinct on {010} Poor on {100}, {110}
Fracture	Irregular, uneven, splintery
Tenacity	Brittle
Mohs scale hardness	5.5 - 6
Luster	Vitreous
Streak	Pale yellow
Diaphaneity	Transparent, translucent
Specific gravity	3.40 - 3.44
Optical properties	Biaxial (-)
Refractive index	nα = 1.700 - 1.705 nβ = 1.716 - 1.720 nγ = 1.726 - 1.728
Birefringence	0.026
Pleochroism	Weak
2V angle	Measured: 70°- 77° Calculated: 70° to 76°
Dispersion	Weak r > v
Ultraviolet fluorescence	None

Wöhlerite, also known as wöehlerite is a member of the amphibole supergroup, and the wöhlerite subgroup within it. It was named after German chemist Friedrich Wöhler. It was first described by Scheerer in 1843, but the crystal structure was later solved by Mellino & Merlino in 1979. Once approved, it was grandfathered by the IMA.

Properties

Wöhlerite shows pleochroic attributes, which is an optical phenomenon. Depending on which axis the mineral is being viewed, it would appear as if it is changing colors. Looking at the mineral from the x and y axis, it appears as a nearly colorless to pale yellow one, while viewing it on the z axis, it takes up a wine-yellow color. It is a granular mineral, and has prismatic crystals that can grow up to 3 cm. It is thick and tabular on {100}. Brøgger listed these forms to be the more frequent ones are {210}, {130}, {120}, {110}, {101}, {100}, {011}, {001}, {-101} and {-111} in 1890, although other forms observed include {720}, {311}, {121}, {111}, {021}, {012}, {010}, {-121}, {-161}, {-201}, {-211}, {-212} and {-221}. It has a paragenetic mode of syenites. Some of the associated minerals include zircon, microcline, nepheline, spreustein, and the pyrochlore group. It is an accessory mineral in nepheline syenites, and in fenites it is associated with alkaline intrusives. In late phase it can be found in alkalic pegmatites, and it can also be found in carbonatites. It mainly consists of oxygen (35.22%) and calcium (19.26%) but otherwise includes silicon (14.21%), zirconium (11.54%), niobium (9.40%), sodium (5.82%), fluorine (2.88%) and iron (1.41%). The fracture of it is brittle and splintery, meaning that the brittle fracture leaves splintery crystal fragments. Since it's granular, it occurs in matrix in anhedral to subhedral crystals. It's a prismatic and tabular mineral as mentioned above, meaning crystal shape is typically a slender prism, and its dimensions are thin in one direction. Upon inspecting it under both long wave and short wave ultraviolet light, the mineral doesn't show luminescent properties. It is also not radioactive.

Formation and mining

The type locality is Løvøya, Norway. It was first described from several syenite pegmatites by Scheerer, and although the type locality is unknown, the island became listed as the type locality since it was mentioned by Scheerer. Although it is described for mainly appearing in alkalic rocks, there are reports of wöhlerite's carbonatite occurrence. This rare mineral occurs in calcite-silicate rocks with varying composition. The composition of the host rocks vary due to their heterogeneous nature. The mixed rocks form the complex's western margin of the central sövitic carbonatite intrusion. They are representing the intrusion's magmatic border facies.