Poudretteite

Poudretteite is an extremely rare mineral and gemstone that was first discovered as minute crystals in Mont St. Hilaire, Quebec, Canada, during the 1960s. The mineral was named for the Poudrette family because they operated a quarry in the Mont St. Hilaire area where poudretteite was originally found, and the quarry is currently owned by the United Kingdom based Salmon Mining Industries Inc. Poudretteite has a barely detectable radioactivity.

Poudretteite

General

Category Cyclosilicate

Formula KNa2B3Si12O30

IMA symbol Pou

Strunz classification 9.CM.05

Dana classification 63.02.01a.08

Crystal system Hexagonal

Crystal class Dihexagonal dipyramidal (6/mmm)

H-M symbol: (6/m 2/m 2/m)

Space group P6/mcc

Unit cell 1,221.72 ų

Identification

Color Colorless, Light pink

Crystal habit Roughly equant barrel-shaped prismatic crystals

Cleavage None

Fracture Conchoidal, splintery

Tenacity Brittle

Mohs scale hardness 5

Luster Vitreous (Glassy)

Streak White

Diaphaneity Transparent

Specific gravity 2.51

Density 2.51

Optical properties Uniaxial (+)

Refractive index 1.511 to 1.532

Birefringence 0.021

Pleochroism Colorless to pink

Etymology

Like many other minerals of the milarite group, poudretteite is very similar in its optical and physical properties to other, widespread minerals such as quartz, apatite or apophyllite. This is probably why it was overlooked for a long time. As early as the mid-1960s, M. Jacques Bradley collected poudretteite-bearing marble xenoliths in the syenite breccias of the Poudrette mine at Mont Saint-Hilaire in Québec, Canada. It took around 20 years before they were recognized and described as a new mineral. Joel D. Grice, T. Scott Ercit, Jerry van Velthuizen and Pete J. Dunn named the new member of the milarite group after the Poudrette family, who operate the quarry at the site.

It took another 14 years before this very rare mineral was recognized and described at another locality. In November 2000, an Italian gem dealer in Mogok, Myanmar, acquired a 3-carat, pinkish-violet, faceted stone from a local dealer who could not mineralogically classify it. The stone was sent to Eduard Josef Gübelin 's gem laboratory in Switzerland for identification, where it was identified as poudretteite. 

Classification

Since poudretteite was only recognized as an independent mineral in 1986, it is not yet listed in the 8th edition of Strunz's mineral classification, last revised in 1977.

In the Lapis mineral catalogue by Stefan Weiß, which out of consideration for private collectors and institutional collections still follows this old form of the system by Karl Hugo Strunz, the mineral was given the system and mineral number VIII/E.22-10. In the “Lapis systematics” this corresponds to the class of “silicates and germanates” and there to the department “ring silicates”, where poudretteite together with agakhanovite-(Y), almarudite, armenite, berezanskite, brannockite, chayesite, darapiosite, dusmatovite, eifelite, emeleusite, faizievite, friedrichbeckeite, klöchite, lipuit, merrihueite, milarite, oftedalite, osumilite, osumilite- (Mg), roedderite, shibkovite, sogdianite, sugilite, trattnerite, yagiite and yakovenchukite-(Y) form the “ milarite-osumilite group ” (VIII/E.22) with the structure of double six-membered rings [Si 12 O 30 ] 12− forms (as of 2018). 

The 9th edition of Strunz's mineral classification, last updated in 2009 by the International Mineralogical Association (IMA), also classifies poudretteite in the division of "ring silicates." This division is further subdivided according to the structure of the rings, so that the mineral can be found in the subdivision "[Si 6 O 18 ] 12− -six-ring double rings." In this division, it belongs to the "milarite group" with the system number 9.CM.05, along with agakhanovite-(Y), almarudite, armenite, berezanskite, brannockite, chayesite, eifelite, darapiosite, dusmatovite, friedrichbeckeite, klöchite, merrihueite, milarite, oftedalite, osumilite, osumilite-(Mg), roedderite, shibkovite, sogdianite, sugilite, trattnerite, and yagiite. 

The Strunz mineral classification (9th edition) is continued by the Hudson Institute of Mineralogy in the mineral database Mindat.org. Here, poudretteite belongs to the class "silicates and germanates" and the division of " cyclosilicates. " This division is further subdivided according to the number and multiplicity of the silicate rings, and poudretteite is listed in the subdivision "six- membered double rings" (Si 6 O 18) with the system number 9.CM, together with the previously listed minerals of the milarite group, the newly added minerals aluminosugilite and laurentthomasite, and the related mineral faizievite. 

The Dana classification of minerals, which is predominantly used in English-speaking countries, also classifies poudretteite in the class of "silicates and germanates," but in the more finely subdivided division of "ring silicates: condensed rings." Here, it is found in the " milarite-osumilite group (milarite-osumilite subgroup) " with the system number 63.02.01a within the subdivision "ring silicates: condensed, 6-membered rings."

Properties

Poudretteite forms isometric or prismatic crystals. It is usually colorless with a glass-like luster and may be pale pink due to trace levels of manganese and lithium. With a Mohs hardness of 5, poudretteite is as hard as apatite. 

Features

Poudretteite is a silicate with the chemical formula KNa₂B₃Si₁₂O30. It crystallizes in the hexagonal system. Its hardness on the Mohs scale is 5.

Crystal structure

Poudretteite crystallizes hexagonally in the structure of milarite in the space group P 6 /mcc (space group no. 192) with the lattice parameters a = 10.239(1) Å and c = 13.485(3) Å and two formula units per unit cell. 

The silicate 6-ring doublets (T1 position) are completely occupied by Si 4+, and B 3+ occupies exclusively the T2 tetrahedral position. The tetrahedral coordination of boron is unusual for minerals that were not formed under high pressure. Most often, boron is incorporated into trigonal-planar coordinated lattice positions, such as in tourmaline.

Poudretteite is the mineral of the milarite group with the highest charge in its tetrahedral framework. Consequently, the positions in the framework channels and cages are either empty or occupied by singly charged cations. Potassium + ions, as in almost all minerals of the milarite group, are located at the 12-coordinate C position. The 9-coordinate B position is empty, and the octahedrally coordinated A position is fully occupied by Na + ions. 

Deposits

Poudretteite forms contact metamorphically in marble that has been altered by alkali-rich magmatites or pegmatitic melts. Only two sites have been documented worldwide (2016).

At the type locality of Mont Saint-Hilaire in Quebec, Canada, poudretteite occurs in marble xenoliths in nepheline syenite along with pectolite, apophyllite, quartz, and aegirine.

In the vicinity of Mogok, Mandalay, Myanmar, syenitic magmas have infiltrated the calcareous sediments. In the skarn mineralizations at the contact of the syenitic melts with the surrounding marble, poudretteite occurs together with diopside, scapolite, spinel, corundum, andalusite, kornerupine, montebrasite, and hackmanite or with nepheline, taaffeite, jeremejevite, corundum, and chrysoberyl. 

Use

The very rare poudretteites from Myanmar reach gemstone quality in terms of transparency, color and size and are traded as gemstones.

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