Vlasovite

At temperatures below 29 °C the stable form of vlasovite is triclinic 1, space group P1. Above 29 °C the stable form is monoclinic 2/m.[3] Most sources simply give it as monoclinic 2/m, space group C2/c. The silicate part of the structure is a chain composed of rings of four SiO₄ tetrahedra linked by sharing a corner oxygen to form a chain of composition [Si₄O₁₁]⁶⁻. These chains of tetrahedra link together with zirconium, Zr, octahedra to form a framework with channels extending along [001], parallel to the c crystal axis. The channels contain the sodium, Na, atoms.[7]

The unit cell parameters are a = 11 Å, b = 10 Å, c = 8.5 Å and β = 100°, with 4 formula units per unit cell (Z = 4).
More accurate values are given by various sources as follows.

For the monoclinic cell:

• a = 10.98 Å, b = 10.00 Å, c = 8.52 Å, β = 100.40°[2][3][5][6]
• a = 11.0390(5) Å, b = 10.0980(4) Å, c = 8.5677(4) Å, β = 100.313(1)°[4][7]
• a = 11.063 Å, b = 10.15 Å, c = 8.60 Å, β = 100.3°[8]

For the triclinic cell:

• a = 10.96 Å, b = 10.01 Å, c = 8.53 Å, alpha = 89.70°, beta = 100.40°, gamma = 90.48°[3]

Vlasovite is colorless, pink, pale brown or silvery in reflected light and colourless in transmitted light.[6] The border zones of large grains are light brown due to dust-like inclusions.[2] It is transparent to translucent, with a white streak and greasy luster, vitreous to pearly on cleavage surfaces. The refractive indices are N_x = 1.607, N_y = 1.623 and N_z = 1.628,[2] similar to those of muscovite.

Vlasovite is not fluorescent, but altered portions of the material fluoresce orange-yellow under both long-wave and short-wave ultra-violet illumination.[2]

Crystals have been found up to 15 cm long, but vlasovite occurs more commonly as irregularly shaped grains and aggregates.[6] It shows distinct cleavage parallel to the plane containing the a and c crystal axes, and this plane can also be a twin plane, although twinning is uncommon. Vlasovite is a brittle mineral, with an irregular to conchoidal fracture, hardness 6 and specific gravity 2.97.[2] It is nearly insoluble in hydrochloric acid HCl and nitric acid HNO₃, but it dissolves easily in a mixture of hydrofluoric acid, HF, and sulfuric acid H₂SO₄.[2]

Vlasovite is a late phase in nepheline syenite and syenitic pegmatites. At Ascension Island it is found in miarolitic cavities in ejected blocks of peralkaline granite (Peralkaline rocks are deficient in aluminium but have sodium and potassium in excess of the amount needed to form feldspar).[5][6] At the Lovozero Massif in Russia it is formed in areas of rock rich in microcline and albite in the contact zone between pegmatites and fenites (metasomatic rocks composed of alkaline feldspar, sodic pyroxene and alkaline amphibole), by the replacement of eudialyte. Associated minerals include arfvedsonite, aegirine, apatite and fluorite, in addition to microcline, albite and eudialyte.[2]

The type locality is the Vavnbed Mountain, Lovozero Massif, Kola Peninsula, Murmanskaja Oblast', Northern Region, Russia, which is also a good locality for zircon crystals. Other localities for vlasovite include the volcanic Ascension Island, in the South Atlantic Ocean, the Kipawa Complex, Villedieu Township, Quebec and the Strange Lake Complex in Labrador. Type material is conserved for reference in two locations in Russia, the Saint Petersburg Mining Institute and the Fersman Mineralogical Museum in Moscow, and also in France, at the National School of Mines in Paris.[6]

JMol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?id=06176