At present i'm not working on anything geologically relevant, but my masters project, which i finished November 2. 1998, involved working on some granitoid pegmatites in the area of Iveland (60 kilometers north of Kristianssand(Norway))
The Li-pegmatites is situated in an amfibolite body of precambrian age. Apparently the amfibolite was more rigid than the rest of the rocks in the area, when the area was metamorphosed in precambrian time, because all the pegmatites is concentrated in the amfibolite, and only verry few is found in the granitoid gneises in the area.
The Li-pegmatite contains a conspicuous horizontal layering, and the layering is made up by biotite single crystals with feldspar and quartz in between. The biotite crystals vary in size, and some of them reach up to 5 meters in length. Although the crystals are long, the individual crystals is also narrow, nowhere is the crystals more than 3 mm. thick. Each layer in the pegmatite is composed of (from the top and down): first a horizontal biotite crystal, (now mostly converted to chlorite) , thereafter a layer of vertical or near vertical biotite crystals with quartz and feldspar in between. As this is followed downwards the biotite diminishes and often (in the thick layers) the biotite crystals stops before the next layer begins. In the narrow layers the vertical biotite crystals extends all the way down to the next layer, and the only indication of a layering is the horizontal biotite.
No rare minerals have been found within the pegmatite, although the pegmatite is rich in large feldspar crystals. The mineralogy of the pegmatite is: Biotite, Feldspar (mostly Microcline, but also cvelandite and normal albite), primary as well as secondary muscovite and secondary chlorite and minor garnet.
A colection of minerals found within the Li-pegmatite. (The yellow coin is a danish 10kr) Note that the beryl-crystal is not from Li, and neither is the large muscovite crystal, although muscovite is pressent within the pegmatite
The model for the genesis of the Li-pegmatite involves sequential nucleation and growth of Biotite and quartz-feldspar. As the pegmatite forming magma had been emplaced, crystallization of anhydros minerals like quartz and feldspar commenced, producing an aplite near the roof of the pegmatite. as fluids (mostly water) became enriched within the melt, rapid crystallisation of quartz and feldspar became impossible, and the crystallisation of large crystals commenced. At one point the concentration of the biotite components became high enough that biotite became stable. At this point crystals which nucleated with the basal plane parallel to the crystallisation front grew in a fluid-enriched zone, inhibiting high growth velocities. This produced large horizontal crystals, whereas the crystals oriented vertical quickly emerged in less fluid-enriched areas of the melt. These crystals began to grow quick, and as the crystals grew, they depleted the melt in biotite components, thus making quartz and feldspar stable again. As the quartz-feldspar grew again a new fluid and biotite enriched zone developed again, repeating the sequence.
The means of identifying the different growth-velocities was done by TEM-investigations in which the polytypes of the biotite crystals was analyzed. In the horizontal biotite crystals only the polytypes 1M, 1Md and 3T was found, indicating slow growth under equilibrium, whereas in the vertical biotitre crystals the polytypes 1M, 1Md, 2M, 3T, 4Tc and 5Tc was found. This indicates rapid growth under nonequlibrium conditions, a fact underlined by the eutectic macroscopic crystal texture.
Different polytypes imaged in diffraction-mode by TEM (from left to right: 1M, 1Md, 2M, 3T, 4Tc, 5Tc)