Rutile is a common mineral in sand and one of the most important sources of titanium. The other important titanium-bearing mineral is ilmenite. It has very simple chemical composition (TiO2). So it is an oxide like ilmenite and magnetite. Crystals are usually elongated and typically deep reddish brown although that color is best seen only in very small crystals. Larger grains are almost opaque and have a metallic luster. Smaller grains have intense adamantine luster because of extremely high refractive index. This index measures how much light bends (or its velocity slows down) when it enters the crystal.
Concentrate of rutile from Pooncarie, New South Wales, Australia. The few transparent grains are zircon crystals. Width of view 4.2 mm.
It is quite stable in the weathering environment and may therefore occur in sand as single crystals or it could be a part of a cryptocrystalline aggregate of several titanium and iron oxides and hydroxides. Such an aggregate is called leucoxene. Leucoxene forms as a weathering product of ilmenite. Hence, rutile as a main component of leucoxene and ilmenite are often mined together as an ore of titanium. Rutile is sometimes concentrated enough to form a placer deposit. Most of this mineral is mined from such placers. It also occurs sometimes as fine needles in quartz or mica crystals.
Rutile occurs in many igneous (mostly plutonic) and metamorphic rocks (especially amphibolite, eclogite, and metamorphosed limestones) but usually as small crystals. Big crystals may grow in pegmatites or hydrothermal veins with quartz and apatite. Rutile grains in sand are also small but their deep red color is pretty distinctive. It is noticed best if the light source is below the microscope slide.
It has two polymorphs (same composition but different structure) – anatase and brookite, which have similar properties but are not so widespread.
The grains on the picture below are all smaller than 125 micrometers. I sieved the sand to remove the larger grains because grains as small as these need high magnification which makes the depth of focus very shallow. So the grains need to be almost uniform in size and the surface as planar as possible. Why did I chose smaller grains instead of larger ones? Because only the small grains or the edges of the bigger ones show the characteristic red color. Grains larger than a quarter of a millimeter are usually practically opaque.
Width of sample 5 cm.
Fine rutile is responsible for blue color in quartz crystals. Bergen, Norway. Width of sample 6 cm.