2. Right: A closer view of ice, in the form of icicles in the Adirondack Mountains of northern New York state, U.S.A. The pale yellowish colour in the ice is most probably due to the build-up of surface-layer impurities, such as ferric iron oxide (from peatland waters) and organic matter. View in January 1997.
"Rock of the Month # 38, posted August 2004"
Most people would probably not think of ice as a mineral. However, many of us have seen magnified images of delicate snow flakes, and the typical six-sided form, even though much embellished by finer structures, is evidence of internal order. The basic definition of a mineral is a material, composed of one or more chemical elements, with a definite crystal structure and a chemical composition which is either fixed or variable within identifiable limits. On this basis, ice is indeed a mineral: it is of hexagonal structure, of fundamental composition H2O, transparent and colourless, with Mohs hardness 1.5 and calculated specific gravity 0.917. The modest (8 percent) difference in density between ice and water has numerous natural and practical implications, from the behaviour of ice floes and icebergs to the cracking of frozen water pipes in winter. The crystal form influences the optical properties of ice particles, which is turn are responsible for unusual weather phenomena such as sundogs and solar pillars.
The wonderful variety of ice crystal habits caught the eye of amateur scientist and Vermont farmer Wilson Alwyn Bentley (1865-1931). He took superb photographs of thousands of snowflakes, 2,453 examples of which were published, after his death, by physicist William J. Humphreys (Bentley and Humphreys, 1931; Anon, 2004).
The very imperfection of ice crystals en masse is comparable to rounded deposits of calcite (travertine) or silica (sinters in hot springs). Ice can also be found in fibrous honeycombs on the undersurfaces of ice formed on a river in a cold winter, and in elongate blobs and tendrils on top of wind-blasted mountain peaks. Stressful environments may produce similarly deformed crystal habits in fibrous and platy minerals deposited in veins and shear zones.
One reason why ice may not at first seem like a mineral is its evident instability with respect to temperature: but this is merely an expression of its low melting point: native mercury is another, fortunately far rarer example of a mineral which exists as a liquid at room temperature and atmospheric pressure.
Ice may accumulate and then persist for long periods. Lake Vostok is a very large lake, 14,000 km2 in area and up to 670 m deep. Its unfamiliarity is the result of its location, beneath roughly 3 km (2 miles) of the ice of the Antarctic ice cap. Elements of the ice hosting this deeply-buried, frigid water body may have gathered for something like 1 million years (Priscu et al., 1999). The age of the basal ice near bedrock in the region of the South Pole is estimated at circa 165,000 years (Price et al., 2000; Siegert and Hodgkins, 2000).
Our familiar terrestrial water-ice occurs in hexagonal form and tends to expel impurities. At lower temperatures, ice in comets may occur partly in a cubic structure or polymorph, while in interstellar space water may occur as an amorphous ice, considered favourable for the accumulation and development of organic molecules, possible precursors of life (Blake and Jenniskens, 2001).
ANON (2004) Flake. Walrus 1 no.10, 94-95, December. For further information on Bentley's amazing accomplishment, and to see an archive of his photomicrographs, visit The Bentley Snow Crystal Collection. Another site, http://www.snowcrystals.com/, has all manner of illustrations, information on snowflake physics, and photographic tips. This site is created by Kenneth G. Libbrecht of Caltech, and is full of information, with details on his seven-or-more illustrated books on the beauty and science of snow crystals (qv).
BENTLEY,WA and HUMPHREYS,WJ (1931) Snow Crystals. Dover Publications, Inc., New York, 227pp., republished in 1962 as a Dover edition.
BLAKE,DF and JENNISKENS,P (2001) The ice of life. Scientific American 285 no.2, 44-51, August.
LIBBRECHT,K (2004) The Little Book of Snowflakes. Voyageur Press, St. Paul, MN, 92pp.
LIBBRECHT,K (2006) Ken Libbrecht's Field Guide to Snowflakes. Voyageur Press, St. Paul, MN, 112pp.
LIBBRECHT,K and RASMUSSEN,P (2003) The Snowflake: Winter's Secret Beauty. Voyageur Press, Inc., Stillwater, MN, 112pp.
PRICE,PB, WOSCHNAGG,K and CHIRKIN,D (2000) Age vs depth of glacial ice at South Pole. Geophys.Res.Letts. 27, 2129-2132.
PRISCU,JC et al. (1999) Geomicrobiology of subglacial ice above Lake Vostok, Antarctica. Science 286, 2141-2144, 03 December.
SIEGERT,MJ and HODGKINS,R (2000) A stratigraphic link across 1100 km of the Antarctic ice sheet between the Vostok ice-core site and Titan Dome (near South Pole). Geophys.Res.Letts. 27, 2133-2136.
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