SPHENE— PREHNITE 421 



SPHENE. 



Sphene or titanite is a combined silicate and titanate of calcium having the formula 

 CaO.SiOg.TiOg. The crystals belong to the monoclinic system, and are frequently finely 

 developed. Sphene of a brown or yellow colour, opaque and unsuitable for cutting as a 

 gem, occurs embedded in many silicate-rocks ; while attached to the walls of crevices in the 

 same rocks are found crystals of the same mineral, which are frequently very clear and 

 transparent. These two varieties are sometimes respectively distinguished as titanite and 

 as sphene : the latter alone is cut as a gem, and that only when perfectly transparent and 

 of fine colour. Sphene is usually green, but may be yellow, brown, or even red : it is always 

 distinctly dichroic. There are many stones which sphene of a green colour may resemble in 

 appearance, such, for example, as chrysolite, idocrase, demantoid, and chrysoberyl. It never 

 resembles emerald, however, since the colour has always a tinge of yellow and is never very 

 deep and intense. Yellow sphene often resembles light yellow topaz in colour. The 

 hardness of the mineral is low : H = 5^. Its specific gravity is somewhat high, ranging 

 from 3"35 to 3'45 ; in pure methylene iodide, therefore, the mineral sinks. 



Fine crystals, suitable for cutting, are found in ci-evices in gneisses and schists at various 

 places in the Alps, especially in the Pfitschthal and the Zillerthal in the Tyrol, and also in 

 the Swiss Alps. In America specially large and fine crystals occur at Bridgewater in Bucks 

 County, Pennsylvania ; these are often over an inch in length, and from those which are 

 perfectly transparent fine stones, varying from 10 to 20 carats in weight, can be cut. 

 Sphene is used as a gem to only a limited extent, and the price is always low. 



PREHNITE. 



This mineral has a fine green colour similar to that of chrysolite, and is therefore 

 sometimes cut as a gem ; but it is used for this purpose only to a very limited extent. 

 Prehnite is a silicate of calcium and aluminium containing a small amount of water. It 

 sometimes crystallises in tabular forms belonging to the rhombic system, but occm-s most 

 frequently as crystal aggregates which take the form of nodular, botryoidal masses with a 

 radially fibrous structure. The mineral occurs, sometimes in masses of considerable size, in 

 the amygdaloidal cavities of older volcanic rocks, such as basalt. It is met with at several 

 places in the Alps, for example, at the Seisser-Alp in the Fassathal, Tyrol, at St. Gotthard, 

 and elsewhere. Prehnite was first discovered at the Cape of Good Hope by Colonel Prehn, 

 the Governor of the Colony, in the latter part of the eighteenth century ; hence the names 

 prehnite and " Cape chrysolite." The mineral is also found in the Lake Superior copper 

 region and at many other places in North America, specially fine specimens being met with 

 at Bergen Hill and Paterson in New Jersey. 



The hardness of prehnite is rather over 6 ; the specific gravity ranges from 2'8 to 3'0. 

 The mineral is usually translucent, rarely transparent ; it has a vitreous lustre, and may be 

 colourless, yellow, or green. The green variety is the only one suitable for use as a gem ; 



