Picrite
PICRITE (from Gr. iriKp6s, bitter, because these rocks are rich in magne'sia, a base which forms bitter salts), a rock belonging to the ultrabasic group, and consisting mainly of olivine and augite often with hornblende and biotite and a greater or less amount of plagioclase felspar. The picrites are of " hypabyssal " origin and in their natural occurrence are connected with dolerites (diabases and teschenites). The distinction between them and the peridotites, which have an essentially similar composition, is not easy to define, but the peridotites accompany the true plutonic rocks, such as gabbro, norite and pyroxenite, are often very coarsely crystalline, and form large bosses and laccolites, while the picrites usually are found in sills or intrusive sheets.
In hand specimens the picrites are dark green to black; the absence or scarcity of lath-shaped plagioclase felspars distinguishes them from diabases and they rarely have the lustremottling which is a characteristic of the peridotites. Since they contain much olivine they readily decompose, passing into deep green and brown incoherent masses in which are embedded rounded lumps of harder consistency. They have a high specific gravity (about 3-0) and may be distinctly magnetic, because they are rich in iron ores. Porphyritic structure is rare though occurring sometimes in the rocks known as picrite-porphy rites; the phenocrysts are olivine and augite. There is seldom any fine-grained or glassy groundmass, and the typical microstructure is holocrystalline, moderately fine grained and somewhat poikilitic. Olivine is abundant in rounded pale green crystals. It may form one half of the rock but rarely more than this. The augite is generally brown or reddish-brown, sometimes violet, and tends to enclose the olivine, yielding poecilitic aggregates. Brown hornblende often occurs as marginal growths' around the pyroxene, and may be so abundant as to replace augite to a large extent; rocks of this class are known as hornblende-picrites. Bright green or pale-green hornblende are less frequently present, and in many cases are really of secondary origin. Deep brown biotite is a frequent accessory mineral and both biotite and hornblende sometimes enclose olivine. A small amount of basic plagioclase occurs in many picrites; apatite, iron oxides, chromite and spinels are minor ingredients seldom altogether absent.
The minerals of picrites are very frequently decomposed. Serpentine partly or wholly replaces olivine, forming radiate fibrous masses which are green, yellow or red in microscopic sections. Sometimes hornblende (pilite), talc, chlorite and mica appear as secondary products after olivine. The augite passes into chlorite or into green fibrous or platy amphibole. Hornblende and biotite are often fresh when the other components are much altered. The felspar is rarely in good preservation but yields epidote, prehnite, sericite, kaolin; calcite and analcite are abundant in some weathered picrites.
Rocks of this type are well represented in Great Britain. In the central valley of Scotland several masses of picrite have been discovered, always in close association with olivine-diabase and teschenite. One of these forms the island of Inchcolm in. the Firth of Forth, another lies near Bathgate (in Linlithgowshire), and there are others at Aberdour (Fife), Ardrossan and Barnton (Midlothian). They belong to the great series of Carboniferous eruptive rocks ot the Scottish midland valley. These picrites are not known to be represented in England, but, on the other hand, there are Devonian picrites in Devon and Cornwall as basic members of the diabase and proterobase series of these counties. Some of them contain much augite like the picrite (often called palaeopicrite as being of palaeozoic age) at Menheniot Station in Cornwall and the picrite of Highweek near Newton Abbot in Devonshire. Others are hornblende-picrites like that of Cartuther near St Germans, Cornwall. Hornblende-picrite occurs also in the island of Sark and several beautiful examples have been described from Anglesey and from Penarfynnydd in North Wales and from Wicklow in Ireland. Picrites occur in several parts of Germany, notably in the Devonian rocks of the Fichtelgebirge and Nassau, where they accompany diabases and proterobases like those of Cornwall and Devonshire. In Silesia and Moravia picrites are found with teschenites like those of Central Scotland. In some of the continental picrites enstatite is present but is rare. In North America picrites occur among the igneous rocks on the Hudson river and in Alabama and Montana. (J- S. F.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
