silicate. The compact variety of pyrophyllite is used for slate pencils and tailors' chalk (“ French chalk ”), and is carved by the Chinese into small images and ornaments of various kinds. Other soft compact minerals (steatite and pinite) used for these Chinese carvings are included with pyrophyllite under the terms agalmatolite and pagodite
Pyrophyllite occurs in schistose rocks, often associated with cyanite, of which it is an alteration product. Pale green foliated masses, very like talc in appearance, are found at Beresovsk near Ekaterinburg in the Urals, and at Zermatt in Switzerland. The most extensive deposits are in the Deep river region of North Carolina, where the compact variety is mined, and in South Carolina and Georgia.
PYROXENE, an important group of rock-forming minerals,
very similar in chemical composition and general characters
to the amphiboles (q.v.). Although crystallizing in three different
systems, they all possess distinct prismatic cleavages, the
angles between which are about 87° (the cleavage angle in the
amphiboles being 56°). They are metasilicates, but, as shown
in the following table, the composition varies widely in the
different species, with corresponding differences in the various
physical characters. The name pyroxene was originally given
by R. J. Haüy in 1796 to the black crystals of augite found in
the lavas of Vesuvius and Etna: he derived the name from the
Greek πῦρ (fire) and ξένος (a stranger), because he thought
that the crystals had been accidentally caught up by the lavas
which contained them. As a matter of fact, the pyroxenes
are, next to the felspars, the commonest constituents of igneous
rocks of almost all kinds, being especially characteristic of
those of basic composition. An igneous rock composed almost
wholly of pyroxene is known as a pyroxenite. Besides being
minerals of primary origin in igneous rocks. the pyroxenes are also
of frequent occurrence in metamorphic rocks, for example,
in crystalline limestones, being then of secondary origin.
At the present day the name pyroxene is used as a group name for all the minerals enumerated below, though sometimes it is also applied as a specific name to include the monoclinic members diopside, hedenbergite, schefferite and augite.
| Orthorhombic Series. | |
| Enstatite | MgSiO3. |
| Bronzite | (Mg, Fe)SiO3. |
| Hypersthene | (Fe, Mg)SiO3. |
| Monoclinic Series. | |
| Diopside | CaMg(SiO3)2. |
| Hedenbergite | CaFe(SiO3)2. |
| Schefferite | (Ca, Mg)(Fe, Mn)(SiO3)2 |
| Augite | Ca(Mg, Fe)(SiO3)2 with (Mg, Fe)(Al, Fe)2SiO6 |
| Acmite | NaFe‴(SiO3)2. |
| Spodumene | LiAl(SiO3)2 |
| Jadeite | NaAl(SiO3)2. |
| Wollastonite | CaSiO3 |
| Pectolite | HNaC32(SlO3)3. |
| Rosenbuschite | Na2Caa[(Si, Zr, Ti)O3]4. |
| Anorthic Series. | |
| Rhodonite | MnSiO3. |
| Babingtonite | (Ca, Fe, Mn)SiOa-Fe2‴(SiO3)3. |
| Hiortdahlite | (Ca, Na)2F[(Si, Zr)O2] |
For details respecting the special characters and modes of occurrence of most of these species reference may be made to the respective headings: others not so treated are briefly mentioned below. Hedenbergite, or calcium iron pyroxene, is a black mineral closely allied to diopside (q.v) and, owing to the isomorphous replacement of iron by magnesium, there is no sharp line of division between them. Schefferite, or manganese pyroxene, is a brown mineral found in the manganese mines of Sweden. Pectolite is a secondary mineral occurring as white masses with a radially fibrous structure in the veins and cavities of basic igneous rocks. Babingtonite is found as small black crystals on felspar in the granite of Baveno in Italy, and in the Haytor iron mine in Devonshire. Rosenbuschite, hiortdahlite, and some other rare members containing zirconium and fluorine, occur as accessory constituents in the nephelinesyenite of southern Norway.
PYROXENITE, a rock consisting essentially of minerals of the pyroxene group, such as augite and diallage, hypersthene, bronzite or enstatite. Names have been given to members of this group according to their component minerals, e.g. pyroxenite (augite), diallagite (diallage), hypersthenite (hypersthene), bronzitites (bronzite), websterite (diallage and hypersthene). Closely allied to this group are the hornblendites, consisting essentially of hornblende. The term perknite (Gr. περκνός, dark) has also been used to designate the whole series.
They are essentially of igneous origin, though some pyroxenites are included in the metamorphic complex of the Lewisian of Scotland; those pyroxene rocks which result from the contact alteration of impure limestones are described as pyroxene hornfelses (calc-silicate hornfelses). The pyroxenites are closely allied to the gabbros and norites, from which they differ by the absence of felspar, and to the peridotites, which are distinguished from them by containing olivine. This connexion is indicated also by their mode of occurrence, for they usually accompany masses of gabbro and peridotite and seldom are found by themselves. They are strictly plutonic and often very coarse-grained, containing individual crystals which may be several inches in length. The principal accessory minerals, in addition to olivine and felspar, are chromite and spinels, garnet, iron oxides, rutile, scapolite. They frequently occur in the form of dikes or segregations in gabbro and peridotite: e.g. in Shetland, Cortlandt on the Hudson river, North Carolina (websterite), Baltimore, New Zealand, and in Saxony. The component minerals often have a close resemblance to those of the surrounding rock. By decomposition the rocks consisting of pyroxene pass into serpentine, which sometimes preserve the original structures of the primary minerals, such as the lamination of hypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende is developed and various types of amphibolite and hornblende-schist are produced. Occasionally rocks rich in pyroxene are found as basic facies of nepheline syenite; a good example is provided by the melanite pyroxenites associated with borolanite (q.v.) at Ledbeg in Sutherlandshire. (J. S. F.)
PYRRHO OF ELIS (c. 360–270 B.C.), a Greek sceptic philosopher and founder of the school known as Pyrrhonism. Diogenes Laërtius (ix. 61), quoting from Apollodorus, says that he was at first a painter, and that pictures by him were in existence in the gymnasium at Elis. Later he' was diverted to philosophy by the works of Democritus, and became acquainted with the Megarian dialectic through Bryson, pupil of Stilpo.
With Anaxarchus, he went to the East in the train of Alexander, and studied in India under the Gymnosophists (q.v.) and under the Magi in Persia. From the Oriental philosophy he seems to have adopted a life of solitude. Returning to Elis, he lived in poor circumstances, but highly honoured by the Elians and also by the Athenians, who gave him the rights of citizenship.
His doctrines are known mainly through the satiric writings
(Σίλλοι) of his pupil Timon of Phlius (the Sillographer) The
main principle of his thought is expressed in the word acatalepsia,
which implies the impossibility of knowing things in their
own nature. Against every statement the contradictory
may be advanced with equal reason (ἰσοσθενεία τῶν λόγν).
Secondly, it is necessary in view of this fact to preserve an
attitude of intellectual suspense (évroxh), or, as Timon expressed
it, οὐδὲν μᾶλλον (i.e. no assertion more valid than another).
The same idea is expressed also by the terms eippexkia' (equilibrium)
and dqbarria. (refusal to speak, non-committal silence).
Thirdly, these results are applied to life in general. Pyrrho
concludes that, since nothing can be known, the only proper
attitude is imperturbability (ataraxia). The impossibility of
knowledge, even in regard to our own ignorance or doubt,
should induce the wise man to withdraw into himself, avoiding
the stress and emotion which belong to the contest of vain
imaginings. This drastic scepticism is the first and the most
thorough exposition of agnosticism in the history of thought.
Its ethical results may be compared with the ideal tranquillity
of the Stoics and the Epicureans. (For its relation to the New
Academy and to scepticism in general see Scepticism and
Megarian School of Philosophy.)
See histories of philosophy by Zeller, Erdmann, Ueberweg; Ritter and Preller, § 364; Waddington, Pyrrhon et le pyrrhonisme (1877); Zimmermann, Darstellung d. pyrrh. Phil. (1841) and Ueber Ursprung und Bedeutung d. pyrrh. Phil. (1843); Wachsmuth, De Timone Phliasio (1859).
PYRRHOTITE, a mineral species consisting of iron sulphide
and crystallizing in the hexagonal system. The formula is
FenSn+1 where n may vary from 5 to 16; usually it is Fe7S8 or
Fe11S12, the latter being also the composition of the artificially
prepared compound. Small amounts of nickel and cobalt
are often present.
