Dictionary of Chemistry [6th Ed.]

concave mirror onto a blackened foil

to which a thermopile is attached.

From the e.m.f. produced by the ther-

mopile the temperature of the source

can be calculated.

pyrones Compounds containing a

six-membered ring system having an

oxygen hetero atom and a carbonyl

group attached to the ring. There are

two forms depending on whether the

carbonyl is in the 1 or 3 position. The

pyrone ring system occurs in many

naturally occurring compounds.

pyrones 446

p

O O O

O

Pyrones

N

2 CH

C

H 2

C

H 2

CH 2

Pyrrolidine

pyrophoric Igniting spontaneously

in air. Pyrophoric alloys are alloys

that give sparks when struck. See

misch metal.

pyrophosphoric acid See phos-

phoric(v) acid.

pyrosilicate See silicate.

pyrosulphuric acid See disul-

phuric(vi) acid.

pyroxenes A group of ferromagne-

sian rock-forming silicate minerals.

They are common in basic igneous

rocks but may also be developed by

metamorphic processes in gneisses,

schists, and marbles. Pyroxenes have

a complex crystal chemistry; they are

composed of continuous chains of sil-

icon and oxygen atoms linked by a

variety of other elements. They are

related to the *amphiboles, from

which they differ in cleavage angles.

pyruvic acid (2-oxopropanoic acid)

A colourless liquid organic acid,

CH 3 COCOOH. Pyruvate is an impor-

tant intermediate compound in me-

tabolism, being produced during

*glycolysis and converted to acetyl

coenzyme A, required for the *Krebs

cycle. Under anaerobic conditions

pyruvate is converted to lactate or

ethanol.

The general formula is X1–pY1+pZ 2 O 6 ,

where X = Ca,Na; Y = Mg,Fe2+,Mn,

Li,Al,Fe3+,Ti; and Z = Si,Al.

Orthorhombic pyroxenes (ortho-

pyroxenes), (Mg,Fe) 2 Si 2 O 6 , vary in

composition between the end-

members enstatite (Mg 2 Si 2 O 6 ) and

orthoferrosilite (Fe 2 Si 2 O 6 ). Mono-

clinic pyroxenes (clinopyroxenes),

the larger group, include:

diopside, CaMgSi 2 O 6 ;

hedenbergite, CaFe2+Si 2 O 6 ;

johannsenite, CaMnSi 2 O 6 ;

augite, (Ca,Mg,Fe,Ti,Al) 2 (Si,Al) 2 O 6 ;

aegirine, NaFe3+Si 2 O 6 ;

jadeite (see jade);

pigeonite (Mg,Fe2+,Ca)(Mg,Fe2+)Si 2 O 6.

pyrrole An organic nitrogen-

containing compound that forms

part of the structure of *porphyrins.

pyrrolidine A saturated hetero-

cyclic compound having one nitro-

gen atom in aÜve-membered ring,

C 4 H 9 N; r.d. 0.87; m.p. –63°C b.p.

87 °C. It is found in certain plants and

the ring structure is present in many

alkaloids.