concave mirror onto a blackened foil
to which a thermopile is attached.
From the e.m.f. produced by the thermopile 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
C
H 2
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 phosphoric(v) acid.
pyrosilicate See silicate.
pyrosulphuric acid See disulphuric(vi) acid.
pyroxenes A group of ferromagnesian 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 silicon 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 important intermediate compound in metabolism, 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 X 1–p Y 1+p Z 2 O 6 ,
where X = Ca,Na; Y = Mg,Fe
2+ ,Mn,
Li,Al,Fe
3+ ,Ti; and Z = Si,Al.
Orthorhombic pyroxenes (orthopyroxenes), (Mg,Fe) 2 Si 2 O 6 , vary in
composition between the endmembers enstatite (Mg 2 Si 2 O 6 ) and
orthoferrosilite (Fe 2 Si 2 O 6 ). Monoclinic pyroxenes (clinopyroxenes),
the larger group, include:
diopside, CaMgSi 2 O 6 ;
hedenbergite, CaFe
2+ Si 2 O 6 ;
johannsenite, CaMnSi 2 O 6 ;
augite, (Ca,Mg,Fe,Ti,Al) 2 (Si,Al) 2 O 6 ;
aegirine, NaFe
3+
Si 2 O 6 ;
jadeite (see jade);
pigeonite (Mg,Fe
2+ ,Ca)(Mg,Fe
2+
)Si 2 O 6 .
pyrrole An organic nitrogencontaining compound that forms
part of the structure of *porphyrins.
pyrrolidine A saturated heterocyclic compound having one nitrogen atom in a Üve-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.
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to which a thermopile is attached.
From the e.m.f. produced by the thermopile 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
C
H 2
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 phosphoric(v) acid.
pyrosilicate See silicate.
pyrosulphuric acid See disulphuric(vi) acid.
pyroxenes A group of ferromagnesian 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 silicon 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 important intermediate compound in metabolism, 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 X 1–p Y 1+p Z 2 O 6 ,
where X = Ca,Na; Y = Mg,Fe
2+ ,Mn,
Li,Al,Fe
3+ ,Ti; and Z = Si,Al.
Orthorhombic pyroxenes (orthopyroxenes), (Mg,Fe) 2 Si 2 O 6 , vary in
composition between the endmembers enstatite (Mg 2 Si 2 O 6 ) and
orthoferrosilite (Fe 2 Si 2 O 6 ). Monoclinic pyroxenes (clinopyroxenes),
the larger group, include:
diopside, CaMgSi 2 O 6 ;
hedenbergite, CaFe
2+ Si 2 O 6 ;
johannsenite, CaMnSi 2 O 6 ;
augite, (Ca,Mg,Fe,Ti,Al) 2 (Si,Al) 2 O 6 ;
aegirine, NaFe
3+
Si 2 O 6 ;
jadeite (see jade);
pigeonite (Mg,Fe
2+ ,Ca)(Mg,Fe
2+
)Si 2 O 6 .
pyrrole An organic nitrogencontaining compound that forms
part of the structure of *porphyrins.
pyrrolidine A saturated heterocyclic compound having one nitrogen atom in a Üve-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.
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