Document 719xJKE0onVovV9MEzk78m4dV
AMPHIBOLE GROUP Orthorhombic, Monoclinic, Triclinic
Composition for the most part near that of a X metasilicate, RSiOa , with R * Ca, Mg, Fe chiefly, also Mn, H & s . Ks , H* Further -often' containing, aluminum and. ferric iron. The composition in the case of some anrahlboles is very complex with extensive replacements taking place between the different elements. It is impossible even to list here the different molecules that have been proposed to account for the variations in composition. Recently, however,-the' atomic structure of the amphiboles has been investigated by X-ray methods (see p. 533) sod formulas suggested that conform both to these facts and to the chemical composition, The type molecule as derived by Warren is found to have a.siliconoxygen ratio of Si^Oji instead of the metasilicate ratio of Si4.0!E.. He finds further that in one-half of trio unit cell
tne number of oxygen atoms or their equivalents is always twenty-four, that the number o f the magnesium atoms, or their equivalents la always five, .that"the- silicon atoms never exceed eight in number, and that a group containing calcium and alkali atoms may vary between two and three. His general formula conforming to these structural' reqiiirements is (:Ca,la,X,L i )E-g(Mg,F e 1*, Pel **,Al,Ti)c (Fer**,Al,Si)g(0,0H,F ). This formula has been modified by Berman and Larsen to the following: (Ca,Ma)sN a o-,Mg{Mg,Al)4 (Al,Si5sSi60g 2 {0,0H,F)B , in which Pe** and:P e { H may replace, Mg and ai, respectively, In the list given immediately below, however, simplified metasilicate formulas have been used in most cases as.giving the essential features of the compositions, of tugciiffrent; rae-mbqrs of the group. Fuller discussions of. thy compositions of each will be found in the succeeding pages.
ABTHOPHYLLITE
Orthorhombic Section (Mg,Fe)SlOa
a:b 0-5137:1
Monoclinic Section
AMPHIBOLE
: 1 ;
2. cummingtonite
3- G r u n e r i t e
4 . Tremolite
5- Actinolite
6 . Richterite
7 Hornblende
0 *5511
MgSiOs
(Mg,Fe)S10a
(Fe,Mg)SiO* ; CaMgaiSiQaJ*
.
Ca{Mg,Fe)g(SiOa )^
( Ha,Mg,Ca,Mn)Si0g
b sc
1 ; 0 -2938
73053'
Edenite Pargasite Common nornblende Basaltic hornblende
) Contain molecules approximating to the
) following in varying proportions.
) Ca(Mg,Fe)5Si40|; Caags{Al,Fe)eSia0|S| ) RaAi.(SiOa)e
^ r tSX'oi3.]? 2.
'
,
' GLAUCOPHANE
RIBBECKITE HASTINGS H E
ARPVEDSONITE
a :b sc
Na(Al,Pe){SiOa)a with
'
(Mg,Pe)SiOs
055 5 3. s 0.29 75
III
NaFe{S10g }a with FeSiOa 0-5^4-75 : 1 i 0.2295 7 6 0 10
CaaNa (F e ,M g ){A 1 ,F e }(O H }a (A 3 ^ i )nOBs
NasP e A l (OH)SI3 Oaa
0 -5^ 9 5 : 1 I 02975 75 1*4
Triellnic Section
`
AKNIGMATITE (F,Naa)(Si,fi)Oa.Na{Al,Fe)(SI08)a
Copied from - .Dana, E* S. and T* 1. Ford, A textbook of
5m i n e r a l o g y Jo h n W i l e y , N. X . i}.th e d , I 9 5 2 , p . ^ * *
0003778