Page 96 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS FELDSPATHOID FAMILY
In sedimentary rocks, apart from plagioclase occurring as detrital <OLOU R Colourless.
grains in many terrigeneous arenaceous rocks, albite may occur as an 0 11ABIT Usually euhedral crystals showing eight-sided sections.
authigenic mineral in some sandstones, forming during sedimentation. clfi.AVAGE Verypooron{110}.
Authigenic albite may be simply twinned but never shows lamellar KELIEF Low.
twinning. I II KINGENCE Isotropic to very low.
In addition to the normal feldspar group of minerals already discus- 1 WINN ING Repeated twinning on { 110} is always present and visible under crossed
sed, some feldspars containing more than 2% BaO are termed barium polars, as a type of cross hatching.
feldspars. The most common barium feldspar (BaA1 2 Si,0 8 ) , celsian, is 1 1111 M 11 A 1 URES At high temperatures, leucite can contain sodium in the structure, which
similar in almost every property to the feldspars, especially orthoclase, must be expelled at lower temperatures. Pseudo-leucite, a mixture of
except that it has greater Ris and a much higher density. This is rare and nepheline and feldspar, forms an intergrowth which completely replaces
tends to be found associated with manganese deposits and stratiform leucite in some rocks.
barite deposits. 111 c t tMMENCE In potassium-rich basic extrusive rocks such as leucite-basanite,
leucite-tephrite and leucitophyre, which are usually silica deficient.
Pseudo-leucite may occur in some alkali basic plutonic rocks but mainly
Feldspathoid family Tektosilicates occurs in extrusive igneous rocks. At subsolidus temperatures, such as
can be attained in plutonic intrusions, leucite breaks down to give
The group of minerals termed 'feldspathoids' include those minerals
nepheline and feldspar, which explains the absence of leucite and the
which have certain similarities with the feldspars, particularly in their presence of nepheline and feldspar assemblages in plutonic alkali rocks.
chemistry and structure. The main feldspathoid minerals given in detail
here are:
hexagonal
leucite KA1Si,0 6 I'Jih •line NaA1Si0 4
nepheline NaAlSiO. (K may replace Na up to 25 per cent) cia 0.838
soda lite Na 8Al 6Si 60 ,.CJ, or 6(NaA1Si0 4 ).2NaCl n 0 1.529-1.546
ne = 1.526-1.542
Other feldspathoid minerals include: a = o.oo3-o.oo5
Uniaxial -ve (crystal is length fast)
hauyne CaNa 6Al.Si.O, •. so. or 6(NaA1SiO.).CaSO. D = 2.56-2.66 H = 5lf2-6
with S replacing S0 4
nosean Na 8Al 6Si 60 , 4SO. or 6(NaA1Si0 4 ).Na,S0 4 ''"ouR Colourless.
II A Il l r Usually anhedral, occurring in the interstices between minerals. Occa-
cancrinite complex hydrated alkali aluminosilicate
sionally found as small exsolved 'blebs' within feldspars, particularly K
with CO,, S0 4 and Cl groups included
kalsilite KAlSiO. feldspars. Euhedral crystals have a hexagonal outline.
1 II AVAGE { 1010} imperfect prismatic cleavage, and poor basal { 0001} cleavage.
Rlt ll!l· Low.
The feldspathoids are all silica deficient compared with the feldspars,
1 11 K liON Nepheline may alter to zeolites such as natrolite Na,Al,Si,OI0.2H,O or
and their occurrence is restricted to undersaturated alkali igneous rocks.
Although analcime is a zeolite it is very closely associated with the analcime, and to feldspathoids such as sodalite e.g.
feldspathoids and has been included here after sodalite.
2NaA1Si0 4 + Si0 2 + 2H 2 0--+ Na,Al,Si 3 0 10 .2H 2 0 (natrolite)
NaA1Si0 4 + Si0 2 + H 2 0--+ NaA1Si 2 0 6 .H 2 0 (analcime)
tetragonal
Leucite KA1Si 2 0 6
(pseudo-cubic) cia 1.054
by the addition of silica, water and other volatiles (chlorine in the case of
n = 1.508- 1.511 sodalite). Nepheline commonly alters to cancrinite (a complex hydrated
a = o.oo1 silicate with sulphate, carbonate and chloride).
If uniaxial, leucite is +ve but sign is virtually impossible to determin • 11 11-INI•I N< I' Low, first order greys. Small inclusions occur within nepheline and give
because of twinning. the crystal a 'night sky' effect under crossed polars.
D = 2.47-2.50 H = 5%-6 11 INN IN<. Rare.
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