Geology Reference
In-Depth Information
Chapter 2
Mineralogy
Potassium-rich silica-undersaturated ma
c igneous rocks are characterised by the
presence of leucite, K-feldspar, clinopyroxene and Fe-Ti
Ti oxides. Plagioclase
feldspar commonly appears in tephrites, tephritic phonolites, and phonolitic teph-
rites. In sodium-rich potassic rocks such as basanites, forsteritic olivine appears
along with nepheline. Whereas in lamproites and lamprophyres, phlogopite is a
very common mineral, K-richterite may also appear along with the above-men-
tioned minerals (Wagner and Velde 1986). In mildly sodium-rich potassic rocks,
nepheline-kalsilite solid solutions are commonly observed. Kalsilite is particularly
present in mafuritic rocks. Melilite, whenever present, is formed by reaction
between clinopyroxene and nepheline (Bowen 1922; Schairer et al. 1962); ha
-
yne
and sodalites may be accompanying phases along with them analcite occurs as an
alteration product of leucite. Pseudo-leucite (an intergrowth of nepheline and K-
feldspar) may occur instead of leucite in rocks emplaced before the Tertiary period.
Priderite and wadeite are two accessory phases commonly associated with lam-
proitic rocks. Mg-spinel (MgAl
2
O
4
), perovskite, titanomagnetite, pseudo-brookite,
ilmenite, chromite, a roedderite-like mineral and apatite are other accessory phases.
Recently, Mihajlovic et al. (2002) have reported the occurence of two accessory
minerals rondor
ü
(Na)
2
(Mn, Fe, Mg)
2
(Be,
Al)
3
[Si
12
O
32
], and a iron-rich wadalite, Ca
12
[(Al
8
Si
4
Fe
2
)O
32
]Cl
6
, from the Beller-
berg (Bellberg) volcano, Eifel, Germany. Carbonates (a solid solution of CaCO
3
,
MgCO
3
and FeCO
3
) occur quite often as groundmass phases. Melanite garnets are
also rarely observed.
te, Ca
8
Mg[SiO
4
]
4
Cl
2
, and almarudite, K
□
2.1 Leucite
This mineral has a cubic symmetry at temperatures higher than 630
C, but below
this temperature, it becomes tetragonal. Most leucite grains therefore, show repe-
ated twinning on the (110) crystallographic plane, which is related to cubic-
tetragonal inversion. The heat capacity measurements indicate that the transition is
continuous and of second order (Lange et al. 1986). Gatta et al. (2008) studied
°
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