Igneous petrology EOSC 321

Laboratory 9:

Alkaline Rocks

Duration: This lab is to be completed within the lab period. The results will be evaluated and returned to you in the following lab period.

Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in Thin Section by Perkins and Henke), c). Handouts on the classification of alkaline rocks and on conoscopy.

Introduction:

Alkaline rocks are generally considered to have more alkalis than can be accommodated by feldspars alone. The excess alkalis then appear in feldspathoids, sodic pyroxenes/amphiboles, or other alkali-rich phases. Alkaline rocks are deficient in SiO2 with respect to NaO, K2O, and CaO to the extent that they become critically undersaturated in SiO2, and nepheline or acmite appears in the norm. Alkaline rocks are extremely diverse; they constitute less than 1% of the total volume of igneous rocks, but account for 1/2 of all igneous rock names. For their classification you should use "Felsic Plutonic" and "Volcanic" triangles. Not included on these triangles are names for ultramafic and mafic alkaline plutonic rocks: urtite, ijolite and melteigite. Urtite is a plutonic Ne-Px rock with over 70% Ne and no Fsp. Ijolite is a plutonic Ne-Px rock with 30-to 70& Ne. Melteigite contains more than 70% mafic minerals and less than 30% nepheline. Alkaline rocks are more common in volcanic and hypabyssal facia, and less abundant as plutonic rocks. Their rock-forming minerals are nepheline, feldspar, clinopyroxene, amphibole, micas, olivine, leucite, melilite. Note that neither quartz nor orthopyroxene can be found in alkaline rocks. Common accessory minerals include zircon, sodalite, analcime, apatite, and magnetite.

Alkali clinopyroxenes are distinguished from other pyroxenes by their strong green and brown colour, stronger pleochroism and higher birefringence. However, the interference colour may be masked by the mineral colour. In thin sections we can identify 2 types of alkali clinopyroxene: those pleochroic in brown or violet are Acmite or Ti- Augite, and those pleochroic in green are aegerines. Acmite is the Fe³⁺-Na pyroxene (NaFe³⁺Si₂O₆), and aegerine is a solid solution between augite and acmite.

Amphiboles in alkaline rocks always contain Na and can be represented by sodic hornblende (strongly coloured in shades of green and brown) or by other sodic amphiboles (kaersutite, richterite, arfvedsonite, all of which exhibit blue pleochroism). The latter are rare and absent in the Reference Collection.

Leucite is found exclusively in K-rich mafic volcanics and associated hypabyssal intrusives. Leucite thus coexists with an anorthite-rich plagioclase, not with a sodic plagioclase. This assemblage makes tephrites rich in K and Ca. It is unusual as in most magmatic rocks enrichment in K is accompanied by enrichment in Na, i.e. the alkalies behave similarly. Leucite is readily weathered and therefore can be observed only in fresh magmatic rocks. With time it breaks down to a mixture of nepheline and K-feldspar. Fresh leucite is easily recognizable by very low birefringence, 8-sided habits and twinning (T/s's 754, 618, 617, 1050)

Nepheline resembles the feldspars, but is uniaxial, lacks good cleavage and twinning, and has lower birefringence. Quartz lacks the clouding and secondary alteration common in nepheline. Nepheline easily breaks down and therefore the presence of secondary alteration is one of several distinguishing features. Common alteration product of nepheline is cancrinite, the easily recognizable "feathers" of yellow interference colours in nepheline rims.

So far we were able to tell minerals in thin sections not using conoscopy - a powerful method of optical mineralogy. However, identification of nepheline requires conoscopic observations, and we will review the basics of this method. You will practice recognition of uniaxial from bi-axial mineral. Conoscopy will be an ultimate test to tell apart uniaxial nepheline from bi-axial feldspars.

In Si-undersaturated rocks you can come across several isotropic minerals with negative relief. They also stand out because of their six-sided cross-sections  - common appearance of dodecahedral crystals in thin sections. They are minerals of the sodalite group (sodalite, nosean and hauyne) and analcime, a zeolite. Minerals of the sodalite group are volatile-bearing framewark silicates. You are very well familiar with a variety of hauyene, lazurite, an intensively blue gemstone found in contact metamorphosed carbonates. Nosean shows less intense blue colours in thin section (T/s 1050). Nosean can be found only in volcanic Si-undersaturated rocks, whereas plutonic rocks commonly contain sodalite, a colourless mineral of the sodalite group. However, sodalite in plutonic alkali rocks can be easily confused with analcime and in this Lab I did not attempt to tell them apart.

Make sure that you see the following minerals in the Reference Collection:

q       Uniaxial character of nepheline and its typical secondary product - cancrinite

q       Aegirine

q       Leucite, nosean and sodalite/analcime

In the third hour you will receive a thin section of an unknown alkaline rock for your independent examination. Write its petrographic description and give a rock name. Determination of the plagioclase composition is a necessary part of the petrographic description. Please write a short explanation what petrographic observations made you think the rock has an alkaline affinity. Your petrographic report should be completed and handed to the TA by the end of the Lab.       

Alkaline Rocks

Reference Collection

Thin Section Description:

Texture:          Hypidiomorphic, with subhedral K-Fsp and anhedral Amph and Cpx

88%  K-Fsp in subhedral large crystals with simple Calsbad twins. All crystals are perthites, with 30-50% exsolution lamellae of albite. 3% of grains show microcline tartan twinning. Irregular rims of albite are common. Replaced by fine-grained grey powder of clay mineral (?).

5%       Albite in subhedral crystals with K-Fsp lamellae, antiperthites. Clear grains of albite also commonly mantle perthjitic K-Fsp.

5%       Alkaline Clinopyroxene, anhedral to subhedral. Pleochroic from green to khaki green => aegirine. Always associated with amphibole. Some distinct cores of aegirine in amphibole.

5%       Hornblende, anhedral to subhedral. Strongly pleochroic from brown to black.    Always associated with Cpx and sometimes rims it.

2%       Opaque minerals, subhedral

Apatite

Secondary Minerals: Fine-grained grey clay mineral after K-Fsp. An excellent marker of K-Fsp under one polar!

Secondary Minerals:

5%       Cancrinite after Ne, replacing its rims. Interference colours are up to first order yellow, N << N Balsam. Typical alteration product of nepheline.

Comment: Chemical evolution of magma in this rock led to later replacement of primary augite by an aggregate of aegirine, hornblende, biotite, an opaque mineral and sphene. Former euhedral augite crystals are now recrystallized into finer-grained intergrowths of these minerals. Thus, we can suggest that magma evolved to become more Ti, Fe³⁺, alkali, and water-rich.

Thin Section   754

Sample Number         P2062

Rock Name:               Leucite tephrite

Location: ?

Thin Section   170

Sample Number         P2902

Rock Name:               Pheno-Tephrite

Location: ?

Thin Section   1161

Sample Number         P1012

Rock Name:               Melteigite

Location: Trepain Law, east of Edinburgh. Said to be first Ne found in Scotland