MT. KENYA VOLCANO
Mt. Kenya is an eroded, extinct volcano in south-central Kenya, just next to the East African Rift Valley system (see photo). The volcano was active principally during the Pliocene and Pleistocene. The summit area of Mt. Kenya consists of majestic-looking, high spires of rock - these rocks are shallow intrusives that filled up the throat of the old volcano. The eroded slopes of the volcano consist of a variety of extrusive rocks, including trachytes, basalts, phonolites, and kenytes.
Kenyte (10.2 cm from top to bottom; 4.1 cm at its widest) - this is the second-rarest lava type on Earth. It is only known from two localities: 1) the extinct Mt. Kenya Volcano in Kenya and 2) the active Mt. Erebus Volcano in Antarctica. Chemically and texturally, kenytes are porphyritic phonolites (intermediate, alkaline-rich volcanic rocks) dominated by large phenocrysts of anorthoclase feldspar ((K,Na)AlSi3O8).
So why aren't kenytes just called porphyritic anorthoclase phonolites? Well, kenytes are different, but a gradational spectrum of lithologies exists between kenyte lavas and porphyritic anorthoclase phonolite lavas. The intended distinction is in the characteristics of the matrix. Kenyte matrix is supposed to be glassy-textured or homogeneously cryptocrystalline-textured or composed of devitrified glass. Kenyte matrix also often shows obvious flow-banding (see above sample).
This kenyte is Pleistocene in age, collected from the southern slopes of the Mt. Kenya Volcano.
Large whitish-grayish crystals = anorthoclase feldspar phenocrysts.
Dark matrix = flow-banded, ~cryptocrystalline material.
Porphyritic anorthoclase phonolite (6.6 cm across) - this second Mt. Kenya rock has the same chemistry and texture as a kenyte (= 1st rock shown above), but the dark-colored matrix of this porphyritic phonolite is more coarsely-crystalline than in kenyte, and also lacks flow-banding.
This rock is Pleistocene in age, and was collected from an outcrop on the southern slopes of Mt. Kenya Volcano.
Large pale-colored crystals = anorthoclase feldspar phenocrysts.
Dark, finely-crystalline groundmass = dominated by sanidine & nepheline.
Porphyritic nepheline syenite (8.3 cm across) - this 3rd rock is derived from the summit spires of Mt. Kenya Volcano. It is a porphyritic nepheline syenite, a shallow intrusive igneous rock that filled up the throat of the old volcano.
This rock was collected from a float boulder on the southern slopes of Mt. Kenya, but derived from the summit spires. The rock contains large phenocrysts of sanidine feldspar, smaller phenocrysts of nepheline, and a groundmass of aegirine pyroxene, alkalic amphibole, aenigmatite, and melanite.
Mostly synthesized from info. provided by Tony Peterson & info. in Baker (1967) and Price et al. (1985).
Baker (1967) - Geology of the Mount Kenya area. Geological Survey of Kenya Report 79. 78 pp. 4 pls. 1 map.
Price et al. (1985) - Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya. Contributions to Mineralogy and Petrology 89: 394-409.