Nisyros rocks greatly vary in range from vitrophyric pumice to holocrystalline rocks. The porphyritic index (volume % of the phenocrystal) ranges from 1-3% in the pumices to 55% in some lava

flows (Francalanci et al, 1995).

Petrographic characteristics of the formations are summarized in the following Table.

Table. Petrographic characteristics of Nisyros volcanites. [■ = abundant (70-90%), ▲ = present (< 20%), ▼ = rarely present, ○ = xenocrystal].

Plagioclase is always the most abundant mineral phase, euhedral or subhedral in shape, with large normal and oscillatory zoning. Many crystals exhibit sieve textures due to numerous molten and solid (generally clinopyroxene and apatite) inclusions.

Olivine phenocrystals are present in the less evolved rocks (basic andesites and andesites) but also in the post-caldera domes and their enclaves. They tend to be anhedral and show reaction rims of clinopyroxene

Clinopyroxene phenocrystals are augitic and usually exhibit only slight zoning. They are abundant in the basaltic andesitic and dacitic rocks and are always larger (up to 2 mm) than olivine. Orthopyroxene is hypersthenic in composition and is always less than 5% of the total phenocrystal abundance.

Hornblende is abundant in virtually all the enclaves, but is also found in the dacitic and rhyolithic volcanic rocks.

Magnetite and ilmenite are commonly present as micro phenocrystals although the post-caldera dacitic-rhyolithic domes lack ilmenite.

Among the secondary mineral phases, apatite is ubiquitous, whereas zircon is only found in the most evolved rocks.

The sedimentary xenoliths consist of clays, lutitic and sparitic limestone, marble and skarn. Small fragments of schist are very rare in the Nisyros rocks.