Mercado and Rose (1992) sampled and examined four kinds of materials from Tacaná:
(1) surficial lava and dome rocks
(2) pumices from fall deposits
(3) lithic fragments from the 1986-1987 phreatic vent, which could represent Tacaná rocks from subsurface
(4) the ash of the February-June 1986 eruptions.
The mineralogy of the first three groups is similar and broadly andesitic in composition.

Mineralogy of the Tacaná volcanic rocks.

Surficial Lava and Dome Rocks

The lavas are two-pyroxene andesites. This andesite is hypocrystalline, containing some glass in the groundmass. It shows a porphyritic texture, with phenocrysts of plagioclase (average: An 40), clino- and orthpyroxene, olivine, hornblende, quartz, and opaques. Sphene was not present in the samples.

Plagioclase is the dominant phenocryst. It is generally subhedral to euhedral, with tabular habit and prismatic elongation. It exhibits a poikilitic texture, with numerous inclusions of opaques and some glass inclusions. Other phenocrysts present are dominantly subhedral and smaller (generally less than 0.5 mm) than the plagioclase crystals, although a few phenocrysts of pyroxene and hornblende measure up to 3 mm. They commonly show a poikilitic texture, often with inclusions of opaques.

Pumices from Fall Deposits

Pumices in ashfall deposits are composed of 40-90% glass. Plagioclase is the most abundant crystalline component of these pumices. Other crystalline components are clinopyroxene and opaques. Orthopyroxene was not observed. Sphene was also absent. All the ash samples are moderately to strongly altered, showing evidence of devitrification and bentonitization.

Ash of the February-June 1986 Eruptions

The ashes from the February 1986 eruptions consist of a variety of altered minerals, including considerable amounts of rounded feldspar grains and quartz. May and June 1986 ashes are less altered and contain a larger variety of rounded and blocky mineral grains, including plagioclase, quartz, hornblende, calcite, olivine, and pyroxene. No vesicular glass pyroclasts were observed in these ashes. Mercado and Rose (1992) interpret these ashes as entirely phreatic.

(Mercado and Rose, 1992)