Encyclopedia of Rocks & Minerals

[16jun12St VBM]

OBSIDIAN (314)

Type: extrusive ignous rock

Chemistry: variable (usually felsic)

Components: essenstials glass, accessories magnetite, ilmenite, oxides & other minerals such as feldspar, as microphenocrysts

Appearance: color shiny black, wit clear conchoidal fracture, texture glassy with rare microphenocrysts & abundant opaque fine dust scattered in concentric zones. Structure massive with, in some cases, the presence of latent concentric fractures (perlitic fractures)

Geotectonic environment: derived from the rapid cooling of a viscous granitic magma. Found as volcanic units, as fragments thrown into the air by erupting volcanos, sometimes as long filaments (Pele’s hair) & as the outer covering of rhyolitic & dacitic domes.

Occurence: very common in recent lava flows, mostly from felsic volcanos (Japan, Java). Found in the Tertiary lva of Lipari isles (Italy). The most striking occurences is in obsidian cliff in Yellowstone national park, Wyoming (USA). Also found in Hungary

Uses: used in prehistoric times for tools & sculptures. Currently used industrially as raw material for rock wool.

PUMICE (315)

Type: extrusive ignous rock

Chemistry: variable (usually felsic)

Components: essentials glass, accessories crystals of various silicates, zeolites, calcite

Appearance: color very light gray, yellowish, red; texture vesicular w/ lots of protuberances & vacuoles, partly closed & partly channellike, which make it very light (it floats on water); structure generally scoriaceous, with vacuoles often oriented, depending on the flow. Frequent zeolite, calcite, etc. amygdule fill

Geotectonic environment: material expelled violently upward into the air, in the initil gas-rich phases of volcanic explosions. Common in felsic volcanos & also on the outermost crust of some flows

Occurence: the Lipari isles (Italy) & most of the volcanos of Sunda (Indonesia) & Japan are rich in pumice. Vesuvius has had emissions of pumice & vocanic ash at various times, in particular at the start of the eruption of AD 79, which destroyed Pompeii

Uses: reasonably good abrasive, esp for soft metals; light insulation in building

IGNIMBRITE (349)

Type: pyroclastic rock

Chemistry: felsic

Components: crystals of quartz, alkaline feldspar (sanidine, albite), biotite & sometimes feldspathoids in a matrix composed mainly of consolidated glass (in part devitrified). Often contain fragments of sedimentary rocks

Appearance: color light, gray or brownish, sometimes reddish or violet due to oxidation; texture clastic, very consolidated; structure massive, with columnar fissuring & abundant pneumatolytic druses, sometimes with flow phenomena made evident by alignment of bubbles or glass shards

Geotectonic environment: deposits of hot incandescently glowing tuff (ash), having rolled down slopes & been derived from highly explosive volcanic activity (eruption clouds or nuée ardente). Consolidated on the spot because of the plasticity of the glass & the presence of residual gases & heat

Occurence: widespread, esp in New Zealand, Asia (Kamchatka peninsula), Armenia & the Caribbean. Forming at the present day from the active volcaanos Bezymianny (Siberia), Katmai (Alaska) & mt Pelée (Martinique)

Uses: locally used as building material, esp when compact

PORPHYRITIC TUFF (350)

Type: pyroclastic rock

Chemistry: felsic or intermediate

Components: together with individual crystals of augite, plagioclase & olivine, they often contain bubby lava fragments (lapilli), volcanic ash & sedimentary material which helps cementation (zeolites, calcite) or glass

Appearance: color light gray, pink, greenish, yellowish or brown; texture markedly clastic, with lapilli & sometimes also sedimentary rocks torn from the volcanic pipe or dislocated during rolling downslope following the volcanic explosion, mixed with ash & partly cemented by authigenic minerals (zeolites) & in part by sedimentary materials (clays & calcite)

Geotectonic environment: interstartified with lava in volcanos of a mixed explosive-eruptive type; large deposits also present at some distance from volcanic centers, in explosive zones

Occurence: associated with the lavas of Vesuvius (Italy). Also present in ancient lavas as in the Ordovician volcanics of the Lake district (England) & North Wales. It is also found in Greece, Turkey, Germany, Indonesia, Japam & the circum Pacific belt of the Americas

Uses: some tuffaceous coherent rocks are used locally as construction stone & as material for the production of special cements

[25jul12R 6ram1433H]

JASPER (336)

Type: chemical sedimentary rock

Class: siliceous rock

Appearance: color very variable; white, gray, red, brown or black; texture compact or microfibrous, with splintery [pecahan bt yg tajam, menyerpih] or conchoidal fracture; stratification thin, often with variously colored zones with folds & constricted [terbatas] areas

Components: mainly chalcedony & quartz with hematite, pyrolusite, clay & sometimes calcite

Geological environment: deposition of silica directly by precipitation from water, possibly enriched by volcanic solutions, at great depth & in mid-ocean. It is also probable that there is diagenesis at the expense [pengeluaran] of organogenetic siliceous sediments.

Occurrence: in the USA jasper is found in the Lake Superior & Mesabi Range iron ore deposits (jaspilites). It is also found in the Jurassic of California & the variety novaculite, from Arkansas, is used for making hones [gerinda] & whetstones [bt asah]. The variety containing volcanic material (porcellanite) is found in England & Michigan (USA)

Uses: sometimes used as a decorative stone

______________________________________________________________
[10-7-14K 13ram1435H lab mikropal]
ANDESITE (305)
Type: extrusive igneous rock
Chemistry: intermediate
Components: essensials: plagioclase (labradorite-andesine in which, by definition & to distinguish them from basalt, anorthite is <50%), biotite. Accessories: magnetite, ilmenite, quartz, hornblende, pyroxene, glass. Accidentals: olivine, orthoclase, anorthoclase.
Appearance: color blackish-brown or greenish, esp if the groundmass is mainly glassy; texture markedly porphyritic, in which plagioclase & biotite phenocrysts stand out in vitreous or fine-grained groundmass, formed by anhedral plagioclase & only locally oriented in fluids forms; structure massive, sometimes w/ abrupt changes of grain size & color; xenoliths are fairly common.
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