Dacite is an effu­sive igneous rock that belongs to the group of mid­dle vol­canic rocks and occu­pies an inter­me­di­ate posi­tion between rhy­o­lites and andesites.

The name dacite orig­i­nates from the for­mer Roman province of Dacia, which was locat­ed between the Danube Riv­er and the Carpathi­an Moun­tains (now mod­ern Roma­nia and Moldo­va).

This name was first used in 1863 by Aus­tri­an geol­o­gists Franz von Hauer and Gui­do Stache in their sci­en­tif­ic work “Geol­o­gy of Tran­syl­va­nia.” They referred to a descrip­tion of the rock pub­lished in the same year by the Ger­man geol­o­gist Fer­di­nand Zirkel in the reports of the Aus­tri­an Acad­e­my of Sci­ences in Vien­na. Dacite was ini­tial­ly defined as a new type of rock to dis­tin­guish calc-alka­line rocks with oligo­clase phe­nocrysts (dacites) from rocks with ortho­clase phe­nocrysts (rhy­o­lites).

Dacite is includ­ed in the list of min­er­als of nation­al impor­tance, approved by Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine No. 827 of Decem­ber 12, 1994, as a raw mate­r­i­al for rub­ble stone and crushed stone.

The gen­er­al­ized min­er­al com­po­si­tion of dacite is inter­me­di­ate between rhy­o­lite and andesite. It is char­ac­ter­ized by a high sil­i­ca con­tent (approx­i­mate­ly 63–68%), which gives it a light color—usually gray, light brown, or green­ish. The main min­er­als of dacite are quartz, pla­gio­clase (andesine–oligoclase), biotite, horn­blende, and some­times augite and pyrox­enes.

The struc­ture is most often por­phyrit­ic: quartz and pla­gio­clase phe­nocrysts, and more rarely oth­er min­er­als, are includ­ed in a glassy or fine-grained ground­mass.

Properties of dacite

The prop­er­ties of dacite deter­mine its use in econ­o­my. Due to its high strength and resis­tance to weath­er­ing, it is used as a build­ing and fac­ing stone, as well as crushed stone in road con­struc­tion. In terms of hard­ness, dacite occu­pies an inter­me­di­ate posi­tion between gran­ite and basalt: on the Mohs scale, it has a val­ue of 6–7. This means that the rock is suf­fi­cient­ly hard but at the same time not brit­tle, which makes it suit­able for pro­cess­ing. The rock shows good resis­tance to mechan­i­cal loads, abra­sion, and atmos­pher­ic influ­ences.

Distribution

Dacite is rel­a­tive­ly wide­spread and occurs in var­i­ous tec­ton­ic and mag­mat­ic con­texts:

• In ocean­ic vol­canic series. Exam­ples: Ice­land (the Geirdar­spordur Ridge) and the Juan de Fuca Ridge.
• In calc-alka­line and tholei­itic vol­canic series of sub­duc­tion zones of island arcs and active con­ti­nen­tal mar­gins. Exam­ples of dacitic mag­ma­tism in island arcs are Japan, the Philip­pines, the Aleut­ian Islands, the Antilles, the Sun­da Arc (Mount Batur), Ton­ga, and the South Sand­wich Islands. Exam­ples of dacitic mag­ma­tism in active con­ti­nen­tal mar­gins are the Cas­cade Range, Guatemala, and the Andes (Ecuador, Peru, and Chile).
• In con­ti­nen­tal vol­canic series, often in asso­ci­a­tion with tholei­itic basalts and inter­me­di­ate rocks.

The type local­i­ty of ori­gin for dacite is the Gisel­la quar­ry near Poeni, Cluj in Roma­nia. Oth­er places of ori­gin of dacite in Europe are: Ger­many (Weisel­berg ), Greece (Nisy­ros and Thera ), Italy (in the quartz por­phyry of Bolzano and Sar­dinia (), Aus­tria (Styr­i­an Vol­canic Arc), Scot­land (Argyle ), Slo­va­kia, Spain (El Oya­zo near Alme­ria ), France (Ester­el mas­sif) and Hun­gary (Mount Cho­di).

Out­side Europe are places such as Iran, Moroc­co, New Zealand (Taupo Vol­canic Region), Turkey, the Unit­ed States and Zam­bia.

Dacite has also been iden­ti­fied extrater­res­tri­al­ly in the caldera of Nili Pat­era in the Syr­tis Major region on Mars.

Dacite on Mars

In 2002, NASA’s THEMIS space­craft entered Mars orbit and began study­ing the plan­et’s sur­face using ther­mal imag­ing tomog­ra­phy. His devices made it pos­si­ble to deter­mine the min­er­alog­i­cal com­po­si­tion of rocks and cre­ate maps of their dis­tri­b­u­tion.

The data showed that the main vol­canic rock on the sur­face of Mars is basalt. Syr­tis Major is a huge basaltic vol­cano about 1,300 km wide near the Mar­t­ian equa­tor, with calderas at the sum­mit and numer­ous vents on the slopes. The erup­tions formed vit­re­ous siliceous dacite flows that formed cones up to 300 m high and lava flows up to 20 km long.

Dacites and obsid­i­ans sim­i­lar to those found on Earth, for exam­ple, in the vol­ca­noes Hood (USA) or Fuji (Japan), were observed in the vol­canic rocks of Syr­tis Major. This indi­cates the for­ma­tion of com­plex mag­mas on Mars, pro­duced as a result of par­tial melt­ing and frac­tion­al crys­tal­liza­tion.

Using dacite

Many dacite sam­ples are char­ac­ter­ized by a fine-grained struc­ture and rel­a­tive com­po­si­tion­al homo­gene­ity. Due to these prop­er­ties, even in ancient times peo­ple used it for mak­ing tools and house­hold items. When worked into spear­heads, scrap­ers, or knife blades, dacite is infe­ri­or to obsid­i­an in sharp­ness, but sig­nif­i­cant­ly sur­pass­es it in strength and dura­bil­i­ty.

In mod­ern times, dacite is wide­ly used in con­struc­tion: as crushed stone for con­crete and asphalt, and as a mate­r­i­al for foun­da­tions and road sur­faces. Due to its dec­o­ra­tive shades, it is suit­able for cladding, the pro­duc­tion of slabs, stairs, and mon­u­ments. In addi­tion, it is used in the pro­duc­tion of min­er­al wool, ceram­ics, and glass.