安(an)山(shan)巖(andesite)是中性(xing)的(de)鈣(gai)堿性(xing)噴出(chu)(chu)巖。與(yu)(yu)閃長巖成分(fen)相(xiang)(xiang)當。安(an)山(shan)巖一詞來源于南美洲西(xi)部的(de)安(an)第(di)斯(si)山(shan)名Andes。分(fen)布(bu)于環太平洋活(huo)動大陸邊緣及島弧地(di)區。產狀以(yi)陸相(xiang)(xiang)中心式噴發(fa)為(wei)主(zhu)，常與(yu)(yu)相(xiang)(xiang)應(ying)成分(fen)的(de)火(huo)山(shan)碎(sui)屑巖相(xiang)(xiang)間構(gou)成層火(huo)山(shan)。有(you)的(de)呈巖鐘、巖針侵(qin)出(chu)(chu)相(xiang)(xiang)產出(chu)(chu)。安(an)山(shan)巖火(huo)山(shan)的(de)高(gao)度最大，一般高(gao)500～1500米，個別可達3000米以(yi)上。

　　安山(shan)巖(yan)(yan)為(wei)中性(xing)噴出巖(yan)(yan)，分布(bu)范圍僅次(ci)于(yu)玄武巖(yan)(yan)。巖(yan)(yan)石一般為(wei)灰、灰綠(lv)、淡(dan)紫(zi)或(huo)紫(zi)紅(hong)色，其中含有少(shao)量(liang)斜長石、角(jiao)閃石、輝(hui)石和黑云母的斑(ban)晶，而且斑(ban)晶常(chang)呈定向排(pai)列，這是(shi)由于(yu)巖(yan)(yan)漿(jiang)是(shi)在流動(dong)中冷卻的。安山(shan)巖(yan)(yan)也會有氣(qi)孔和杏仁構造。

　　安山(shan)巖(yan)(andesite)一種中(zhong)(zhong)性的鈣堿性噴出(chu)巖(yan)。其成(cheng)(cheng)分相當于閃長(chang)巖(yan)。手(shou)標本新(xin)鮮(xian)面深灰-褐灰色，風(feng)化或蝕(shi)變后呈灰綠(lv)-紫紅色。色率(lv)一般為(wei)20～35，斑(ban)(ban)狀結構(gou)(有時可見(jian)巨斑(ban)(ban))，斑(ban)(ban)晶(jing)主(zhu)要(yao)為(wei)斜(xie)長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)及暗色礦物(wu)。其中(zhong)(zhong)斜(xie)長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)以(yi)中(zhong)(zhong)長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)﹑拉長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)為(wei)主(zhu)，常具環帶及熔蝕(shi)結構(gou)。常見(jian)暗色礦物(wu)有輝石(shi)(shi)(shi)(shi)(shi)(shi)(普通(tong)輝石(shi)(shi)(shi)(shi)(shi)(shi)﹑紫蘇輝石(shi)(shi)(shi)(shi)(shi)(shi))﹑角閃石(shi)(shi)(shi)(shi)(shi)(shi)和黑(hei)云母。基(ji)質(zhi)主(zhu)要(yao)為(wei)交(jiao)織結構(gou)及安山(shan)結構(gou)(玻(bo)(bo)基(ji)交(jiao)織結構(gou))，由(you)斜(xie)長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)(更長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)﹑中(zhong)(zhong)長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)為(wei)主(zhu))微晶(jing)﹑輝石(shi)(shi)(shi)(shi)(shi)(shi)﹑綠(lv)泥(ni)石(shi)(shi)(shi)(shi)(shi)(shi)﹑安山(shan)質(zhi)玻(bo)(bo)璃等組成(cheng)(cheng)﹐堿性長(chang)石(shi)(shi)(shi)(shi)(shi)(shi)﹑石(shi)(shi)(shi)(shi)(shi)(shi)英(ying)少見(jian)﹐僅個別(bie)填充于微晶(jing)間(jian)隙中(zhong)(zhong)。副(fu)礦物(wu)以(yi)磷灰石(shi)(shi)(shi)(shi)(shi)(shi)及鐵的氧化物(wu)為(wei)主(zhu)。氣孔﹑塊狀構(gou)造，有的氣孔被方解(jie)石(shi)(shi)(shi)(shi)(shi)(shi)﹑石(shi)(shi)(shi)(shi)(shi)(shi)英(ying)﹑綠(lv)泥(ni)石(shi)(shi)(shi)(shi)(shi)(shi)等充填﹐形成(cheng)(cheng)杏(xing)仁構(gou)造。

　　安(an)(an)山(shan)巖(yan)中SiO2含(han)(han)量(liang)變(bian)化較大(52～63%)﹐平均含(han)(han)量(liang)為(wei)(wei)(wei)58.17%。98.5%的(de)安(an)(an)山(shan)巖(yan)的(de)SiO2過(guo)飽(bao)和，出現標準礦物(wu)石英(多小(xiao)于(yu)15%)。安(an)(an)山(shan)巖(yan)按SiO2含(han)(han)量(liang)可分(fen)為(wei)(wei)(wei)兩種﹕含(han)(han)52～57%的(de)為(wei)(wei)(wei)玄(xuan)(xuan)武(wu)安(an)(an)山(shan)巖(yan)﹔含(han)(han)57～63%的(de)為(wei)(wei)(wei)安(an)(an)山(shan)巖(yan)﹐安(an)(an)山(shan)巖(yan)的(de)里(li)特曼指數﹐即(K2O+Na2O)2/(SiO2-43)比值﹐一(yi)般(ban)小(xiao)于(yu)3.3﹐屬鈣堿(jian)性﹐安(an)(an)山(shan)巖(yan)平均化學(xue)成分(fen)為(wei)(wei)(wei)SiO2=52.4%﹐Al2O3=17.17%﹐CaO=7.92%﹐Na2O=3.67%﹐K2O=1.11%﹐以SiO2較低﹐CaO較高﹐全堿(jian)小(xiao)于(yu)5.5%﹐Na2O>K2O為(wei)(wei)(wei)特征。安(an)(an)山(shan)巖(yan)與(yu)玄(xuan)(xuan)武(wu)巖(yan)常不易區別﹐一(yi)般(ban)認為(wei)(wei)(wei)﹐SiO2>52%﹐色率<40%的(de)為(wei)(wei)(wei)安(an)(an)山(shan)巖(yan);反之(zhi)為(wei)(wei)(wei)玄(xuan)(xuan)武(wu)巖(yan)。

　　由巖(yan)漿冷卻形成(cheng)的安山巖(yan)會在不同(tong)(tong)的環境下，會有不同(tong)(tong)的改(gai)變，將之歸為兩種(zhong)(zhong)途徑，一(yi)(yi)種(zhong)(zhong)是(shi)化學成(cheng)份上(shang)的改(gai)變，另(ling)一(yi)(yi)種(zhong)(zhong)方式則是(shi)物理結構(gou)上(shang)變化。

　　化學成份的變化

　　主要(yao)有溶(rong)(rong)解、取代(dai)及(ji)A、B成份(fen)上的(de)(de)(de)互換(huan)。溶(rong)(rong)解就(jiu)是所(suo)謂的(de)(de)(de)淋溶(rong)(rong)作(zuo)用，溶(rong)(rong)液將某些離子(zi)從礦(kuang)物(wu)中帶走;取代(dai)作(zuo)用則是礦(kuang)物(wu)中當某些離子(zi)被帶走后(hou)，又充填進(jin)來(lai)新的(de)(de)(de)離子(zi);A和(he)B礦(kuang)物(wu)內成份(fen)的(de)(de)(de)互換(huan)，則是產生新的(de)(de)(de)礦(kuang)物(wu)。這就(jiu)是一般所(suo)謂巖石(shi)(shi)風化、換(huan)質(zhi)(zhi)、變(bian)(bian)(bian)(bian)(bian)質(zhi)(zhi)等作(zuo)用的(de)(de)(de)原(yuan)因了。B、物(wu)理結(jie)(jie)構的(de)(de)(de)變(bian)(bian)(bian)(bian)(bian)化：當巖石(shi)(shi)所(suo)處的(de)(de)(de)溫度、壓力條件改(gai)變(bian)(bian)(bian)(bian)(bian)或離子(zi)改(gai)變(bian)(bian)(bian)(bian)(bian)時，會影響原(yuan)子(zi)之間的(de)(de)(de)鍵長(chang)和(he)排列(lie)位置，這就(jiu)是所(suo)謂礦(kuang)物(wu)結(jie)(jie)構上的(de)(de)(de)改(gai)變(bian)(bian)(bian)(bian)(bian)，例如石(shi)(shi)墨再(zai)超高壓的(de)(de)(de)環境下，即可變(bian)(bian)(bian)(bian)(bian)成鉆(zhan)石(shi)(shi)。它的(de)(de)(de)成份(fen)雖(sui)然沒有改(gai)變(bian)(bian)(bian)(bian)(bian)，但鍵結(jie)(jie)方式完全不一樣了，表現出來(lai)的(de)(de)(de)物(wu)理性(xing)質(zhi)(zhi)也(ye)跟著(zhu)改(gai)變(bian)(bian)(bian)(bian)(bian)了，這就(jiu)是變(bian)(bian)(bian)(bian)(bian)質(zhi)(zhi)巖的(de)(de)(de)成因。任何物(wu)質(zhi)(zhi)隨著(zhu)溫度壓力的(de)(de)(de)改(gai)變(bian)(bian)(bian)(bian)(bian)，分子(zi)之間會發生變(bian)(bian)(bian)(bian)(bian)化的(de)(de)(de)。