實際上并不是一個礦物種,而是針鐵礦、纖鐵礦、水針鐵礦、水纖鐵礦以及含水氧化硅、泥質(zhì)等的混合物。化學(xué)成分變化大,含水量變化也大。
(1)針鐵礦 α-FeO(OH),含Fe 62.9%。含不定量的吸附水者,稱水針鐵礦HFeO2·NH2O。斜方晶系,形態(tài)有針狀、柱狀、薄板狀或鱗片狀。通常呈豆狀、腎狀或鐘乳狀。切面具平行或放射纖維狀構(gòu)造。有時成致密塊狀、土狀,也有呈鮞狀。顏色紅褐、暗褐至黑褐。經(jīng)風(fēng)化而成的粉末狀、赭石狀褐鐵礦則呈黃褐色。針鐵礦條痕為紅褐色,硬度5~5.5,比重4~4.3。而褐鐵礦條痕則一般為淡褐或黃褐色,硬度1~4,比重3.3~4。
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