▄Distinguished from▄: The similarly appearing ores by its insolubility in acids and the possibility of making it disappear by heating. REMARKS: Graphite is widely distributed in the oldest rock formations, especially in the schists. It is often associated with rutile and the iron oxides. Graphite is not acted on by acids. H., 1 to 2. Sp. gr., 2.09 to 2.25. It is burnt with great difficulty in thin sections on platinum foil; but this test may vary, in many cases the graphite (when in bladed flakes) not being consumed even after long heating. When heated it may expand into worm-like forms. _Carbonaceous Matter._—Occurs in opaque, grayish-black particles having no lustre; and is found finely disseminated, sometimes in larger aggregations, in clay slates, limestones, etc. HEMATITE. HEXAGONAL. COMPOSITION: Fe_{2}O_{3}. ▄Usual Appearance in Sections▄: Irregular scales, minute grains or earthy. Distinct crystalline forms not often observed in rocks. _Opaque_, and by reflected light, black with metallic lustre, or red without lustre. May also be transparent in red tints. No marked pleochroism observed. REMARKS: Found widely distributed in acid eruptive rocks, crystalline schists, etc. Also as inclusions in minerals, and as a red pigment in many rocks. It is insoluble in hydrochloric acid, and non-magnetic, unless attached to grains of magnetite. H., 5.5 to 6.5. Sp. gr., 4.9 to 5.3. ILMENITE, Menaccanite. HEXAGONAL. COMPOSITION: (FeTi)_{2}O_{3}. ▄Usual Appearance in Sections▄: Irregular masses, without crystallographic outline, rhombohedral crystals, or skeleton-like growths. Also in brownish, translucent mica-like forms. _Opaque_, and by reflected light, iron-black with metallic lustre. When _translucent_: pleochroism brown to yellow; double refraction not very strong; optically (−). ▄Alteration▄: Often takes place to a whitish, strongly refracting, substance only slightly transparent, called _leucoxene_. This alteration product frequently develops along definite rhombohedral directions, Fig. 33 C. Also a change to titanite or rutile may occur, or the ilmenite may be surrounded by these minerals. ▄Distinguished from▄: MAGNETITE and HEMATITE.—By whitish, strongly refracting decomposition product. At times the distinction may be very difficult. REMARKS: Ilmenite occurs principally in the soda-rich and basic eruptive rocks. The mica-like form is limited to the porphyritic eruptives. The brown pigment in the plagioclase of certain gabbros may be ilmenite. It is attacked slowly by hot hydrochloric acid, and the solution when heated with tin becomes violet. Pure ilmenite is indifferent towards the magnet, hence strong magnetic properties would indicate a mixture with magnetite. H., 5 to 6. Sp. gr., 4.5 to 5.