LIMESTONE AND LIME.

New Zealand Journal of Agriculture, Volume XX, Issue 3, 20 March 1920, Page 157

LIMESTONE AND LIME.

MINERALOGY, TESTING, AND SAMPLING

Extracts from “ The Limestone and Phosphate Resources of New Zealand ” : Geological Survey Bulletin No. 22, Part I, 1919, by P. G. Morgan Director, assisted by officers of the Survey. '

LIME-BEARING MINERALS.

Carbonate of lime occurs in two distinct mineral forms, alike in chemical composition but physically different. These two forms are known to the mineralogist as calcite and aragonite. There may also be other forms of calcium carbonate.

Aragonite' forms hard parts of reef-building corals, the shells of gasteropods, and the inner pearly layers of bivalve shells, but is otherwise a somewhat uncommon mineral of little importance as a source of lime. Authoritative information concerning aragonite in organic structures is difficult to obtain, and it is somewhat doubtful how far it does or does not enter into the composition of shells, corals, &c.

.Calcite, on the other hand, is a very abundant mineral, widely distributed through the earth’s crust. / There are numerous varieties, differing considerably in general appearance. All the varieties are characteristically of a light colour, but the presence of impurities may cause variations in tint from white or transparent to black. For the purposes of the present publication the chief forms in which, carbonate of lime occurs may be enumerated and described as follows : —

(•1.) Pure or nearly pure calcite, occurring in more or less perfect crystals, and then known as dog-tooth spar, nailhead spar, &c. Iceland spar is a perfectly transparent form.

(2.) Carbonate of lime, occurring in the form of calcareous concretions, which are usually very fine-grained and tough, but as a rule are not of great purity. Concretions are small masses of mineral substances which have collected round a nucleus. The nucleus may be a grain of some mineral other than that forming the mass of the concretion, a piece of vegetable matter, or it may be merely the point at which the concretion has begun to form. The late Alexander McKay’s felicitous definition of a concretion may here be quoted : “A concretion is some-

thing that has gathered itself round about something else ; sometimes there is nothing for it to gather about, but that does not prevent its being a concretion all the same, only there is no foreign substance in its heart.”* Calcareous concretions occur in many forms—globular, ovoid, kidney-shaped,- cylindrical, disk-like, dumb-bell shaped, or wholly irregular. They are common in claystone and the allied rocks, which in New Zealand are often called “ papa.” Calcareous concretionary bands or layers, .usually very tough and impure, are not uncommon also in the calcareous claystones of this country.

(3.) Massive limestone, occurring in beds or layers. Such material forms the principal subject of this bulletin. Argillaceous or hydraulic limestone is an impure sub-variety, containing a considerable amount of clayey matter. As the clay increases, the rock grades into calcareous claystone. Arenaceous limestone is a limestone containing a considerable proportion of sandy matter. With an increase of the sand to, say, 50 per cent, arenaceous limestones become calcareous sandstones. Shelly . limestone is a limestone composed very largely of visible fragments of shells. Some of the so-called shelly limestones of this country contain numerous pebbles, . and are rather to be called shelly or calcareous conglomerates.

(4.) Marble is a highly crystallized altered limestone, suitable for ornamental use. The coarser-grained marbles are sometimes called crystalline limestones.

(5.) Chalk is a soft easily disintegrated variety of limestone, formed chiefly from the remains of the minute organisms known as Foraminifera.

(6.) Coral-rock is formed principally of the remains of corals.' (7.) Calcareous marl is a soft earthy deposit, formed chiefly in freshwater lakes by the accumulation of the remains of calcareous algae, fresh-water shells, &c. It grades into ordinary marl, which is simply a notably calcareous claystone.

(8.) Stalactite and stalagmite are materials formed in caves or under overhanging rocks by the deposition of carbonate of lime from water that has percolated through limestone or other calcareous rock. Moisture excluded, they are usually practically pure carbonate of lime.

(9.) Calc-sinter, calcareous tufa or tuff, and travertine are names applied to one and the same thingnamely, carbonate of lime deposited by springs or, more rarely, by streams.

(10.) In New Zealand shells form a somewhat important source of carbonate of lime. As a rule, they consist of almost pure calcium carbonate, either in the form of calcite or of aragonite. A few shells are highly phosphatic. The shells of Crustacea are more phosphatic than those of ordinary shell-fish. As already indicated, corals, Bryozoa, Foraminifera, and some algae are also important sources of carbonate of lime ; and their remains, together with those of Mollusca (shell-fish), form the great bulk of the world’s limestones.

Immense amounts of carbonate of lime exist in less pure forms than those enumerated above. If the percentage of carbonate of lime falls below 85 the limestone, as already explained, may be called “ arenaceous ” or “ argillaceous,” according to whether sand or clay is the

chief impurity. Calcareous claystones, sandstones,, and conglomerates are very common in New Zealand. The rock, or rather group of rocks, popularly called “ papa ” has a widespread distribution in both North and South Islands. Though in general only slightly or moderately calcareous, “ papa ” in places approaches or becomes argillaceous limestone. There are many rocks which, as first formed, contain lime only in the form of silicate,, but by alteration come to contain several per cent, of carbonate of lime. In rare cases carbonate of lime in the form of calcite is an original constituent of igneous rocks. • .

Dolomite is a mineral composed of carbonate of lime and carbonate of magnesia, with possibly other carbonates. Typical dolomite, if pure, contains 54-35 per cent, of carbonate of lime and 45-65 per cent, of carbonate of magnesia. The union of the two substances is probably analogous to that of two metals in an alloy (especially such an alloy as Muntz metal) rather than to a strict chemical combination, such as that of calcium, carbon, and oxygen in carbonate of lime. It is not analogous to an ordinary mixture, because dolomite has a definite crystalline form and definite physical properties of its own. Dolomite forms the main constituent of extensive rock-masses in various parts of the world. Limestones with a moderate percentage of magnesia are not uncommon, and are termed dolomitic or magnesian limestones. Such rocks on microscopic examination are found to be mixtures of dolomite, calcite,-and other minerals.

Various other minerals containing more or less carbonate of lime need not be mentioned here. There are, however, many minerals which contain a proportion of lime in the form not of carbonate, but of silicate. The chief of these are various varieties of feldspar, augite, and garnet. In addition there are two important mineral substances of which lime is one of the principal constituents. .These are gypsum and phosphate of lime. ■ . .

Gypsum is hydrated sulphate of lime, and is represented by the symbol CaSO 4 .2H 2 O. Varieties of it are known as satin-spar, alabaster, and selenite:

Several phosphates of' lime, differing somewhat in chemical composition, and each having its own name, are known to the chemist. In ordinary usage the name phosphate of lime ” is applied to tricalcic calcium phosphate, with the symbol Ca 3 P 2 O 8 . This substance does not occur pure in nature, but forms the essential constituent of the minerals apatite, phosphorite or collophane, and ordinary phosphate rock. It is also the most important constituent of green bones, and may be considered to form the whole of bone-ash, impurities excepted.

■ There are two varieties of — -fluor-apatite, with the composition represented by the compound symbol 3Ca 3 P 2 O 8 • CaF 2 ; and chlor-apatite, represented symbolically by 3Ca 3 P 2 O 8 • CaCl 2 . Phosphorite ” and various other names have been applied to the form of phosphate of lime that forms the bulk of ordinary phosphate rock. Undoubtedly the mineral here present is an amorphous substance which has combined with it a small amount of carbonate of lime. Austin F. Rogers* proposes that the old name of “ collophane ” should be applied and restricted to this mineral.

TESTS FOR LIME AND LIMESTONE.

The simplest test for limestone or any other form of carbonate of lime consists in the application of any of the ordinary acids (in liquid form). If carbonate of lime is present numerous bubbles of gas will form. This gas is no other than the carbon dioxide (CO 2 ) already mentioned as the substance driven off from limestone by heating strongly.

The most satisfactory acids for general use are nitric acid (spirits of hartshorn) or hydrochloric acid (spirits of salt) diluted with one or two parts of water. Good vinegar will answer quite well. It is not generally known that various solid acids, such as tartaric and citric, can be used by placing a tiny pinch of the powdered material upon the stone together with a drop or two of water. •

Ordinary limestone gives off gas quite freely when cold acid is applied, but fine-grained hard calcareous concretions, especially if magnesian, may effervesce very slowly unless powdered and gently heated. Dolomite is hardly affected by cold acids, but if it is powdered and gently heated brisk effervescence soon begins.

It will be observed that the acid test is for carbon dioxide rather than lime, but in practice this test, combined with the general appearance of the stone being tried, is quite sufficient. For full information regarding tests for lime and its various compounds the reader must be referred to works on chemistry and mineralogy, or to teachers of. those subjects.

If a small piece of fairly pure limestone is placed in. a glass or porcelain dish (say, an ordinary saucer) and covered with some acid in liquid form (preferably dilute nitric or hydrochloric) it will be seen that the fragment of stone' as it effervesces diminishes in size, and after a short time breaks up and practically disappears, except that a little sand will be left in the dish or other vessel used. The effervescence ceases, the. carbon dioxide of the limestone having now escaped, while the calcium oxide or . lime formerly combined with it has united with the acid to form a new substance (calcium nitrate or chloride, according to the acid used), into the exact nature of which it is not necessary here to enter. A stone which, though it effervesces freely, does not break up when treated with acid is of poor quality, and is useless for the manufacture of quicklime. • ' .

The object of describing the above experiment is to show the reader how he may roughly ascertain the quality of a sample of limestone — namely, by comparing the residue after acid treatment with the origina material. For this purpose the following directions may be given : —

Reduce what is considered an average sample of the limestone to a fine powder in any. convenient way. The lumps may be broken into small pieces with a hammer on an anvil or on a hard flat stone, and then crushed to powder by placing them between two folds of brown paper and striking with the hammer. An iron pestle and mortar, if available, will, of course, be found much better than the brown paper and hammer. A very small sample may be powdered by pressure between two large coins. Take as much of the powdered limestone as will lie on a sixpence, place it in a saucer, dampen it with water, and then add a few drops -of semi-dilute nitric or hydrochloric acid (one part of water by measure -to one part of acid as bought from the

chemist or drug-manufacturer). When effervescence ceases add two or three more drops of acid to make sure that all the carbonate of lime has dissolved, then gently pour off the acid or most of it, and compare the bulk of the sandy residue with that of the powdered limestone that may be conveniently placed on a sixpence. If there is clayey matter in the- limestone the comparison will not be very satisfactory, because it will be impossible to decant the acid off the residue left after treatment without some loss. Should the residue, however, be sandy, and clearly less than one-tenth the bulk of the limestone taken for the test, then the limestone is of good quality. .

If the powdered limestone taken for the test is weighed on a chemical balance, and the residue after acid treatment with aid of heat is transferred to a filter-paper, washed with pure water, dried, collected (preferably after igniting--that is, strongly heating the material and burning the -paper to an ash), and weighed, the difference of the two weights, due allowance being made for the ash of the filter-paper when this has been ignited, represents with a moderate degree of accuracy the amount of carbonate of lime in the stone, plus any other carbonate, such as that of magnesia or iron, that may be present. ■ The result thus obtained, however, is usually too high, because some of the silicates present will almost certainly have gone partly into solution. The weight of the residue left after acid treatment, reduced to a percentage of the weight of limestone taken, is the “ insoluble ” of some analyses.

SAMPLING LIMESTONE.

The operation of sampling a substance with a view to making an analysis is most important, for if the sample is not properly taken the analyst’s skill and time , will be wholly or largely wasted. The complete analysis of a rock takes days of the chemist’s time ; it is therefore irrational, in most cases at least, to spend only minutes in taking the sample. The sampling of an agricultural limestone need not be so elaborate as when the stone is to be used in the manufacture of glass' or in certain other chemical industries, but it will be- admitted that the more carefully the work is done the better. Hitherto , in New Zealand the. samples of limestone and of many other substances collected in the field have consisted of single lumps (which may be called fortuitous or “ grab ” samples), or of a few pieces of stone broken 'here and there from the deposit, so as to form as representative a sample . as the collector knew how to obtain under the particular circumstances. Such samples may be termed “ empirical ” samples, and represent most of the samples collected by the Geological Survey in the course of its ordinary work.

'Concerning the . types of samples termed above “ grab ” and “ empirical,” Orton and Peppel write : “ Experiment has shown that it is difficult, if not impossible, for any person to select a sample without bias, even where not interested in the result, if he is . acquainted with the effect of his actions in putting in or rejecting portions.”* This statement is somewhat unhappily worded, for it may be construed to imply that the less a person knows the more likely he is to select a

true sample, nor does it differentiate between the grab sample and the empirical. The meaning, however, is this : An interested person —for example, a prospectoris hardly able to resist the temptation of putting a large proportion of the best-looking material in his sample, instead of selecting average-looking material. Some men, perhaps with the best intentions, select nothing but the best material, and then persuade themselves that they have collected average samples. On the other hand, a person with a judicial temperament, if possessed of the requisite technical knowledge, and keenly interested in obtaining a true average sample—for example, a minemanager or a batterysuperintendentmay be trusted to select samples that on the whole will yield an average result, with possibly a slight error one way or the other, due to the personal equation.

The sampling of limestone and other nearly homogeneous substances does not present the same difficulty as the sampling of variable material such as auriferous quartz. In the case of limestone to be used for agricultural purposes only, the method of collecting empirical samples by chipping off average-looking fragments with hammer and chisel will give good results in the hands of an experienced person ; and even an inexperienced person, by , taking pains, employing. common-sense, and resisting the temptation to select the best material, ought to obtain fairly representative samples. In this method of sampling the following points have to be observed : —-

(i.) In the case of an unworked limestone deposit the places to be sampled ought to be selected with a view to access and convenience of working.

* (2.) The limestone outcrop ought to be carefully observed, with a view to determining whether it is fairly uniform in quality from top to bottom. - >

(3.) If the limestone stratum is thick, and especially if the different layers show want of uniformity, it must be divided into sections of measured thickness.

(4.) The' material to form the sample ought to be selected along lines at right angles to the plane of the depositthat is, along lines that measure the thickness. Such lines will be at right angles both to strike and dip.* It is true the latter condition meed not be exactly fulfilled, but the nearer one can approach to it the better.

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(5-) In sampling the various sections care must be taken to select a proper proportion of each kind of material, as nearly as can be judged by the eye. Thus if the stone is partly hard and partly soft a due proportion of hard rock must be selected. If there are argillaceous, arenaceous, or greensandy layers present, a fair amount of the impure material must be taken. . The inexperienced or non-judicial sampler is very apt to neglect this point.

(6.) If the outcrop is large, samples should be selected along several lines, duly divided into such sections as necessary, and spaced at equal distances from one another.

(7.) Careful notes concerning the locality of each sample, arid the thickness and class of stone it represents, should be taken.

(8.) Each sample should be carefully labelled and placed with its label in a stout canvas bag, which also should be labelled or numbered.

(9.) The analyst should be given full particulars concerning the locality of the deposit, and such other information as may seem desirable or be asked for. Do not send him single lumps of stone or carelessly selected samples. Do not be so foolish as to attempt to mislead him as to the locality or nature of the deposit. Those who misinform or deceive the analyst are bound to be the losers in the long-run.

Quartering down. In general. a sample of limestone if properly taken will be too large to send by parcel-post to the analyst. It can safely be reduced in size by the method of quartering down. This consists in breaking all large pieces of stone to a small —say, that of hazel-nuts or —mixing the sample well, and piling it on a flat smooth surface in a somewhat low conical heap, which is then carefully spread out in a circle. The material is then divided into four quarters by drawing two narrow channels at right angles through the centre of the material. Two opposite quarters are then removed, and the operations of mixing, piling, spreading out, and quartering repeated. If a small sample is desired the lumps must be broken still smaller before quartering is repeated. The best procedure of all is to reduce the whole sample to a coarse powder before quartering down, but in the field. this is not practicable. . . .

Quartering down may also be employed where duplicate samples are required. The method of obtaining supposed duplicates by breaking a single lump in half, or of dividing a rough sample in two.by picking out a few lumps, need only be mentioned to be condemned ; yet there are frequent cases where persons have used this method, and have been surprised when the analyses of the two samples did not agree. Quite commonly the analyst is blamed for his lack of skill. If two analysts have been employed, the one who gets the lower results will get the credit of being unskilful or ignorant. As a matter of fact, an inexperienced sampler will obtain concordant duplicate samples only by accident.

Quarry sampling.— Where a quarry has oeen opened the facilities for sampling are generally better than in the case of ordinary outcrops. If only one or two general samples are desired the broken rock may be sampled, but usually it is better to sample the rock-faces in sections, as described above. When a rock-breaker is at work it is feasible to obtain a good average sample from the material passing through the

machine, care being taken to select proper proportions of coarse and fine. The sample, which should be large and selected at intervals, may then be broken and quartered down as described. If a fine-grinding machine forms part of the equipment, as in a limestone-pulverizing plant or cement-mill, no difficulty need be experienced in getting good average samples.

Systematic sampling.- The ideal method of securing true samples of a hard deposit is to bore it from top to bottom with a diamond drill, and save all the core. If the deposit is soft it may be bored in some other way, and all the drillings saved for analysis. Alternating hard and soft layers will prevent thoroughly representative samples of the whole being obtained by drilling methods. The usual method of sampling an exposed face of rock or mineral is to cut a uniform groove from top to bottom, and save all the cuttings. The face, if .high, and especially if heterogeneous in composition, is divided into sections'; An experienced sampler will alter his methods according to circumstances ; but the following details, mainly quoted or paraphrased from Orton and Peppel,* illustrate the general methods of systematically sampling' an outcrop or quarry-face : —

(i.) The sample should be cut from the strata in place rather than taken from the stock pile of the quarry. The latter is liable to fluctuate from hour to hour, as the product of one stratum or horizon happens to form its surface layer, as may be the case after a large shot is bred. Very often it is not possible to tell whether the pile of debris at the foot of the face is representative of the whole face or not.

(2.) The part to be cut should be first well cleaned from top to bottom by sweeping with a broom or brush.

(3.) A large canvas sheet (one 9 ft. square was used by Orton and Peppel) should be spread close under the place selected, so that the cuttings will fall and be collected on the sheet.

(4.) A groove, uniform in width (2 in. or 3 in.) and in depth (1 in., 2 in., or 3 in.), should be cut from top to bottom of the face to be sampled. The tools used may consist of a blacksmith's hammer of about 4 lb. weight, one or two cold-chisels mounted on handles, . and a few heavy stone-cutters, gads, &c. It is desirable that the groove should be cut in a straight line from top to bottom of the section sampled ; but if this is impracticable, then by following the edges of bedding-planes, and collecting from different strata in one small area, a full cross-section of the limestone may be obtained. When on account of the nature of the rock being sampled, or its location at great height requiring climbing and working with scant footing, a groove cannot be cut, the canvas may be removed from the foot of the face and large lumps of rock broken out of the various layers. From each of these a section of the' proper size may .be dressed.

■' Where narrow strips of shale, greensand, or other. impurity occur, these should be carefully cut to the proper section, and .all the cut material allowed to drop on the sampling-sheet. If the impure bands are wide it is better in some cases to sample them separately. The width, of course, must be carefully measured.

(5-) All the cuttings should be saved on the sheet, but pieces of stone known to be accidentally dislodged from points adjacent to the groove should be rejected.

(6.) The total weight of the sample may range from 5 lb. to 150 lb. In special cases it may be greater. Apparently Orfon and Peppel, as a rule, took only one sample of a high quarry-face. It would be better in many cases to divide the face into measured sections and take several samples.

(7.) Every sample should be labelled plainly and legibly with a number and other particulars inside and outside of the sack in which it is placed. The inside number may be written or cut on a piece of soft wood and then wrapped in cloth. Whatever the method, care should be taken that the marking cannot be obliterated or the label lost.

(8.) If the sample is taken from a stock pile, bins, rock-breaker, or any unusual source, all the circumstances should be noted.

(9.) Every detail of the sampling and every observation of the material sampled should be noted at the time the sample is taken. The entries in the notebook used should be clearly worded and legibly written. Abbreviations should be sparingly used.

Orton and Peppel give further details, which need not be quoted, of how the main sample, by crushing and quartering, is reduced to the small sample of about | lb. weight used by the analyst for his work.

As previously stated, for agricultural purposes limestone need not be sampled in quite so elaborate a manner as that just described, but for some industries sampling of that kind is very necessary. In reconnaissance work and in general geological survey it is usually impracticable to take samples by the method of cutting a continuous groove. Provided the geologist or other explorer has experience, skill, and a knowledge of the general principles of sampling, he can without much difficulty select empirical samples that are representative of the locality sampled; but during the preliminary examinations it is generally not feasible to take all the samples that may be required for a thorough knowledge of the deposit, the expense and time involved in such work being usually prohibitive. It may be added that where a deposit is being worked on a large scale, and small variations in quality do not matter, numerous empirical samples may safely take the place of a smaller number of systematic samples.

K.W. Gorringe,

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., * “ On the Prospects of finding Coal on Rowley’s Farm, near Shag Point Railway-station.” Rep. of Geol. Explor. during 1890-91, No. 21, 1892, p. 48.

* " A Review of the Amorphous Minerals.” Journal of Geology, Vol. 25, No. 6, Sept.-Oct., 1917, pp. 530 el seq.

* Orton, Edward, and Peppel, S. V. : " The Limestone Resources and the Lime Industry in Ohio.” Geol. Surv. of .Ohio, 4th Series, Bull. No. 4, 1906, p. 26.

* Orton, Edward, and Peppel, S. V. : loc. cit., pp. 27, 28.